DIYLithiumBatteriesHowtoBuildYourOwnBatteryPacks

WrittenbyMicahToll

Copyright©2017byMicahL.Toll

All rights reserved. Except for noncommercial uses as permitted under UScopyright law, no part of this publication may be reproduced, distributed, ortransmittedinanyformorbyanymeans,includingphotocopying,recording,orotherelectronicormechanicalmethods,withoutthepriorwrittenpermissionofthepublisher,exceptinthecaseofbriefquotationsembodiedincriticalreviews.For permission requests, email to the publisher, addressed “Attention:PermissionsCoordinator,”atMicahLToll@gmail.com.

Trademark notice: Products, companies or corporate names referenced in thisbook may be trademarks or registered trademarks, and are used only foridentification purposes without any intent to infringe upon the rights of thetrademarkholders.

TomywifeSapir,whotoleratesmemorethananybodyshouldhaveto…

TableofContents

Chapter1:Introduction

Usesforlithiumbatteries

Howlithiumbatterycellswork

Chapter2:Formfactorsoflithiumcells

Pouchcells

Prismaticcells

Cylindricalcells

Chapter3:Typesoflithiumcells

Lithiumion(Li-ion)

LithiumManganeseOxide(LiMn2O4orli-manganese)

LithiumCobaltOxide(LiCoO2,li-coorli-cobalt)

LithiumNickelManganeseCobaltOxide(LiNiMnCoO2orNMC)

LithiumNickelCobaltAluminumOxide(LiNiCoAlO2,NCA,orNCR)

Lithiumpolymer(li-polyorlipoorRClipo)

LithiumIronPhosphate(LiFePO4)

Generallithiumbatterycellsummary

Chapter4:Sourcinglithiumbatterycells

Buyingnewcells

Usingsalvagedcells

Buyingusedelectricvehiclebatterymodules

Chapter5:Cellratings

Capacity

Maximumdischargerate

Crate

Maximumchargerate

Maximumnumberofcycles

Chapter6:Combininglithiumbatterycellstomakebatterypacks

Increasingvoltageusingseriesconnections

Increasingcapacityusingparallelconnections

Combiningseriesandparallelconnections

Chapter7:Safety

Thedangerofshortcircuits

Theeffectoftemperatureonlithiumcells

Handlingandstorage

Chapter8:BatteryManagementSystems

WhywouldabatteryneedaBMS?

DisadvantagesofaBMS

ConnectingaBMS

Chapter9:Constructionmethods

Physicallyholdingcellstogether

Joiningcellsinseriesandparallel

Cell-levelfuses

Chapter10:Batterylayoutanddesign

Voltage

Capacity

Maximumcontinuouscurrent

Choosingappropriatecells

Currentrating

Cyclelife

Weight

Size

Safety

Groupingcellsinabattery

Makingdifferentbatteryshapes

Chapter11:Makingbatteryconnections

Safetyconsiderations

Cellmatching

Cellalignmentandcontainment

Boltedconnections

Wireconnectorconnections

Spotweldedconnections

Seriesvsparallelconnections

ConnectingtheBMS

Connectingchargeanddischargewires

Connectingthebalancewires

Addingabalanceconnector

Performingfinalqualityandperformancechecks

Chapter12:Sealingthebattery

Hardcases

Heatshrink

Coolingissues

Chapter13:Charginglithiumbatteries

ConstantCurrent,ConstantVoltagecharging

ChargingwithaBMS

ChargingwithoutaBMS

Chargingtemperature

Chapter14:Increasingcyclelife

Chargetoalowermaximumvoltage

Dischargetoahigherminimumvoltage

Chargeanddischargeatlowercurrents

Keepcellscool

Chapter15:Disposingofoldlithiumbatteries

Chapter16:Examplebatterybuildingprojects

5VbackupbatteryandUSBdevicecharger

12sRClipoelectricskateboardbattery

10s(36V)electricbicycletrianglebattery

38s(120V)LiFePO4prismaticcellelectriccarbattery

14s(52V)18650homeenergystoragebattery

4s(14.8V)18650RCdroneorFPVbattery

Conclusion

References

Index

Safetydisclaimer:

Lithiumbatteriescanbedangerous.Theycontainalargeamountofenergyinasmallvolume,andarespecificallydesignedtoreleasethatenergyquickly.Whenused properly, they can be a safe and efficientway to power nearly anything.However, when used improperly or carelessly, lithium batteries have thepotentialtocausedevastatingfiresthatcanandhaveresultedinlossofpropertyandlife.

Thisbookismeanttobeaneducationalguide.Pleasedonotattempttorecreateanythingyousee in thisbookoron the internetwithoutprofessionalguidanceand training.Alwaysuseproper safety equipment.Always engage in propertysafetypractices.Neverleavelithiumbatterieschargingunattended.Alwaysuseyourhead,besmartandbesafe.

Chapter1:Introduction

Lithiumbatterieshaveexistedinvariousformssincethe1970’s,andinnovationsinthe80’sand90’shaveledtothefamiliarlithiumbatterycellsthatweknowtoday.Currentresearchonlithiumbatterieshasproducedbatterycellscapableofextreme performance, for example, 100% recharging in just a few seconds.However, these current advances are strictly experimental and won’t seecommercializationformanyyears,potentiallydecades.Theinformationin thisbookcoversthetypesoflithiumbatteriesthatarecommerciallyavailabletodayandwilllikelyremainavailablewellintothefuture.

Usesforlithiumbatteries

Today,lithiumbatteriesareusedforaseeminglyendlessnumberofapplications.TheycanbefoundeverywherefromelectricvehiclestoNASA’sspacesuits.Duetotheirlightweightandenergydenseproperties,lithiumbatteriesareperfectforanincrediblywiderangeofapplications.

In the past, lithium batteries were mostly used by original equipmentmanufacturers (OEMs) foruse inconsumerproducts.Thesebigmanufacturersbuiltlithiumbatteriessuitedtotheirneedsforspecificproductsorlargeclients.Ifahobbyistwantedabatterysizeorshapethatdidn’texist,heorshewasoutofluck.However,todaytherearemanylithiumbatteriesandcellsthatarereadilyavailabledirectlytoconsumersforusein,well,whateverwewant!

Iwasintroducedtotheworldofcustomlithiumbatteriesduringmytimespentworking in the do-it-yourself (DIY) electric bicycle industry. I have beenbuilding lithium batteries for electric bicycles for years, largely because thenumber and variety of lithium batteries available to the consumermarket hasalways been frustratingly small. If Iwanted a specific size battery pack but itdidn’talreadyexist,Ihadnootherchoicethantomakeitmyself.Thisopenedupawholenewworldtome.Suddenly,Icouldbuildbatteriesofanyvoltage,anycapacityandmostimportantly,anysizeandshapethatIwanted.

ButDIY lithiumbatteries aren’t only limited to theworldof electricbicycles.TherearethousandsofapplicationsforDIYlithiumbatteries!

Even thoughelectriccarsarebecoming increasinglyavailable in theconsumermarket, itcanstillbecheaper(andmorefun) tobuildyourown.Manypeopleconvertallkindsofvehiclesintoelectricvehicles,andtheyneedbatteriestodoit.Unlessyouwanttobuyanexpensive,purposebuiltelectriccarbattery,you’llneedtoknowhowtoassembleyourownlargebatterypackfromlithiumbatterycells.

Just like electric vehicles, home batteries are also becoming increasinglypopular.Alithiumbatteryinthebackofyourclosetorhiddeninyourgaragecanpowerahousefordaysintheeventofapoweroutage.Theyarealsogreatforstoringenergythathasbeengeneratedonsite,suchasfromsolarpanelsorwindturbines.HomebatterystoragesystemsliketheTeslaPowerwallaregreatOEMproducts,butyoucanstillbuildyourowncustomsystemsuitedforyouruniqueneeds.Allyouneedisthebatteryknow-how!

Drones, wearables, backup batteries, toys, robotics and countless otherapplicationsareall ripeforcustomDIYlithiumbatteries.Thisbookwill teachyou how to design and build lithium batteries for all of these uses andmore.Prepareyourself,becausebythetimeyoufinishthisbook,youaregoingtobefullofknowledgeandrearingtogooutandpowertheworld!

Howlithiumbatterycellswork

Despite undergoing years of research and development, the electrical andchemicalprocessesthatallowlithiumbatterycellstofunctionisactuallyfairlysimple. As lithium ion batteries are by far themost common form of lithiumbatterycells,we’lltakealookathowatypicalcellworkshere.

Alithium-ioncelliscomposedoffourmainparts:

Cathode(orpositiveterminal)

Anode(ornegativeterminal)ElectrolytePorousseparator

The cathode varies between different types of cells but is always a lithiumcompoundmixedwithothermaterials.Theanodeisalmostalwaysgraphite,andsometimesincludestraceamountsofotherelements.Theelectrolyteisgenerallyan organic compound containing lithium salts to transfer lithium ions. Theporousseparatorallowslithiumionstopassthroughitselfwhilestillseparatingtheanodeandcathodewithinthecell.

Whenthecellisdischarged,lithiumionsmovefromtheanodetothecathodebypassing through the electrolyte. This discharges electrons on the anode side,powering the circuit and ultimately any device connected to the circuit. Thisprocess isdemonstrated in thediagrambelow.When thecell is recharged, thisprocessisreversedandthelithiumionspassbackfromthecathodetotheanode,whichisoppositetothediagrambelow.

Theactualprocessisquitesimple.Themajordifferences(andwherethingsgetmorecomplicated)areintheshapeofthecellsandtheirslightchemicalchanges.We’llcoverallofthatinformationinthenexttwochapters.

Chapter2:Formfactorsoflithiumcells

Lithiumbatterycellsareavailableinanumberofdifferentformfactors,yettheirunderlying construction is always the same. All lithium battery cells have apositiveelectrode(cathode),anegativeelectrode(anode),anelectrolytematerialandsometypeofporousseparatorinbetweenthatallowslithiumionstomovebetween the cathode and the anode. We’ll talk about how changes in thechemistryofdifferent li-ioncellscanaffect themin thenextchapter.Fornow,themaindifferencebetweenvariousshapesoflithiumcellsisthewaytheyareassembled.

Pouchcells

Pouchcellsare the simplest formof lithiumbatterycells.They look likea tinfoil bag (or pouch, get it?) and have two terminals at an edge of the pouch.Inside the pouch is a cathode and anode on opposite sides separated by theporous separator andwith the electrolyte on either side of the separator. Thiscathode-electrolyte-anodesandwichisfoldedbackandforthmanytimeswithinthepouchtoincreasethecapacityofthebattery.

Therearenostandardsizesforpouchcells.Theyareproducedbymanydifferentcompanies and areoftendesigned to exact sizes for specificproducts, such ascellphones,toensurethattheytakeadvantageofthemaximumpossibleusablespace.Productionatsuchhighvolumesallowsforthelackofstandardizationofsizes.Whenyoucanaffordtohaveamillionbatterycellsmade,itbecomeslessimportantifadealerhasyoursizeinstockornot.

Theadvantageofpouchcellsisthattheyarelightweightandcheaptoproduce.

Themaindisadvantageisthattheyhavenoexteriorprotectionandthuscanbedamaged if they aren’t enclosed in some form of protective case.A lack of ahard exterior casemeans they are the lightest andmost space efficientway toproducea lithiumbatterycell.Pouchcells areoftenused inconsumerdevicessuchaslaptopsandcellphonesduetotheirefficientuseofspace.Thesedevicesalsoserveasprotectionforthefragilepouchcellinside.

Pouch cells actually perform better when they are contained in some time ofrigid or semi-rigid structure that can apply a slight amount of pressure to thecells.Thishelpskeepallofthelayersofthecellsinclosecontactandpreventsmicro-delaminationwhichcandegradecellperformanceovertime.

Whenapouchcellagesitcanbegintoexpandor“puff”asitissometimescalledintheindustry.Thisisoftenduetosmallinteriorshortsthatoccurovertimeasthe battery ages, creating gas that puffs up the cell. Because pouch cells areentirelysealed,thegashasnowheretoescapeandthuscreatesthepuffy,pillow-likeappearance.

Theexpansionofthepouchcellresultsinareductioninperformanceofthecellasthelayersofthecellfurtherdelaminate.Somedegreeofgasbuildupcanberetainedbythepouchstructure,butwhenthegasbuildupbecomestoogreat,thepouchcanruptureexplosively.Thisisarareyetwelldocumentedphenomenon.Therupturereleasesalargeamountofflammablegas-notagreatsituationtobein.

Prismaticcells

Prismaticcellsarequitesimilartopouchcells,exceptthattheyhavetheadditionofa rigid rectangularcaseoutsideof thecell.Thisgives thecella rectangularprism (or prismatic) shape. Prismatic cells are therefore slightly less spaceefficient than pouch cells, but are also more durable than pouch cells.Whilepouch cells must be handled carefully, prismatic cells can withstand morejarring,thoughtheycanstillbefragile.

Unlike pouch cells that have thin tab terminals, prismatic cells often havethreaded terminals that allow a nut or bolt to be used for connections. Thismakes it easier to join prismatic cells into larger battery modules. Largeprismaticcellsof20Ahto100Ahormoreareoftenusedinverylargeenergystorage devices such as home batteries or DIY electric vehicles. There aren’tstandard dimensions for prismatic cells, but they often come in variouscapacitieswith5-10Ahincrements.

Cylindricalcells

Cylindrical cells are theAA-style batteries that we are all familiar with fromremote controls, flashlights and other consumer electronics. They come in avariety of sizes (most are larger than standardAA batteries) but all share thesamecylindricalshapeandrigidmetalcase.

Cylindricalcellsareproducedbyrollingupwhatamountstothesamecontentsofapouchcell,thenplacingitinsideofametalliccylinderwithapositiveandnegative terminal at either end of the cylinder. These cells are not as spaceefficient due to the rolling of the inner layers and the addition of the cylinderwallandendcaps.However,cylindricalcellsarethemostrobusttypeoflithiumbatterycellanddon’trequireanyexternalframeorsupport.

Unlikepouchcellsandprismaticcells,cylindricalcellsareproducedinstandardsizes.Themostcommonlithiumbatterycylindricalcellisthe18650cell,namedforits18mmdiameterand65mmlength.The18650isthecylindricalcellmostcommonlyusedinlaptops,powertools,flashlightsandotherdevicesthatrequirecylindrical lithium cells. Two other common sizes of cylindrical cells are the14500,whichis14mmindiameterand50mminlengthandisthesamesizeasastandardAAbattery,aswellasthe26650,whichis26mmindiameterand65mm in length. The 18650, which falls right in the middle of the three mostcommon cylindrical standard sizes, has seen the most widespread use and isavailablefromthehighestnumberofmanufacturers.

In 2017, Tesla began producing the new 21700 cell format that they co-developedwithPanasonic.The21700isaslightlylargercellthanthe18650andcomeswithadecentboostincapacitycomparedtocurrent18650s.Thecellwasdesignedspecifically forTesla’svehiclesandso itwill likely takea fewyearsuntil it becomes available for DIYers to incorporate into their own batteryprojects.

There are also a series of LiFePO4 cylindrical cells made by the companyHeadway that are available in the38120and40152 sizes,whichare38mm indiameter, 120 mm in length and 40 mm in diameter, 152mm in length,respectively.Theseareobviouslymuchbiggercylindricalcellsandhavemuchhighercapacitiesthan18650cells.ThesecellsaretheonlycylindricalcellsthathaveboltedterminalsforeasyconnectionsMostothercylindricalcellsmustbespotweldedtoconnectthemtogether.

Chapter3:TypesoflithiumcellsAll lithium battery cells aren’t created equally. There are a few differentchemistries of lithium batteries that have very different properties andspecifications.Theyallhavetheirownuniqueadvantagesanddisadvantages,solet’scomparethemhere.

Lithiumion(Li-ion)

Li-ionisthemostcommontypeoflithiumbatteryusedinconsumerelectronicslike cellphones, laptops, power tools, etc. They have the highest energy toweightratioandarealsosomeofthemostenergydensecells,meaningyoucanpackalotofenergyintoasmallvolume.

Dependingontheexacttype,li-ioncellsarerelativelysafecells,atleastasfaraslithium batteries go. Most li-ion cells won’t just burst into fire if they arepuncturedorthecellisotherwiseheavilydamaged,thoughthiscanhappenwithsome typesof li-ionandhasbeenobservedmany times.Thechanceof fire isalwayspresentinlithiumbatteries,butisusuallycausedbynegligenceorabuseofalithiumcellorbattery.Shortcircuitingabatteryisonecommonexampleofsuchnegligence,butwe’lltalkmoreaboutshortcircuitsinChapter7onsafety.

Li-ion cells also have relatively long cycle lives. The shortest are rated foraround300cyclesuntiltheyreach70-80%oftheirinitialchargecapacity,whilethelongestcanlastforover1,000cycles.Thereareofcoursewaystostretchthenumberofcyclesthatyoucangetoutofalithiumcellevenfurther,whichwe’lltalk about in Chapter 14. Just based on manufacturer’s ratings though, li-ioncellsaremiddleof the roadforcycle life,ascompared to theother twomajorchemistriesthatwe’lltalkaboutnext.

Cost isalwaysan importantfactorwhenchoosingcomponentsforanyproject.Li-ioncellsfallinthemiddlerangeoflithiumcellprices(youmightbenoticingthatli-ionissomethingofthe“Goldilocks”chemistry-it’srightinthemiddleon

manyofthesespecifications).Therearecheaperchemistries(RClipo)andmoreexpensive chemistries (lithium iron phosphate), which leaves standard li-ionsomewhereinthemiddleintermsofprice.

Whereli-ionreallyshinesisinavailability.Becausethisisthemostwidelyusedlithiumbatterychemistry, it’salso themostwidelyavailable indifferent sizes,shapes,capacitiesandslightchemicalvariationsthathavedifferenteffectsontheperformance.

Oneofthemostcommonandeasiesttoworkwithformatsofli-ioncellsisthe18650 cylindrical cell thatwe talked about in the previous chapter. There aredozensanddozensofgreatquality, topbrand18650li-ioncells,plushundredsofotheroff-brandandgeneric18650li-ioncellsaswell.Because18650saresocommonlyusedinOEMproductsincludingeverythingfromelectricvehiclestopowertools,theyhavebeendevelopedwithawiderangeofspecifications.Youcanfindcheap,lowpower18650li-ioncellslikeSamsungICR18650-26Fcellsthat are perfect for simple, low power projects, or you can find insanelypowerfulSonyUS18650VTC5,whichhavethesameapproximatecapacity,sizeandweightbutcanprovideover600%morepower!

Anyonewhomakesuseofhighpower18650cellsowesabigdebtofgratitudeto theelectricpower tool industry,by theway.Theywere someof the first todemandhigherpowercylindricalli-ioncells,whichspurredthebatteryindustrytorespondandmeetthatdemandwithnewandeverhigherpowercells.Thanksto power drills, you can now find li-ion cells that contain a massive amountpowerinsomethingthesizeofyourthumb.

It’sdifficulttosaywhichprojectsarebestsuitedforli-ionuse,mostlybecausedifferent li-ion cells span such a large range of specifications and properties.However,ifyourprojecthasspaceandweightlimitationsaswellasmoderatetohighpowerneeds,li-ionislikelyagoodoptionforyou.

Most li-ion cells have a nominal voltage of between 3.6 V to 3.7 V and areusuallyratedforadischarge-chargevoltagerangeof2.5V-4.2V.Li-ioncellsareusuallyratedformaximumcapacityatthisvoltagerange(i.e.chargingto4.2V,thendischargingdownto2.5V)butitisrecommendedtoavoiddrainingli-ioncellsallthewayto2.5Vveryoften.Theycanhandleit,butitreducestheirexpectedlifetime.Mostbatterymanagementsystems(BMSs)forli-ionbatteriescutoffdischargeataround2.7V-2.9Vpercell.Dischargingbelow2.5Vwillcause irreparable damage to the cell, resulting in the cell not holding its ratedcapacityorsustainingitsrateddischargecurrent.

Some newer li-ion chemistries are becoming commercially available that aredesignedtobechargedashighas4.3V-4.4V.Thesearestilltheexception,andmostli-ioncellsshouldneverbechargedtohigherthan4.2V.Alwayscheckthemanufacturer’s recommendations for highest rated charging voltage.Overchargingali-ioncellnotonlyreducesitslifetime,butcanbedangerousaswell.

Thereareanumberofuniqueli-ionchemistriesthatareallcontainedwithinthelarger class of li-ion cells. They all share very similar or identical anode(negative terminal) materials but have unique cathode (positive terminal)materials.Thedifferentli-ionchemistriesarelistedbelow.

LithiumManganeseOxide(LiMn2O4orli-manganese)

LiMn2O4 gets its name from the use of a manganese matrix structure in thecathode.Itwasdevelopedinthelate1970’sandearly1980’s,makingitoneofthe first commercial li-ion chemistries. LiMn2O4 can handle relatively highpowerinveryshortburstsandoffershighthermalstability.Thismakesitoneofthe safer li-ion chemistries because higher temperatures are required to causethermalrunaway.

LiMn2O4cellscanalsobetweakedforeitherhigherpowerorhighercapacityattheexpenseofeachother.ThedownsidetoLiMn2O4isitsrelativelylowercyclelifecomparedtoother li-ionchemistries.AnexampleofaLiMn2O4cell is theLG18650HB2.

LithiumCobaltOxide(LiCoO2,li-coorli-cobalt)

LiCoO2wasdevelopedaroundthesametimeasLiMn2O4andwasalsooneoftheearliestformsofcommerciallyavailableli-ioncells.Itusesalayeredcobaltstructure in its cathode. LiCoO2 is known for its relatively low cost and highcapacity,butgenerallyhasalowercurrentratingandonlymoderatecyclelife.Italso has a lower thermal runaway temperature, making it somewhat less safethanotherli-ionchemistries.

LiCoO2 is also the basis for themuchmore dangerous RC lipo batteries thatwe’lldiscusslaterinthischapter.InRClipobatteries,thechemistryisalteredtoproduce a much more powerful cell capable of sustaining extremely highdischargecurrent.Thisincreasedpowercomesat theexpenseofsafety,weightandcyclelife.

LithiumNickelManganeseCobaltOxide(LiNiMnCoO2orNMC)

LiNiMnCoO2 is a fairly new chemistry that is still undergoing constantdevelopment.NMCfallsinthesweetspotofimprovinguponthedrawbacksofmany previous types of li-ion chemistrieswhile retaining their benefits.NMCsharesmany of the advantages of both LiCoO2 and LiMn2O4 chemistries. Bycombining cobalt and manganese, then including nickel, NMC cells havedemonstratedrelativelyhighpower,capacity,andsafety.

Byadjustingtheratioofcobalt,manganeseandnickelinthecathodeaswellasincludingothertraceelementsinboththecathodeandanode,NMCcellscanbetweakedfor improvedperformance innearlyanymeasurementcategory.Otherchemistriesarecapableofachievingbetterperformanceinsomecategories,butNMChave someof the highest all-aroundperformance figures of any lithiumbatterychemistry.

ThismakesNMSanexcellent“allaround”chemistry.Itdoesn’thavethehighestperformance in any single category, but it has some of the highest averageperformance of any chemistry. An example of an NMC cell is the SamsungINR18650-25R, which is optimized for relatively high power and mediumcapacity.

LithiumNickelCobaltAluminumOxide(LiNiCoAlO2,NCA,orNCR)

LiNiCoAlO2 is very similar to theNMCchemistry above, butwith aluminumswapped formanganese in the cathode.The additionof aluminumhelpsNCAcellsachievethehighestcapacityofallli-ionchemistries.Thedownsidesareaslightdecrease incycle lifeandpowerascompared tomostotherchemistries.AnexampleofanNCAcellisthePanasonicNCR18650Bcell,whichwasusedformostorallofTesla’searlyelectricvehicles.

LikeNMC,NCAisaverypromisingchemistryforfuturedevelopmentofli-ioncells. It isbest suited forhighcapacity, energydensepurposes.This iswhy itwasselectedbyTesla foruse in theirelectriccars.NCAexcels inpacking themost energy into the smallest space. With a large enough battery, its lowerrelativepowercanbemitigated.However, continued researchand incrementalimprovementsarehelping to increase thepowerof this typeofcell,making itquitecompetitivewithNMC.

Lithiumpolymer(li-polyorlipoorRClipo)

There’s an immense amount of confusion out there regarding lithiumpolymerbatteries.Thisismostlybecausethecellsthatthetermwasoriginallycreatedforandthecellsthatmostpeopletodaycalllithiumpolymercellsarenotthesamething.

Rememberwhenwediscussedhowlithiumcellsaremade?Howtheyhaveananode,acathode,andaliquidormorecommonlygelledelectrolytematerialinbetweenthetwo?Right.Sotheoriginalterm“lithiumpolymerbattery”referredtoanewtypeoflithiumcellsthatusedasolid(sometimesreferredtoas“dry”)electrolyte instead of the common liquid or gelled electrolyte. The solidelectrolyteused in these experimental cellswas apolymer,orplasticmaterial,givingrisetothename“lithiumpolymerbattery”.

This new technology for dry electrolyte batteries promised incredibly safebatteries.However,itnevermadeitoutofthelaboratoryonanylargescale.Theproblem was that the dry electrolyte didn’t conduct electricity very fast atambienttemperature.Thatmeantthatthebatterieshadtobeconstantlyheatedtofunctionproperly.Thiswasobviouslyadealbreakerformostapplications.Whowantsabigheaterbuiltintotheircellphoneorlaptop?

So the original lithium polymer batteries never really went anywhere. Theproblem with the name began when some manufacturers started referring toother cells that had a polymer packaging, namely pouch cells, as “lithiumpolymer”cells.Thisbecameconfusing,asthesecellsdidn’treallyhavepolymer

electrolytes, but instead had their liquid electrolytes gelledwith the use of anexternal polymer.These should really be called “lithium-ionpolymer” cells todistinguishthemfromtheoriginal,noncommercialized“lithiumpolymer”cells.However,oncepeoplestartedcallingthemlithiumpolymercells,theconfusionbegan.

Buttheconfusiondoesn’tstopthere!Becausewhatpeoplebegancallinglithiumpolymer batteries (whichwere actually lithium-ion polymer batteries in pouchcell formats) are actually nearly identical to the standard lithium-ion batteriesthatalreadyexisted.Theyhavethesameorsimilarcathodeandanodematerialsand similar amounts of electrolyte. The main difference is that lithium-ionpolymer batteries use amicro-porous electrolyte instead of the normal porousseparatorlayerplacedintheelectrolyteofli-ioncells.

Thatmeansthatall“lithium-ionpolymer”and“lithium-ion”batteriesavailabletodayaretechnicallyli-ionbatteries.They’reallsimilarandtheyallfunctionbytransporting lithium ions back and forth through an electrolyte. But the term“lipo”,whichisshortforlithiumpolymer,hasnowcommonlybeenusedtorefertotheshapeandstyleofcells,namelypouchcellswhosepouchesaretechnicallyapolymermaterial.Because thisuseof the term“lipo” tookoff,manypeoplenowthinkthatalipocellisanothernameforapouchcell.Infact,apouchcellissimplyatypeofbatterycellstructureanditcanbeusedtomakeli-ion,LiFePO4orpotentiallyothernewchemistriesinthefuture.So“pouchcell”describestheshape, not the chemistry. But now everyone seems to be running aroundreferringtopouchcellsas“lipos”.

Lastly,thereisanentireclassofli-ioncellsusedforradiocontrolled(RC)toysandvehiclesthataregenerallyreferredtoaslipobatteries.Theseareextremelyhigh power li-ion cells that are specifically used in the RC industry for their

abilitytoprovidethehighestpossiblecurrent.

The most common usage of the term lipo nowadays is to refer to these RCbatteries.Forthatreason,intheremainderofthisbook,IwillrefertothesehighpowerRCbatteriesas“RClipo”batteries.Thisisnottheoriginalhistoricaluseofthetermlithiumpolymer,butitisthecommonlyusedconventiontodayandthusitishowIwilluseit.WheninRome...

Butpleasebeaware that there ismuchconfusion in the industryregarding thetermslithiumpolymer,li-ionandlipo.Forourpurposes,“RClipo”willrefertoli-ionbatteriesspecificallydesignedforRCpurposes,andallother lithiumionbatterieswillbereferredtoasli-ion.Iwon’tusethetermlithiumpolymerasanycellonthemarketbeingcalled“lithiumpolymer”todayisreallyjustlithiumion,andthereal“lithiumpolymer”cellsneverreallymadeitoutofthelab.

Phew! Ok, I’m sorry that took so long, but it’s important to point out theconfusion and try tomake senseof it.Now let’smoveon to actually learningaboutRClipobatteries.Whichshouldbefun,becausethesearetheonesthatgokaboomwhenyoumessupwiththem.

Let’sget thisoutof theway immediately:RC lipobatteriesare thedangerousones.Thesearetheonesthatareitchingtoburnyourhousedownifyoudon’tfollowpropercharginganddischargingprocedures.Theycanbesafe,but theyarealsoincrediblyvolatilewhenusedimproperly.

Alright,nowthatwe’vegotthatoutoftheway,let’slookatwhatmakesRClipobatteriesspecial.RClipocellsareaspecialtychemistrybasedonlithiumcobaltthatissuitedforhighpowerapplications.Theycanprovidesuperhighdischargeratesforlongperiodsoftime,andinsanelyhighdischargeratesforshortperiods(beforetheyoverheatandwegetbacktothatunfortunatefirescenarioIwarnedyouabout).

RClipocellsarealmostexclusivelyusedintheremotecontrolvehicleindustryforapplicationssuchasRCdrones,helicopters,planes,cars,etc.Thesedevicesrequireveryhighdischargeratesfromasmallandlightweightbattery.RClipocellsaren’tthelightestcells(thosearevariantsofconventionalli-ioncells),buttheycanprovidemuchhigherpowerforonlyaslightlyhigherweight.

RClipocellsarealsothecheapestlithiumcellsavailable.Theycostmuchlessthanli-ionandLiFePO4cells(we’lllearnaboutLiFePO4shortly),makingthemattractiveforotherapplicationsthatmakeuseofDIYbatteries,suchaselectricbicycles.

Onemajordrawback(besidesthefactthatRClipoarebasicallylittlebombsthatcan also power electronics) is that RC lipo cells have very short cycle lives.Reaching 200 cycles on a RC lipo cell would be considered fairly goodperformance.SomeRCliposcanbepushedcloserto300cells,butdon’tlastaslongasli-ioncellsandcan’tevencomenearLiFePO4cells.(Note: thesecyclecountsarebasedoncompletecharginganddischargingcycles.We’lltalkabouthowpartial charginganddischargingcanextend the lifeofnearlyall typesoflithiumbatterycells.)

Another issue with RC lipo cells is their more complicated charging process.Whileli-ionandLiFePO4cellsareprettyeasytocharge,especiallywhenusingabattery management system (BMS), RC lipo cells require more expensivebalancechargerstoensurethatallcellsinabatteryaremaintainedatthepropervoltageandbalancedwithoneanother.

ThereasonforthisisthatwhenRClipobatteriesstrayfromtheirratedvoltagerange, they become incredibly volatile. Do a quick search on YouTube for“overcharging RC lipo cell” to get an idea of what I mean. It is criticallyimportant that RC lipo cells are charged within their specified voltage range.Theyshouldalsoneverbedischargedtoolow.DischargingaRClipocellbelow2.5 volts and then charging the cell can result in combustion of the cell,especially at higher charging currents. For this reason, RC lipo cells must bemonitored carefully during discharge as well to ensure that they are neverdrainedtoofar.

It is possible to rechargeRC lipo cells that have been over discharged, but itmustbedoneatverylowcurrentsandcaneasilyresult infire,dependinghowdamagedthebatterycellwas.Ideallythiswouldn’tbeattempted,butifitwas,itshouldbedoneinamonitoredenvironmentandawayfromanythingflammable.

RC lipocellsareelectro-chemicallysimilar to li-ioncells,andhaveanominalvoltageof3.7V.However,becausecaremustbetakennottoover-dischargethecells, it isnot recommended todischarge themlower than3.0V.Aimingforahighervoltageof3.2Visconsideredsafer.IntheRCaircraftfield,manypilotswillstopflyingwhenabatteryreachesashighas3.5V,thusmaintainingalarger

safety margin. The maximum voltage of RC lipo cells should never exceed4.2V.

It should also be noted that these voltages are considered the “under load”voltage.Dependingonthecurrentload,alithiumbatterycell(ofanychemistry)willseeadropinvoltage.Thisdropinvoltageisknownasvoltagesag.AnRClipocellshouldneverdropbelow3.0Vwhileinuse.Ifdischargingstopsat3.0Vunderload,thevoltagewhenmeasuredaftertheloadisremovedwillreturntoahighervoltage,likelyinthe3.3V-3.5Vrange,thoughanevensaferlevelforatrestvoltageisaround3.7V.Forthisreason,itiscriticallyimportanttomonitorRClipocellsunderloadtoensuretheyneverover-dischargebeyondasafelimit.

LithiumIronPhosphate(LiFePO4)

Lithiumironphosphate is technicallyasubsetof themoregeneral li-ionclass,but it isuniqueenoughthat it isoftenlistedseparately.LiFePO4cellsarebothheavier and less energy dense than most li-ion cells. This means that batterypacksbuiltfromLiFePO4cellswillbebulkierandmoremassivethanli-ionorRC lipo batteries of the same voltage and capacity. The exact amount variesdependingonthecellformat,butyoucanexpectaLiFePO4batterytobearoundtwiceaslargeandtwiceasheavyasacomparableli-ionbattery.

LiFePO4cellsarealsosomeofthemostexpensivecells.Theircostvariesbasedonmany factors including cell size, format, vendor and location, but you canexpect to pay around 20%more for LiFePO4 cells than for li-ion cells of thesamecapacity.

Most commonly available LiFePO4 cells also have a lower discharge rate,meaning they can’t provide asmuchpower, though this isn’t always the case.Somecells,suchasthosemadebythehighqualitybatterycompanyA123,canprovidehighpowerlevelsbutcostapremiumandarehardtosource.ThosearemostlysoldtoOEMsforuseinconsumerproductslikepowertools.

ItiscommontohearLiFePO4beingtoutedforitshighdischargerate,butunlessyousourceLiFePO4cellsthatarespecificallydesignedforhighdischarge,mostLiFePO4cellshaverelativelylowdischargerates.

With all of these downsides,whywould someonewant to useLiFePO4cells?ThereareactuallytwobigadvantagesforusingLiFePO4-cyclelifeandsafety.LiFePO4 have the longest rated cycle life of all commonly available lithiumbatterycells.Theyareoftenratedforover2,000cycles.Theyarealsothesafestlithiumbattery chemistry available.While fires fromLiFePO4 cellshavebeendocumented, they are incredibly rare. The electrolyte used in LiFePO4 cellssimply can’t oxidize quickly enough to combust efficiently and requiresexceedingly high temperatures for thermal runaway, often higher than thecombustiontemperatureofmanymaterials.

SowhenshouldyouuseLiFePO4cells?ThebestapplicationsforLiFePO4cellsare projects that require long cycle lives and high safety, don’t have criticalspaceorweightlimitations,anddon’trequireveryhighlevelsofpower(unlessyouspecificallysourcehighpowerLiFePO4cells).

LiFePO4cellshaveanominalvoltageof3.2Vpercellandadischarge-chargevoltage range of 2.5V - 3.65V. Just likewith li-ion cells, discharging below2.5 V will cause irreparable damage to the cell, though it isn’t necessarilydangerous,likeintheRClipocellsthatwejustlearnedabout.

Generallithiumbatterycellsummary

All of the information I have described for the lithium chemistries above isgenerally true for the majority of lithium battery cells that are commerciallyavailable today.Thatbeingsaid, thereareexceptions inallof thesecases.Thenewesttechnologyforlithiumbatterycellsfaroutpaceswhatyoucancurrentlybuy.Rightnowtherearelithiumioncellsonlabtablesincleanroomsthatcancompletely charge in a few seconds, weigh a fraction of what previous cellsweighedandperformallsortsofamazingfeats.

However, these are all experimental batteries and have not yet beencommercialized.Thebatterieswecanbuyandusetodayweredevelopedyearsago, sometimes decades ago, and have undergone extensive development andcommercializationprocessestoreachourworkbenches.

Ina fewyears thepropertiesdescribed in theprevioussectionsof thischapterwill likely begin slowly changing and improving as we enter a new era oflithiumbatteries.Thatmightbeinthenext5years,oritmightbeinthenext20years. For now though, the above descriptions cover the batteries that aregenerallyavailableforustousetoday.

Chapter4:Sourcinglithiumbatterycells

Thereareanumberofdifferentwaystofindthelithiumcellsthatyou’llneedfora DIY battery project. Lithium battery cells aren’t cheap, and thus the mainfactors thatwill likelyaffectyourdecisionwillbecost andavailability.Whilebuyingnewcells isalwaysthebest(andsafest)method, thereareanumberofoptionsavailable.We’llcovermanydifferentoptionsinthischapter.

Buyingnewcells

Thefirstmethod,andtheoneIgenerallyrecommend,istobuynewcellsfromestablished vendors. Lithium battery cells are not cheap, but by buying newcells,you’llbe sure thatyou’regettinghighquality, safecells thatwill last aslong as you expect them too. That might be a few hundred cycles or a fewthousand,dependingon the typeofcells,butat leastyouaren’t likely tohaveanysurprises.

Dependingwhereyoulive,youmightnothaveagoodlocalsourceforbuyinglithiumbatteries.Don’tworry though - there aremany sources to buy lithiumbatteries online. For years, lithium batteries were almost exclusively made inAsia,meaningthatthebestmethodtobuycellswastopayastrangeronlineandwaitamonthforthemtoarriveinthemail.Nowthereareanincreasingnumberofresellersincountriesaroundtheworld,makingiteasiertodealwithavendorthatisatleastlocatedinthesamecountry,eveniftheyaren’tlocaltoyourarea.

IstillmostlybuymybatterycellsstraightfromChina,whereyoucanusuallygetthe best price. If you’re buying small cells, like RC lipo packs or 18650batteries, then buying online from halfway around the world can be a goodoption. Such lithium cells are usually small and can be shipped easily andcheaply. However, the constantly changing shipping regulations for lithiumbatteriesmightnotmakethistrueforever.

Igenerallybuyboxesof18650cellsbythehundred,whichusuallyresultsinaprettysignificantprice reductionofaround10-30%compared tobuyingsingle

18650cells.

Acoupleof themostcommonsourcesIuseforbuyingsmaller lithiumbatterycells are Alibaba and their retail division AliExpress. Alibaba is good forwholesalepurchaseswhereyou’llbebuying100+cells,thoughsomevendorsonAlibabadon’twanttosellquantitiesoflessthan1,000cells.OnAliExpress,youcanbuycellsanywherefromsingleunitsuptomanythousands.

Alwayslookforavendorthathasbeeninbusinessforatleastafewyearsandhaslotsoffeedbackratings.It’struethatgoodfeedbackcanbeboughtorfaked,but generally these vendors are selling in high enough volume that any badfeedbackwill stillmake it through.Vendorswithhundredsof transactionsandgenerallypositivefeedbackareusuallygoodsources.

Thereisaverylargeindustryoffakelithiumcells,whichareusuallyoff-brandcells that have been rebadged to look like brand name cells. When buyingdirectlyfromAsia,itcanbehardertodetermineifthevendorissellinggenuineor counterfeit cells. Only buying from reputable vendors with years of goodfeedbackhelpsstacktheoddsinyourfavor,butit’sstillhardtobe100%sure.Online forums for people building similar projects as you (such as electricbicycleorDIYpowerwall forums)oftenhave largecommunities thatcanhelpyoufindthemosttrustworthydealersusedbymembersofthatindustry.

Ifyou’rereallyworriedaboutgettingbadorfakecells,orderjustafewcellsatfirsttotestthevendor.Thatwayyoudon’tcommittoalargeorderfromanewvendoronlytolaterfindoutthatthecellsaren’tgood.Youcouldalsoconsider

justgoingwitha localvendor thatguarantees theauthenticityof thecells theyprovide. Itwillbemoreexpensive,butyoucanbesure thatyou’regetting therightcells,andyou’llpresumablyhaverecourseifyoudon’tgetwhatyoupaidfor.

If you’re buying large cells, such as those used in many electric vehicleconversions, your sources are going to be more limited. Most commonlyavailablelithiumbatteriescomeinsmallformfactors.Bigcellsexist,butthereare fewermanufacturers and vendors.You’ll likely do better by looking for avendorinyourcountrythatcanimportlargelithiumcellsdirectlyforyou.Youcanalwaysusemanysmallcells(Teslaelectricvehicleshavethousandsofsmallcells),but forbigprojects likeelectricvehicleconversionsorvery largehomeenergystoragebatteries,largecellscansaveyoualotofworkintheassemblyphase.

Lastly, ifyouaregoingtospendthemoneytobuynewcells,youshouldonlybuy name brand cells made by recognizable companies like Panasonic,Samsung, Sony,LG, etc. There are a number of off brands out there that sellcheaperlithiumcells,buttheydon’thavethesamequalityasthebigplayers.

In the thousandsofPanasonic andSamsungcells that I’vepurchasedover theyears, I’ve never once gotten a bad cell. The name brand companies all haveexcellent quality control. They test and remove any bad cells at the factorybeforeshipment.Othercompanieshavelowerstandards,andit’snotuncommontogetabadcellortwoinashipmentofafewhundred.Whilethismightseemnegligible, it can actually have a devastating effect if you’re building a largebattery.A singleweakcell canendupdraggingdownanentireparallelgroupconnectedtoit,ruiningmanymorecells.

Therearealsoseveralbrandsofcellsthatsimplybuyrejectedcellsfromthebigcompanies and repackage them with ridiculously overrated specifications.Ultrafireistheposterchildforthispractice,butotherbrandssuchasTrustfire,Surefireandothersdothesamething.Ifyouseecellsthatarecheaperthannamebrandyetclaimtohavehighercapacities,youcanbesuretheyaren’tlegitimatecells.Ifitsoundstoogoodtobetrue,itprobablyis.

Usingsalvagedcells

Anothercommonsourceforlithiumcellsistofindsalvagedcells.Lithiumcellsused in consumer electronics like laptops and power tools are rated formanyhundredsoreventhousandsofcycles.Thiscanbegreatwhentheproductsaredesigned to last, but somany products end up dying prematurely for variousreasons. The lithium battery cells in these products are often then discardedalongwiththeelectronicslongbeforethecellsareactuallydead.Alaptopmaydie after a few years, but the batteriesmight have only gone through 20%oftheirratedchargecycles.

ManyscrupulousDIYershavefoundthattheycancollectthesediscardedOEMbatteries,open themupandsalvage the lithiumcells inside.Thiscanbe fairlylaborintensivedependingonhowthecellsareheldintheirOEMpackagingandhowmanycellsareincludedineachbattery.However,thepricesimplycan’tbebeat. Salvaged batteries are likely the cheapest way to get a large supply ofbatterycells.

Manyofthesediscardedbatterypackscanbeacquiredforfreefrombusinessesthat accumulate huge quantities, such as computer repair stores, recyclingcenters, hospitals and clinics that use battery powered devices, etc. In manyplaces it is technically illegal to throw away these lithium battery packs innormal municipal garbage, and so a lot of these businesses actually paycompaniestocomeandtakethesebatterypacksawayforrecycling.Byofferingto take the batteries off of their hands, you are saving them the expense andheadacheofdealingwithwhattheyconsidertobeelectronicwaste.

It’sliketheyalwayssay,“oneman’selectronicwasteisanotherman’streasure”.Orsomethinglikethat.

As this practicehasbecomemore common, someplaceshavebegun chargingfortheirdiscardedbatterypacks,butthepricesarestillmuchcheaperthanyou’dspendonnewcells.Afewdollarsperpoundisacommonfigureforthesebulkbatterypacks.

As Imentioned above, I always recommend buying new cellswhen possible.Thisisbecausethereareafewmaindownsidestousingsalvagedcells.

1) Youneverknowwhatqualityyou’llgetfromsalvagedbatteries.Aretheyname brand? Are they counterfeit or inferior quality cells? Did thepreviousownerabusethem?

2) Youdon’tknowhowmuchcapacityeachsalvagedbatterycellhas.Whenyouusesalvagedbatteries,youneedtogothroughandcheckeverysinglecell for capacity to determine which cells hold a charge and havereasonable battery capacity. Old cells lose their capacity, so it’s notuncommontothrowoutoverhalfofthesalvagedcellsfromasinglehaul.Theprocessofcheckingeverycellforcapacitycantakeanywherefromasingleafternoontomanymonths,dependingonhowmanycelltestersyouwanttoinvestin.

3) Youdon’tknowhowmuchlifeisleft ineachsalvagedbatterycell.Wasthecellonlyusedfor20%ofitstotalratedcycles,orwasitusedfor90%ofthem?Ifyouknowtheoriginalratedcapacityofthecell,youmightbeable to approximately gauge this by how close it still is to its ratedcapacity.However,manysalvagedcellsaren’tmarkedwiththeiroriginalcapacity or even any identifying information. That means you’re leftguessing. Furthermore, if you build a pack out of cells with differentremaininglifeexpectancies,thefirstcellstobegindyingwillstarttodragdowntherestofthecellswiththem.Thegoodcellswillhavetopickupthe slack,whichmeans that the good cellswill get overworked and diequicklyaswell.

Dependingonyourgoalsandrequirements,thesedisadvantagesmightbemoreor less of an issue. If you want a powerful battery for an electric vehicle orelectricbicycle,salvagedcellsarenotthebestwaytogo.Therearesimplytoomanyunknowns.Ifyou’rebuildingabackupbatteryforyourhouseortostoreenergyfromyoursolarpanelsanduseitareasonablerate,salvagedcellscanbea great option. In fact, a huge portion of theDIY powerwall community usessalvagedcellsexclusivelyintheirbatteryprojects.

Themainthingtorememberwhenusingsalvagedcellsisthatyoushouldalwaysuse themconservatively -don’t try todraw toomuchpoweroutof them. It isbetter to build a larger capacity battery thanyou thinkyou’ll need in order todrawlesscurrentfromeachsalvagedcell.We’lltalkmoreaboutcellratingsandhowmuchpowercellscanhandleinChapter5.

Buyingusedelectricvehiclebatterymodules

This option is something of a compromise between buying new, name brandcellsandfindingfreeorcheapsalvagedcells.Usedelectricvehiclebatterypacksare becoming increasingly available on the second handmarket through siteslike eBay. With more electric vehicles on the road, more of these batterymodulesareavailableduetocrashes,replacementsandupgrades.

Buying used electric vehicle batterymodules stillmeans that you don’t knowexactlyhowmuchlifeisleftinthebatteries,butyouatleastknowtheyareallatthe same level of charge cycles (life expectancy) and that they are all namebrand,genuinecells, assuming theycame fromacar that isknown forhavinggood cells. Most electric vehicles are made with good cells though - there’ssimplytoomuchatstaketoskimponcellquality.

Some electric vehicles, like those made by Tesla, have batteries built fromthousandsofsmallercells.Otherelectricvehiclescanhavebatteriesmadefromjustafewlargercells.Dependingonwhatyourrequirementsare,you’llhavetodecidewhichtypeofbatteryisbettersuitedforyourproject.

Chapter5:Cellratings

Nowthatwe’vetalkedaboutdifferenttypesofcellsandwheretogetthem,weneed to learn about different cell specifications andwhat theymean. Just likecrayonsintheCrayolaBigBox,thereareaseeminglyinfinitenumberoflithiumcellsout there.Manyof them look similar,but their specificationsand ratingsarewhatsetthemapart.

There’sactuallyaverylonglistofdifferentspecificationsandratingsforcells.Herewewill look at themost important specifications that affectwhich cellsyou’lluseforagivenproject.

Capacity

Thecapacityofacell isprobablythemostcriticalfactor,asitdetermineshowmuch energy is available in the cell. Capacity of lithium battery cells ismeasuredinamphours(Ah)orsometimesmilliamphours(mAh)where1Ah=1,000mAh.Lithiumbatterycellscanhaveanywhere froma fewmAh towellover100Ah.

Occasionallytheunitwatthours(Wh)willbelistedonacellinsteadoftheamphours. Watt hours are another unit of energy, but also consider voltage. Todetermine the amp hours in this case, simply divide the watt hours by thenominalvoltageofthecell.Forexample,tocalculatethecapacityinamphoursofalithiumioncellwithanominalvoltageof3.7Vandacapacityratinginwatthours of 11.1 Wh, simply divide 11.1 Wh by 3.7 V to get 3.0 Ah (or3,000mAh).

Amphoursofcell=watthours÷nominalvoltageofthecell

Orinourexampleabove,

Amphoursofcell=11.1watthours÷3.7volts=3.0amphours

Capacityratingsonlytellyouhowmuchenergythecellcanstoreandprovide.Theydon’tgiveyouanyinformationaboutthepowerofthecelloritslongevity.Infact, thehighestcapacitybatteriesusuallyhaveonlymoderatepowerlevels.Thereisoftenatradeoffbetweenpowerratingandcapacity.Therefore,theonlythingyoucanusethecapacityratingforistodeterminehowmuchenergyisinacell.

Onelastnoteaboutcapacity-don’texpectyourcellstoachievetheirentireratedcapacity.Manufacturersusuallytestthecapacityoftheircellsusingtwo“tricks”toeekoutasmuchcapacityaspossible.Theytestatanincrediblylowdischargerate,usuallyabout0.2C(we’lltalkaboutCrateslaterinthischapter).Theyalsodischarge the cells all thewaydown to their absoluteminimum ratedvoltage,often 2.5V formost li-ion cells. Discharging that low is possible, but it willdecreasethelifetimeofthecellifitisdonetoooften.Mostcommercialproductsusingli-ioncellsdischargedowntoaround3.0V, ifnothigher, inorder togetlongerlifeoutofthecells.

Sojustbecauseyourcellmightsayit’srated3.0Ah,don’tbesurprisedtoseeitperformcloserto2.9Ahwhenyoutestit.

Maximumdischargerate

Themaximumdischargeratingtellsyouthemaximumload,whichistosaythemaximum current, that can be drawn from the cell. There are actually twocommondischargeratings,the“maximumcontinuousdischargecurrent”andthe“maximumpeakdischargecurrent”.Themaximumcontinuousdischargecurrentis thebetter figure tousewhenmakingcomparisonsbetweencells.This is themaximumcurrent that thecell cansupplycontinuouslywithoutoverheatingordamagingitself.Ifthethemaximumcontinuouscurrentratingis10A,thenthecellcanprovide10Aofcurrentcontinuouslyfromitsfullchargestateuntilitsemptystate.

Themaximumpeak discharge current is the amount of current that a cell canprovideforashortburst.Everymanufacturerratesthisdifferently,whichiswhyitishardtousethisnumberforcomparison.Somemanufacturersconsiderthistobea2-3secondburst,whileothersconsidera10secondorlongerperiodforthemaximumpeakdischarge.

Regardless, you should never exceed the maximum peak discharge rating formore than a couple seconds, and if possible, you should try to design yourbattery to be sufficiently large that you never even reach the maximum peakdischarge(we’lldiscussbatterydesigntominimizecurrentloadinChapter6).

Becausemanufacturersusually rate theircellsat theextremeendofwhat theyarecapableof,itisneveragoodideatopushthemtothelimitoftheseratings.Furthermore, battery cells that are operated near their rated limits tend to get

veryhotandoperateinefficiently,robbingthemofupto10%oftheirdesignedcapacity.Soifyouwanttousetheentirecapacityofabatterycell,don’tpushittoitsmaximumdischargecurrentlimit.

Crate

TheCrateofabatterycellisameasurementoftheratethatthebatterycellcanbedischargedorcharged in relation to thecell’scapacity.TheCratedoesnotchangebasedonthecapacityofthebatterycell;rather,itisanintrinsicpropertyofthebatterycellitself.ThatmeansthattwocellsthatareidenticalineverywayexceptfortheirratedcapacitieswillalsohaveidenticalCrates.

If that sounds confusing, then don’t worry. We’re going to work out someexamples.

TheC rate is calculated as amultiple of the capacity rating of the battery.Abatterycellwitharatedcapacityof2Ahandamaximumcontinuousdischargecurrentof4AhasaCrateof2.Thiswouldbeknownasa2Cbattery.Inthisexample,wefoundtheCratebydividingthemaximumdischargecurrentratingbythecapacity,whichiscalculatedas:

Crate=4A÷2Ah=2

This givesus aC rate of 2. If that same2Ahcell had amaximumdischargeratingof5Ainsteadof4A, itwouldbea2.5Cbattery. If ithadamaximumdischarge rating of 10 A, it would be a 5 C battery. If it had a maximumdischargeratingof10Abutithadacapacityof5Ahinsteadof2Ah,itwould

bebacktoa2Cbattery.Gotit?Ifnot,tryworkingoutthoseexamplesonpaperusingthesameequationasabove.We’llalsohavesomemoreexamplescomingupsoon.

TheCrateisimportantbecauseitisusedtocomparetherelativepowerofcells,evenwhencellshavedifferentratings.Abig10Ahcellmightberatedfor10Amaximumdischargewhileasmaller2.5Ahcellisonlyratedfor5Amaximumdischarge.At first itmight seem like the big cell ismore powerful, as it canprovidetwicethecurrentthanthesmallcellcanprovide(10Ainsteadof5A).However,itisthesmallercellthatisinfactmorepowerfulrelatively,asithasahigherCrating.ThesmallercellhasaCratingof2whilethelargercellhasaCrating of 1. If we combined four of the smaller cells together in parallel, wewould make a 10 Ah battery that was rated at 20 A maximum discharge.Comparethattotheoriginal“bigger”10Ahbatterycellandwecanseethatthebigger cell is in fact the weaker cell, as it is rated for only 10 A maximumdischarge.

Occasionally lithiumbattery cells aremarketedwith just aC rating and not amaximumcurrentrating.Thiscanmakeiteasiertocomparethepowerlevelofbatterycellsofdifferentcapacities.Aslongasyouknowthecapacityofthecell,youcanuse theC rate toquicklycalculate themaximumcurrent ratingof thecell.

Maximumchargerate

Themaximumchargerating issimilar to themaximumdischargeratingand isalsofairlyselfexplanatory-it’sthemaximumratethatyoucanchargethecell.Mostcellswillhaveacharge ratingofnotmore thanabout0.5C.Chargingacellataratenearitsmaximumchargeratingwillshortenthelifeexpectancyofthecell.Itisrecommendednottochargemostlithiumcellsatmorethan0.5C,andchargingcloserto0.2Cismuchbetterforthecell’shealth.

RememberhowwecalculatedtheCratingaboveforthedischargerate?Itworksthesameforthechargerate.A5Ahcellchargedat0.5Cwouldbechargedat2.5 A. However, if you charged a 2.5 Ah cell at 2.5 A, that would be 1 Ccharging(andveryfastchargingaswell,bylithiumbatterystandards).

Maximumnumberofcycles

Depending on the type of lithium battery, the number of cycles could beanywherefrom200to3,000ormore.Cycleratingscanbedifficulttocomparefromonecelltothenextthough,asmanufacturer’sdon’talwaysuseastandardratingsystem.

Themostcommonratingsystem is thenumberofcyclesbeforeacell reaches80%ofitsoriginalratedcapacity.Thecapacityoflithiumcellsslowlydegradesover timewith increasingchargecycles.At80%of theoriginal ratedcapacity,manymanufacturersconsiderthecelltohavereachedtheendofitsusefullife.Some manufacturers give a rating of charge cycles until 70% capacityremaining.Somedon’tevenspecify,andjuststateanumberofcyclesuntil“endoflife”,leavingitunclearexactlywhattheyconsidertheendoflifetobe.

Regardless,whenacellreaches80%oreven70%ofitsoriginalratedcapacity,it isn’tnecessarilydead, it just isn’tgoing toperformaswell.Notonlywill itobviouslynothaveasmuchcapacity,butitwillalsohavealarger“voltagesag”,ordropinvoltageunderload.Thiswillresultinlesspowerandanevenshorterworkinglifeforthebattery.

There are many other ratings and specifications for most cells, includingeverything fromdimensions to temperature ranges.However, the ratings listedabove: capacity, discharge/charge ratings and cycle life are generally themostimportantparametersforchoosingbatterycellsformostprojectsandalsohelpwithcomparisonbetweenmultiplecells.

Chapter6:Combininglithiumbatterycellstomakebatterypacks

Larger lithiumbatterypacksaremadebycombining individual lithiumbatterycells. By combining multiple cells, different voltages and capacities can beachieved. The way these cells are combined determines the ultimatespecificationsforeachresultingbatterypack.

Increasingvoltageusingseriesconnections

Individual lithium battery cells are usually 3.7 V nominal (for li-ion cells) or3.2 V nominal (for LiFePO4 cells). This voltage is acceptable for some lowpower devices, such as cellphones and other small electronics, but it can’tprovide enough power for anything more substantial. For bigger projects,includinganythingfromanelectricskateboardtoanelectriccar,multiplelithiumcellsarewiredinseriestoincreasethevoltageofthebattery.

Inaseriesconnection, thepositive terminalofonebatterycell isconnected tothe negative terminal of the next battery. If you’ve ever slid more than onebattery ina row intoa flashlight tube, that isaseriesconnection.Thepositiveterminalofonecellalwaysconnectstothenegativeterminalofthenextcell.

These series connections can combine just twocells orhundredsof cells.Thenumberofcellswiredinseriesdependsonwhatvoltageisrequired.Tocalculatethe voltage of a set of battery cells connected in series, simply multiply thevoltageofonecellbythenumberofcellsintheseriesconnection.

Totalvoltageofcellsinseries=nominalvoltageofasinglecell×#ofcellsinseries

Ifwe are using 3.7Vnominal li-ion cells andwe connect two cells in series,

we’llendupwitha7.4Vnominalbatterycalculatedasfollows:

Totalvoltage=3.7V×2cells=7.4Vtotal

Ifweaddedonemorecell into that seriesconnection fora totalof threecells,we’dhave an11.1Vbattery.Tencells in serieswouldgiveus37Vnominal.Fifteencells inserieswouldgiveus55.5V,and I thinkyouget the idea fromthere.

Animportantthingtorememberisthatthenominalvoltageofabatterycelloralargerbatterypackisjustthat-“nominal”,whichcomesfromthesamerootastheword“name”.Basically, thesecells are“named”3.7Vcells,but in realitytheyspanamuchwidervoltagerangeduringuse.

A single 3.7 V nominal lithium-ion cell can be charged up to 4.2 V anddischargedaslowas2.5V,whichisaverybigrange.Imagethatweconnect10of thesecells inseries tocreatea37Vnominalbattery.Thatbattery’svoltagewillactuallyrangefromafullychargedvoltageof42Vdowntoaminimumof

25Vifdischargedto0%stateofcharge.

Ifwehaveadevice that requiresat least30V tooperate, thenwewouldstopdischargingat3.0Vpercellinthis10cellbattery,eventhoughthebatterycouldhavekeptdischargingdownto25V.Thatequatestonotusingabout5%ofthepack’s total capacity. You might not think 5% is a big deal, but what if thatdevice required 35V?We’d stop discharging at 3.5V per cell in this 10 cellbattery, which would equate to leaving about 40% of the battery’s capacityunused.Thisiswhyitisimportanttoconsidertheentirerangeofthevoltageofabatterywhencalculatinghowmanycellsinseriesarerequiredforyourproject.

Many electronics such as inverters, electric motors and other DC devices aredesignedforvoltages in12Vincrements,suchasa12Vheadlampora48Velectricbicycle.Thisisaholdoverfromthemanyyearswhenleadacidbatterieswereusedtopowerthesetypesofdevices.Leadacidbatteriesusecellsthathavea nominal voltage of 2 V, and six are usually connected together in series tocreate12Vleadacidbatteries.Those12Vbatteriesaretheneasilyconnectedinseriestocreateanyothersizebatterywith12Vincrements.

The problem this old system has created for us is that most lithium batteriesdon’tconformwelltothisarbitrary12Vincrement.

Mostelectronics(butnotall!)arecapableofhandlingasmallrangeofvoltagesaboveandbelow their ratedvoltage.For example, a12VLEDheadlampcanlikely function with a voltage of between 9 V - 15 V, thoughmore sensitiveelectronicswillhavesmallerpermissiblevoltageranges.

Thisvoltagerangeallowsustousealithiumbatteryvoltagethatisclosetothe12V increments thatmanyelectronics are rated for, even if it isn’t exact.Forexample, electric bicycles are usually designed for either 24V, 36V or 48Vbatteries.Again, this isbecausemostof the ebikecomponentswereoriginallydesignedforleadacidbatteriesandthenomenclatureintheindustryjuststuck.

Themost commonly accepted lithium-ion battery for 24V ebike systems is 7cellsinseries,whichcreatesa25.9Vnominalbatterythatactuallyrangesfromapproximately 21V - 29V during use. For 36V lithium batteries, nearly allebikemanufacturersuse10cellsinseriestocreatea37Vnominalbatterythatranges from approximately 30 V - 42 V during use.When it comes to 48 Vbatteriesthough,theindustryisfairlysplit.Batterieswith13cellsinseriesusedto be the most popular configuration for a 48 V battery. This resulted in anominalratingof48.1Vandavoltagerangeunderuseofapproximately39V-54V.However,withvoltagesag,thebatterywouldactuallyspendthemajorityofitstimebelow48V,whichresultsinlesspower.

For this reason,many48Vbatteries forebikesarenowmadewith14cells inserieswhichgivesanominalratingof51.8Vandhasahighervoltagerangeofapproximately 42 V - 58.8 V. These batteries are often referred to as 52 Vbatteries instead of 48 V batteries to signify that they are indeed of a highervoltagethan“standard”48Vlithiumbatteries.

Otherindustriesdon’talwayshavethis12Vincrementissueandcanessentiallyuseanyvoltage that theydesign theirdevices for.Batterypowered tools area

greatexample.Manypowerdrillsuse11.1Vnominalbatterieswhichconsistofthreelithium-ioncellsinseries,thoughmanytoolmanufacturersstillcallthese12Vbatteries.This isn’t really fair,as theyspendvery little timeabove12V.However, because they charge up to 12.6V, the 12V badge isn’t technicallyuntrue.Rather,it’sjustmisleading.Thenextstepupinpowertoolsisusuallyan18Vnominalbattery,whichconsistsoffivelithium-ioncellsinseries.

OnethingtonoteisthatalloftheaboveexamplesIgaveusedlithium-ioncells,as these are the most commonly used cells in these applications. However,LiFePO4 cells actually lend themselvesmore easily to 12V increments.With3.2Vnominalcells,combiningfourLiFePO4cellswillcreatea12.8Vnominalbattery,whichisprettydarncloseto12V.LiFePO4cellsare fairlypopular forDIYelectricvehicleconversions,largelyduetoacombinationofhighcyclelife,excellent safety and only moderate space restrictions (who needs that trunkspaceanyways?).Asmanyelectricvehiclecomponentswereoriginallydesignedforleadacidbatteries,theyarealsooftenratedin12Vincrements,whichmakesusingLiFePO4cellsabiteasierwhenyou’reaimingforaspecificvoltage ina12Vincrement.

Increasingcapacityusingparallelconnections

Combiningcellsinseriesincreasesthevoltage,yetithasnoeffectatallonthecapacityofthebattery.Combining10li-ioncellsthatareeachratedfor3.5Ahinserieswillresult ina37V3.5Ahbattery.That’sadecentlyhighvoltage,butverylowcapacityformostapplications.Inordertoincreasethecapacityofthebattery,wemustcombinecellsinparallel.

Parallel connections can be thought of as the opposite of a series connection.Insteadofconnectingthepositiveterminalofonecelltothenegativeterminalofthe next cell as in series connections, parallel connections are made byconnecting the same terminals together. To connect two cells in parallel, yousimplyconnect thepositive terminalof thefirstcell to thepositive terminalofthe second cell, and then connect the negative terminal of the first cell to thenegative terminal of the second cell. This essentially creates one larger cell,because the twocells arenow sharing the same terminals and function as onebatterycell.

Oneimportantsafetynote:beforeyouconnectanybatterycellsorbatterypacksin parallel, youMUST ensure that they have nearly identical voltages. If thevoltagesareoffbyalargeamount,itmeansthatonecellisatahigherstateofchargethantheother.Whenyouconnectcellswithmismatchedchargelevelsinparallel,thehigherchargedcellwilltrytochargethelowerchargedcell.Ifthedifference in charge is large, the higher charged cellwill try to dump a largeamountofenergyatonceintothelowerchargedcell.Thishighcurrentflowwilldamage both cells and can result in the cells overheating or catching on fire.Alwayscheckthatcellsareverycloseoridenticalinvoltagebeforeconnectingtheminparallel.

Ok,nowlet’slookatsomesimplemath.CalculatingthetotalcapacityinAhofbattery cells connected in parallel is easy: just multiply the number of cellsconnectedinparallelbythecapacityoftheindividualcells.

Capacityofparallelcells=the#ofcellsinparallel×capacityofasinglecellinAh

Forexample,let’ssaythatwehavetwoli-ioncells,eachwithanominalvoltageof 3.7V and rated for 3.5Ah. Ifwe connect them inparallel by joining theirpositive terminals together and then their negative terminals together,wewillhavecreateda3.7V7.0Ahbatterypack,asshowninthediagram.Ifweaddonemore cell in parallelwith the first two, thatwill create a 3.7V10.5Ahbatterypack. If we connect 10 of these cells together in parallel, that will create a3.7V35Ahbatterypack.Easy,right?Great!

Combiningseriesandparallelconnections

We’venowseen that series connections increase thevoltageof abatterypackbutdon’taffectthecapacity,whileparallelconnectionsincreasethecapacityofthe battery pack but don’t affect the voltage. So howdowe increase both thevoltage and the capacity simultaneously?We simply combine both series andparallelconnectionstogether.

Let’slookatanexample.We’llstartwithparallelconnections.Let’stakethosesame3.7V3.5Ahli-ioncellsfromthepreviousexampleandwirethreeofthemin parallel. Thiswill effectively turn those three cells into one larger cell thatmaintains its 3.7 V nominal voltage but now has a combined capacity of10.5Ah,whichcanbecalculatedas:

Totalcapacity=3cellsinparallel×3.5Ahpercell=3×3.5=10.5Ah

Nowlet’sdoitagain.We’lltakeanadditionalthreecellsandcombinetheminparallel,justlikethefirstthree.Nowwe’vegottwosmallbatterypacksofthreecellseach.Bothpacksare3.7Vnominalandhavecapacitiesof10.5Ah.

Alright,areyoureadytogonuts?Nowlet’scombinethosetwopacksinseries.Thatmeanswewillelectricallyconnectthepositivesetofterminalsonthefirstthree-cellgroupto thenegativesetof terminalson thesecondcellgroup.Thisconnection can be made with wires, or with metal tabs, or even simply bypressingtheterminalsagainsteachother.Byjoiningthesetwoparallelgroupsinthisway,we’vecreateda seriesconnectionbetween the two three-cellparallelgroups.

Tocalculatethenewvoltageofthecombinedbatterypackswejustmultiplythevoltageofeachcellbythenumberofcellgroupsinseries,whichinthiscaseistwo(thetwoparallelgroupseachcountasonebigcell,notthreeindividualcells,because they are connected in parallel and function as one cell). Our totalvoltageisthengivenby:

Totalvoltage=3.7V×2cellgroupsinseries=3.7×2=7.4V.

Remember that the series connection that we just performed only affects thevoltage, not the capacity. That means that our 10.5 Ah capacity remains thesame.Therefore,byperformingtheparallelconnectionsfollowedbytheseriesconnection,wehavecreatedasix-cellbatterypackthatisratedat7.4Vnominalvoltageand10.5Ahnominalcapacity.

Now let’s learn some terminology and abbreviations. Referring to the abovebatteryas“threecellsinparallelandtwocellsinseries”isamouthful,sowe’llusetheindustryabbreviationsof‘s’forseriesand‘p’forparallel.Thebatterywecreatedintheexampleabovewouldbereferredtoasa2s3pbattery,asithastwoseriesgroupsofthreeparallelcellseach.

A3s3pbattery couldbemadeby following the exact same instructions as theabove example, except that we’dmake three parallel groups in the beginninginstead of two and then connect that third group in series as well. That 3s3pbatterywouldbean11.1V10.5Ahbattery.Ifweusedthesamecellstocreatea10s4pbattery,itwouldberatedat37V14Ah,whichwecancalculateas:

Totalvoltage=10cellsinseries×3.7Vpercell=37V

and

Totalcapacity=4cellsinparallel×3.5Ahpercell=14Ah

Thisseriesandparallelconnectionschemeisthebasisofbatteryconstruction,soit is important that you completely understand it. Screwing up the series andparallel connections will not only result in getting your battery specificationswrong, but it can also very likely lead to a short circuit in the battery pack.Creating a short circuit in a lithiumbattery is a dangerous situation thatwe’lltalkmoreaboutinChapter7.Sufficeittosay,youwanttogetthisstuffcorrectevery time. To double check that you’re good to go on series and parallelconnections,let’sdoalittlepopquiz.

Determinethenominalvoltageandcapacityofthefollowingconfigurations:

A. 13s4pbatterypackusing3.7V3.0Ahli-ioncells

B. 8s2pbatterypackusing3.2V5.0AhLiFePO4cells

Determinethepackconfiguration(thenumberofseriesandparallelcells)forthefollowingspecifications:

C. 37V11.6Ahbatterypackusing3.7V2.9Ahli-ioncells

D. 51.8V100Ahbatterypackusing3.7V10Ahli-ioncells

Determine the capacity of the cells used in a battery pack of the followingspecifications:

E. 25.9V17.1Ahbatterypackofconfiguration7s6pbuiltusing3.7V li-ioncells

F. 74V 20Ah battery pack of configuration 20s8p built using 3.7V li-ioncells

Answers:A.48.1V12Ah;B.25.6V10Ah;C.10s4p;D.14s10p;E.2.85Ahcells;F.2.5Ahcells

Chapter7:Safety

Propercaremustbetakenwhenhandlingandworkingwithlithiumbatteriesandcells.Ifusedproperly,theycanbequitesafe.Butifmisused,theycanpresentaserious firehazard.Let’s takea lookathowthingscangowrongwith lithiumbatteriesinordertolearnwhatweshoulddoright.

Thedangerofshortcircuits

Perhapsthebestwaytocausealithiumbatterytocatchfireistoshortcircuitit.Ashortcircuitoccurswhenthetwoterminalsofabatterypackorsinglecellareconnectedtogether.Essentially,abatteryisput inserieswithitself,connectingits positive terminal to its own negative terminal. This creates a short loop ofcurrent that flows directly through batterywith nothing to slow it down otherthanthebattery’sownlowinternalresistance.

Abatteryshouldneverbeshortcircuited.Thisisrarelydoneonpurpose(outsideofperhapstryingtolightanemergencyfirewithabatteryandsomesteelwooloragumwrapper).Rather,mostbatteryshortcircuitsaretheresultofacarelessmistake.Theseaccidentscanbeincrediblycostly.Bestcasescenario: theshortcircuitonlyoccursforafractionofasecondandresultsinonlyminordamagetothe battery. Worst case scenario: the short circuit is maintained, causing thebattery to overheat, catch fire and start a chain reaction with other cells orbatteriesaroundit,eventuallyburningdownyourhome,car,airplaneoranythingelsecontainingthebattery.

SobynowIthinkthemessageonshortcircuitsisprettyclear:don’tdoit.Youmight be thinking, “that’s easy, Iwon’t do it.”But it’s not that simple.Like Isaid, most short circuits occur accidentally, usually due to carelessness orabsentmindedness,andoftenduringtheassemblyprocess.

Rememberhowwetalkedaboutseriesconnections,wherethepositiveterminalof one cell is connected to the negative terminal of the next cell? Well,

performingseriesconnectionsisagreatopportunitytoaccidentallyshortcircuitabattery.Ifyourbatterycellsarelinedupendtoend,likeinaflashlighttube,it’s a bit harder to accidentally create a short circuit as you’d have to findsomethingconductive that could span the lengthof the cells and reacharoundtheirendstocontactbothterminals.

But imagine ifwe rearranged those cells so that instead of being in a straightline,theywerenexttoeachotherwithonecellflippedupsidedown.Wecouldcreateasimpleseriesconnectionbyconnectingtheterminalsononeendsincethepositiveandnegativeterminalsarerightnexttoeachother.Butnowlookattheotherendofthebattery,ithasapositiveandnegativeterminalrightnexttoeachother,separatedbyjustafewmillimeters.Anymetallicobjectlargeenoughtoreachbothterminalscouldeasilyshortcircuitthisbattery.

Imagine this battery sitting on your workbench. If you dropped a piece ofexposedwireorsolderontothoseterminals,you’dcreateashortcircuit.Ifyoulaid the battery down on a screwdriver, pliers or any other metal tool, you’dcreateashortcircuit.Youcouldshortcircuitthatbatterybytouchingyourringtoitwhenyoupickupthatbatteryandholditinyourhand.I’mevenspeakingfrom experience on that last one: a gold wedding band makes an excellentconductorandleavesaveryuncomfortableburnring.

For that reason, you should always wear gloves when working with lithiumbatteries.Evenifyoudon’thavejewelryonyourhands,sweatypalmscaneasilyconductcurrentandyou’llgetquiteashock(literallyandfiguratively)thefirsttimeyouexperienceit.Mechanicsglovesworkwell,butIsometimesliketouselatexornitrileglovesastheygivemebetterdexterity.Also,speakingofjewelry,remember to remove everything metallic. Watches, necklaces, earrings, noserings,etc.,basicallyanythingmetallicthathangsordanglesoverexposedbattery

connectionscanbeahazard.

Nowthat’sjustasimpletwo-cellbattery.Imaginealargerbatterywithdozensofcells andmanymore series connections.Everynon-series connectionbetweencellgroupsisanotheropportunitytoaccidentallyshortthecellsinabatterylikethis.Idon’twanttomakethissoundtooscary,becauseitshouldn’tbe,butitisimportant to keep in mind that the potential for short circuits is there duringdifferent stages of a build. From the moment you begin creating seriesconnections until the moment the battery is sealed and the terminals are nolongerexposed,thereisthepotentialtocreateanaccidentalshortcircuit.

Inordertoprotectagainst thesekindsofaccidentalshortcircuits,remembertoalwaysworkonaclean,clearsurface.Ifyourworkbenchisanythinglikemine,it’sconstantlyclutteredandcoveredwithbitsandpiecesofthelastfew(ormorethanafew)projectsyou’vebeenworkingon.Thatseemstoworkfineformostprojects, but it’s an accident waiting to happen when you’re working withlithiumbatteries.Asinglesmallscreworstapleonyourworkbenchcancauseashortcircuitwithpotentiallydisastrousresults.Anytimeyouworkwithlithiumbatteries,completelyclearoffyourworkstation.

IhavearollofcheapbutcherpaperandlaydownabigpieceontopofmyworkbenchanytimeI’mworkingwithlithiumbatteries.NotonlydoesitensurethatI’mstartingwithasurfacefreeofanymetallichazards,butthewhitepaperalsomakes it easy to seeanynewhazards I create, likedropsof solderor snippedwireendsthatcouldshortmybatteryifIaccidentallysetitdownontopofoneofthem.

Butshortcircuitsdon’tonlyhappenfromaccidentalcontactwithforeignobjectsbridging between neighboring cell terminals. Many short circuits occur fromcarelesswireplacement.Whenwiringyourbattery,you’ll likelyhavemultiplelengthsofwirefloppingaroundatdifferent timesonyourworksurface.Thesewires will be connected to different parts of your battery and have differentvoltage potentials, meaning that contact between the ends of these wires cancauseashort.Thishappensmoreeasilythanyou’dthink,asmanypeoplearesofocused on their battery that they don’t pay attention to loose wires movingaroundontheirworkbench.

This is why you should never leave exposed ends on any wires that areconnectedtoyourbattery.Whenpossible,Iliketoattachconnectorstomywiresfirst before I ever attach them tomy battery. Thismeans that the ends of thewiresarealwayscoveredfromthesecondtheyareattachedtomybattery.Ifthisisn’tpossible,Iatleasttrytoputapieceofelectricaltapeorheatshrinkovertheendsofthewires.Ifyou’redoingrepairormaintenanceworkandhavetosnipwires,alwayscovertheendsimmediatelywithtapeorheatshrink.

Also,bewareofthespringinessofmanywires.Ifyouaren’tholdingontoawirewhenyousnipit,itcanoftenspringbackafterithasbeencut.Allittakesisalittle bad luck for it to land anywhere on your exposed battery terminals andcreateashortcircuit.Soalwaysholdontoawirethatyouareactivelycutting.Iliketoholdwithmultiplefingersandcutinbetweenmyfingers.ThatwayIcanholdontobothendsof thewireonce it hasbeencut.When this isn’tpossiblethough,alwaysmakesureyouholdontotheendthatremainsconnectedtoyourbattery,asithasahigherchanceofcausingashort.

Anotherexcellentwaytoshortcircuityourbatteryisbycuttingmultiplewiresatthesametime.Whendisassemblingabatteryordoingmaintenance,you’lloftenneedtouseawirecuttertosnipoffsectionsofwireorremoveconnectors.Forthelazyamongus,itcanbetemptingtosnipmultiplewiresatonce,butdoingsocan easily create a short circuitwhen twowireswith different potentials bothtouchthemetallicjawsofyourwiresnipsatthesametime.

Again, thismaysoundobvious to some,but it is easilyoverlooked,especiallywhenworkingquickly.I’mnotsoproudthatIcan’tadmitthatIhaveapairofwiresnipswithasizeablechunkofthejawsmissing.Theywerevaporizedfromstupidlysnippingoffachargingconnectoratonceinsteadofonewireatatime.Inmydefense, Iwasyoung andnaive, but that excusedoesn’t bring that toolback (or the few charge cycles it probably took off of my battery’s lifeexpectancy).

Soalwaysremembertocutwiresindividually.Itcanbeannoyingwhenyou’vegotadozenormorewirestosnip,butitonlytakesafewmoresecondsandcanpreventyoufromdamagingyourbattery,yourtools,orboth.

Yet another great way to short circuit your battery is when you’re addingconnectorstoyourbattery.Ifyoudon’taddyourconnectorstothewirebeforeitisattachedtoyourbatterythenyou’llbeworkingwithpotentially“hot”wires,meaning the wires have the potential for current to flow if the circuit iscompleted.That iswhy it isbest toaddyourconnectorsbeforeyouattach thewirestothebattery.

However, sometimes adding connectors first just isn’t possible. If youneed toadd connectors after the wires are already attached to the battery, add eachconnectoroneatatimeandmakesuretoinstallwhatevertypeofinsulatingshellcomeswithyourconnector, ifnecessary. Ifyouhaveconnectorssuchasbulletconnectors,AndersonPowerpoleconnectorsorothers that require solderingorcrimping the baremetallic connector first followed by the insulator, then it isevenmorecriticaltodotheseconnectionsoneatatime.Otherwise,you’llhavemultiple exposed connectors dancing out in space together, just asking for ashortcircuit.

Again,I’munfortunatelyspeakingfromexperienceonthisone.ManyyearsagowhenIwasjustgettingstartedwithbatteries,Isolderedtwo6mmgoldplatedbulletconnectorsontothedischargewiresofa48V15Ahbatterybeforeaddingtheirinsulators.Iletonedropanditmusthavecontactedtheother.Isay“musthave” because I didn’t see anything other than an intense ball of lightning. Infact,Ineverevensawtheconnectorsagain.Theysimplyvaporized.Theyweregone. What had just a few seconds earlier been a 1 inch (2.5 cm) electricalconnectorhadturnedintojustgasanddustbeforemytemporarilyblindedeyes.

Soplease,learnfrommymistake.Don’tletbareconnectorsdanglefreely.Don’tcutmultiplewiresatonce.Don’thandleanexposedbatterywithbarehandsandwhilewearingawedding ring.Theseall sound like stupidmistakes.And theyarestupidmistakes.Butthey’restupidmistakesthatareeasytomakewhenwelosefocus.Allittakesisafewsecondsoflettingyourminddrifttoallowyoutoaccidentallymakeamistake.

When you’re working with lithium batteries, and especially when performingconnectionsbetweenmultiplebatterypacksand/orcells,thereisnoroomforalack of focus. Always think about what you’re doing. In carpentry we say

“measure twice, cut once”. In battery building, the saying should be “thinktwice,actonce”.Wheneveryou’remakingaconnection,cuttingawireorelseperforming any other type of action on a lithium battery, double check thatyou’remakingtherightaction,andthendoit.

You’llnoticethatIdevotedalotoftimetotalkingaboutshortcircuits.Thatisbecauseshortcircuitsareaconvenientcombinationofboththeeasiestandmostdangerouswaystogowrongwithlithiumbatteries.Butshortcircuitsaren’ttheonlywaystoscrewupandcreateadangeroussituation.LikeamorbidversionofRonPopeil,I’mheretosay“butwait,there’smore!”

Theeffectoftemperatureonlithiumcells

Heatistheenemyoflithiumbatterycells.Moderatelyhighheatwillcauseyourbatteries tooperate lessefficiently,meaningyourcellswilldie soonerandnotdelivertheirfullratedcapacities.Moderatelyhighheatshouldbeavoidedwhenpossible,butitisn’tnecessarilydangerous.Lithiumbatteriescandischargeundertemperatures as high as 60°C, though it’s better to keep that at a lowertemperature, if possible, to increase their cycle life. Lithium batteries shouldneverbechargedattemperatureshigherthan40°C,butwe’lltalkaboutchargingspecificsmoreinChapter13.

The actual danger occurs when lithium cells are brought to much highertemperatures.Lithiumbatterycellsshouldneverexceed130°C.Attemperaturesslightlyhigher than130°C, some lithiumbattery cells riskundergoing thermalrunaway,whichiswhentheelectrolyteinthecellsoxidizesataratethatcreatessomuchheatthatitincreasesitsownrateofoxidization,essentiallyfuelingitsownfire.Asthecellgrowshotterunderthermalrunaway,anynearbycellscanalsoheat up to the point of thermal runaway, causing a chain reaction limitedonly by the number of cells nearby. Once a cell reaches thermal runaway,nothingcanbedonetostopit.Thecellwillcombustuntilithasconsumeditself.

The temperature atwhich thermal runaway beginswill vary from cell to cell.Lithiumcobaltcellscanenterthermalrunawayattemperaturesaslowas150ºC,while NMC cells generally reach thermal runaway at closer to 180ºC. Bothchemistries can reach temperatures of over 500ºC at the peak of thermalrunaway. The onset of thermal runaway varies greatly for LiFePO4 cells, butsome will begin thermal runaway at around 200ºC, and most won’t peak athigherthanapproximately230ºCduringthermalrunaway.

Thephysicaleffectonthecellslargelydependsonthetypeofcell.Cylindricalcellslike18650cellshaveaventmechanismonthepositiveterminalofthecellthat allows gas to escapewhen the cell overheats and nears thermal runaway.Some prismatic cells have built in ventingmechanisms. Other prismatic cellsand all pouch cells don’t include vents and will have no way to release thebuildupof pressure in the cell. If thepressure exceeds the strengthof the cellwall,thesecellscanrupture,oftenviolently.Uptohalfofthegasventingfromlithiumbatterycellsismadeupofhighlyflammablegasesincludinghydrogen,methaneandethylenegas.

Itisrareforcellstoheatuptothethepointofthermalrunawayduringnormaluseor storage.Yourattic likelydoesn’t reach130°C, though it’s still better tostore lithiumbatteries inacoolplaceat room temperature.Storing inacoolerlocationhelpstoincreasetheusefullifeoflithiumcells.

Thebiggerriskofthermalrunawayiswhenusinglithiumbatteriesunderlargeloadsthatresult inahighcurrentdrawontheindividualcells.If thecurrent islarger than thecellcanhandle, itwillbegin toheatup. If thisgoeson for toolong,thecellcanreachthermalrunaway.Forthisreason,itisalwaysimportanttodesignyourbatterytooperatewithinthedesignspecificationsofthecellsyouareusing. If youhave a20Acontinuous load and arebuilding abatterywithfour cells in parallel, those cells better be rated for at least 5 A continuous.Preferably,thecellswouldberatedforevenmorethan5Acontinuoustoensurethatyouaren’tpushingthecellstothelimit.

On the other end of the spectrum, extreme cold temperatures aren’t good forlithium cells either. Most cells can discharge in temperatures down to about-20°C,buttheyshouldneverbechargedattemperaturesbelow0°C.Butagain,we’lltalkmoreaboutcharginginChapter13.

Handlingandstorage

Dependingonthetypeofbatterycell,lithiumbatteriescanbeeithersurprisinglyrobustorincrediblyfragile.Cylindricalcellsareusuallyfairlystrongduetotheirrigidmetalcaseandcanactuallystanduptoquiteabitofabuse.Youshouldn’ttoss them aroundwilly-nilly, but a few cylindrical cells bouncing around in aboxisn’tgoingtodestroythem.Whenpossible,youshouldtrytokeeptheminsomekindofplastic(notmetal!)enclosurewhenyouaren’tusingthem,buttheybasicallycomeintheirownprotectivecasesforallintentsandpurposes.

Also,withtheirpositiveandnegativeterminalsonoppositesidesofthecell,it’sprettyhardtoaccidentallyshortcylindricalcellsbyhavingthemlooseinaboxorabag.Still,it’sbesttoenclosetheminsomekindofcase,justtobesafe.

Prismatic cells can be fairly strong, depending on how they aremade. Largercells in the 20 Ah to 100 Ah range that are designed for electric vehicles orhome/off-grid energy storage are usually encased in a fairly substantialenclosure. You should avoid dropping them from any decent height, but theygenerallyaren’ttoofragile.

Pouch cells, on the other hand, need to be handled with care. They can’t betossed inaboxlikecylindricalcellscan.Notonlydo they lackanyprotectionfrombeing stabbed,crushed, foldedor torn,but theyalsogenerallyhave theirpositive and negative terminals on the same side of the cell, making itdangerously easy to short circuit them if they are stored in or near anymetalobjects.Aloosepaperclipcouldburndownahouseif itshortsacouplepouch

cellsinadeskdrawer.

Pouch cells are generally shipped and sold in formed plastic sleeves or cases.Theseplasticholders shouldbeused to store thecells safelywhen theyaren’tbeing used.Never stack a big pile of pouch cells on top of each other unlessyou’reactivelyconnectingthemandarebothcarefulandawareofwhatyou’redoing.It’sjusttooeasytoaccidentallyshortthewide,flattabsofpouchcellsoneachotherwhentheyareleftinastack.

To summarize, lithium battery cells are inherently dangerous by design. Theycontainalargeamountofenergyinasmallpackageandaredesignedtodeliverthat energy quickly. But by using proper precautions and safe operatingprinciples, we can ensure that the cells are as safe as possible for our uses.Alwayspayattentiontowhatyou’redoinganddon’tworkwithlithiumbatterycellsunlessyoucandevoteyourfullfocustoyourwork.

Chapter8:BatteryManagementSystems

BatteryManagement Systems (BMSs), or as they are less commonly known,Protection Circuit Modules (PCMs) or Protection Circuit Boards (PCBs), arecircuits that can be added to a lithium battery to protect the health of theindividualcellsinthebattery.Whileoptional,theyaregenerallyagoodideatoincludeinmostbatteries.

WhywouldabatteryneedaBMS?

Aswe’vealready learned,batterypacksareassembledbyconnectingmultiplecellsinseriesandparallel.Andaswealsolearned,parallelconnectionsbetweenmultiplecellsmakethecellsactasiftheywereonebigcell.Ifanyonecellinaparallel group has a load applied to it, then all of the cells in that groupexperience the load equally. In this way, the voltage in all cells of a parallelgroupremainsidentical.Assoonasonecell’svoltagetriestodrop,currentflowsinfromitsneighboringcellstomaintainthevoltageequilibrium.Thisisknownasbalancing.Cellsthatarebalancedwithrespecttoeachotherallhavethesamevoltage(andthuschargestate)aseachother.

Butthinkaboutthoseseriesconnections.Theydon’tworkthesameway.Whenaloadisappliedtomultiplecellsconnectedinseries,allcellsintheserieschainwill experience that load fairly equally. However, because the cells in seriesaren’t connected at both terminals to each other (as that would make themparallelcellsinstead),theycan’tbalanceeachother.

Now notice in the previous paragraph that I said those cells in series willexperiencea load fairly equally. I didn’t say identically, and that is important.Small differences between the connections to the cells as well as the cellsthemselveswillmeanthateachcellhasaslightlydifferentresistance.Thatsmallresistancemeans that a slightly different amount of current will flow throughthem as different amounts of current are burned off as heat in the form ofresistance.The same current passes through the loop of series cells, but somecellsburnoffalittlemoreorlessofthatcurrentasheat.Thatresultsinslightlydifferentstatesofchargesforeachcellorparallelcellgroupinaserieschain.

Goodqualitycellswithstrongelectricalconnectionswillhavenearly identicalresistances and thus they will stay very closely balanced. However, at highercurrentlevels,evengoodqualitycellswilleventuallybecomeunbalancedafterdozensorhundredsofchargecycles.Forlowqualitycellswithlargedifferencesininternalresistance,orevenforbatteriesmadewithgoodqualitycellsbutwithpoorelectricalconnections,thecellscanbegintolosetheirbalanceafteronlyafewchargecycles.

Theproblemwithbecomingunbalanced is thatunlessyouareusingaspecificchargerknownasabalancingcharger tomonitoreverycellgroupandactivelybalanceitduringachargecycle,thenthecellswillbecomechargedtodifferentlevels. This is because simple, non-balancing chargers don’t monitor theindividualcellgroupswhentheychargeabattery.Theyjustsupplyavoltageandacurrent, thenwait for thebattery tochargeup to thatvoltage,atwhichpointchargingiscompleted.

With a non-balancing charger and an unbalanced battery, some cells willinevitablybecomeoverchargedwhileothercellsdon’tgetchargedall theway.This results inmultiple problems for the battery. First, the lower voltage cellshave a lower state of charge because theyweren’t completely charged. Thosecells will get drained even lower the next time the pack discharges, causingirreparable damage to those cells. Next, the higher voltage cells will havechargedover4.2V(theproperfullvoltageforli-ion)or3.65V(theproperfullvoltage forLiFePO4). Spending any amount of time over themaximum ratedvoltagewillalsocauseirreparableharmtothecells.

Unbalancingofcellsisalsoarunawaycondition.Ascellsbecomeunbalanced,they support a disproportionate amount of the load, which causes them tobecome further unbalanced, and the vicious cycle continues, increasing theimbalanceuntilthebatterydestroysitself.

Alright,Iknow,yougetit.Unbalancedcellsareabadthing.Sowhatcanwedotopreventcellsfrombecomingunbalancedaftermanychargecycles?

Oneoptionistouseabalancingcharger,likementionedabove.BalancechargersarecommonlyusedforRClipobatteriesintheRCvehicleworldwheretheyareconsideredanecessity.Mostbalancechargersstartbychargingalithiumbatterythe simple way, by providing the appropriate total voltage and current to theentirepackacrossallcellsevenly.Thisusuallyoccursbypluggingdirectlyintothe discharge wires and reversing the current to use the discharge wires forcharging.Butbalancechargersalsoconnecttosmallerwires(knownasbalancewires)thatinturnconnecttoeachindividualcellinthebattery(oreachparallelgroup,iftherearemultiplecellsinparallel).Thechargermonitorseachcell(orparallelgroup)compared to theothersandcandrainorbleedoffvoltagefromthehighervoltagecellsuntilallcellsarebalanced.

Most balance chargers are capable of performingboth this balancing chargingscheme as well as just the simple, “total voltage to the entire pack withoutbalancing”scheme.Thelatterisknownas“bulk”charging.Inbulkcharging,thecells do not get balanced and the issue of some cells becoming slightlyoverchargedwhileothercellsbecomeslightlyunderchargedstillexists.Forgoodquality battery cells though, the amount of unbalancing that occurs duringnormaldischargeandchargecyclesisquitesmall,meaningthatthebatteriescanreasonablygoafeworevendozensofchargecyclesusingbulkchargingbeforetheyneed tobebalancedcharged.Theadvantageofbulkcharging is that it is

faster-youdon’thavetowaitforthecellstobalanceattheendofthechargingoperation. The disadvantage is of course that the cells will slowly becomeunbalanced. Depending on the specific battery, project requirements and userrequirements,thecorrectratioofbulktobalancechargingcanbeselected.

Ok, you’re probablywonderingwhy I haven’t talked aboutBMSs in awhile.This is after all the BMS chapter. Well, you need the background theory tounderstandwhyBMSs are so important and useful. Now that you understandwherewe’vebeen,let’slookatwherewegoing.EnterthemightyBMS!

The job of aBMS is to allow a simple bulk charger to be used to supply thecorrecttotalvoltagetoabatterywhilestillmakingsurethecellsdon’tbecomeunbalanced.TheBMS sits between the bulk charger and the battery cells andregulates the cells. It allows the total pack voltage from the charger to passthrough the cells and charge them to the correct voltage. But during thatcharging,theBMSconstantlymonitorsthecellsandwillstarttodrainoffsomeenergyfromanycellorparallelgroupthatstartstogetchargedtoohigh.

TheBMSbasicallydoesthejobofabalancecharger,butinsteadofresidinginthe charger, it usually sits inside the battery in the form of a separate circuitboard. Larger BMS units, on the other hand, often have their own separateenclosurestohelpdispersetheheatcausedbythebalancingprocess.

The type of balancing described above is known as ‘top balancing’ because itbalancesthecellsatthetopofthechargecurves.ThisisthemostcommontypeofBMSbalancingscheme.Analternativeisknownas‘bottombalancing’andit

works just like it sounds. Instead of balancing cellswhen the cells are full, itbalancesthemwhentheyareapproachingtheiremptystateofcharge.There’salotofdebateinthebatteryindustryaboutwhichformofbalancingisbetter.Forthemostpartthough,topbalancingistheindustrystandardandwhatyou’llfindinmostBMSs.

The fact that the BMS is always connected to the battery provides somesignificant benefits. BMS designers have taken advantage of this to developBMSswithmanyadditionalfeatures.NearlyallBMSshaveprotectioncircuitryfordischargingaswell.Becausetheyareconnectedtoeverycellorcellgroup,theycanmonitorthevoltageofeverycell.WheneverthefirstcellhitstheLowVoltageCutoff(LVC),theBMSwillcutthedischargecircuitandstopthebatteryfrom being discharged any deeper. This protects the cells in the battery fromoverdischargingandsufferingirreparabledamage.

Interestingly, some of the cheapestBMS’s only have this LowVoltageCutoff(LVC)feature,butnotabalancingfeature.Thisissomethingtowatchoutforifyou’rebuyingabudgetBMS.Alwaysdoublecheckwiththevendorthatitdoesinfactincludethebalancingcircuitry.

ManyBMS’s also protect the pack fromover-drain,which iswhere theBMScutspowerfromthebatteryiftheloadonthebatteryexceedsacertainthreshold.Thisprotects thecellsfromworkingtoohardanddamagingthemselves.Itcanalsoprotectagainstshortcircuitingthebattery,dependingontheBMS.

SomeBMS’s have thermal protection built-in, where an included temperature

probemonitorstheinstantaneoustemperatureandcanstopadischargeorchargeprocess if the battery becomes too hot. The temperature sensor is often on alengthofwireallowingtheusertoplaceitagainstthecellsofthebatterywhereitwillbemoreresponsivetoasuddenincreaseincelltemperature.

MoreexpensiveBMSscanincludefeaturessuchasBluetoothconnectivityandanti-theftoralarmfeatures.Bluetoothconnectivityallows theuser to remotelymonitorthestatusandhealthofthecellsorentirebatterypackinrealtimefromtheir cell phone or other Bluetooth-enabled device. Anti-theft features canpreventthebatteryfromprovidingcurrentuntilasignalisreceived.SomeBMSsinclude a display screen to give an accurate readout of battery specifications.This is a rare feature though, andmost BMSs use a simple LED indicator todisplaywhetherornotchargingofacellhascompleted,iftheyhaveanindicatoratall.TherearealsoprogrammableBMSsthatallowtheendusertoupdatetheBMS’ssettingsathisorherleisure,thoughtheseBMSsareconsiderablymoreexpensive.

Ultimately,most applications only require a simpleBMS thatwill protect thecells during charging anddischarging. If you’d like something fancier though,therearemanydifferentBMSsouttheretochoosefrom.

DisadvantagesofaBMS

IntheprevioussectionwetalkedabouttheadvantagesofusingaBMSinsteadofabalancecharger.However,therearealsosomedownsidestousingBMSs.

OneofthemaindisadvantagesisthataBMScanoccasionallyfail,andwhenitdoes,itwilloftencausetheveryproblemitwasdesignedtoavoid.ABMSthatfails can slowly drain the cells in a battery, often at a rate that makes itunnoticeable to the user. This can result in the cells becoming degraded ordestroyedovertime(orquitequickly,dependingonthetypeoffailure).Batteriesthat aremass producedwith an emphasis on cost reduction aremore likely tohave these problems due to cheap BMS units with poor quality controlstandards.

Whileitisarareproblemtohave,theebikeindustryhasseenafairnumberofthese cases. Manufacturers have sometimes tried to reduce the cost of thealready expensive batteries by using cheaper BMSs or skimping on qualitycontrol.ThishasresultedinsomebatteriessufferingaprematuredemisewhentheircheaporlowqualityBMSfails.

For this reason, BMSs have gotten something of a bad reputation in certaincircles.Likeanything,yougetwhatyoupayfor,andifyouwantagoodqualityBMS,youshouldexpecttopayareasonableprice.Ifadeallookstoogoodtobetrue,thenitprobablyis.AcheapBMScanendupcostingalotofmoneyinthelongrunifitdestroysanexpensivebattery.

BMSs also limit the maximum amount of power that a battery can provide.BecausethedischargepathofabatteryhastopassthroughaBMS(toallowtheBMS tomonitor and cut off dischargewhen necessary), the discharge currentlimitofabatterywithaBMSis limitedtothecurrent limitof thecomponentsused in theBMS.BetterqualityBMSscanhaveveryhighdischargerates,buttheirpricesriseaccordingly.

This reason, alongwith the extraweight of aBMS, are themain two reasonswhy the RC vehicle industry has not adopted BMSs in their batteries. Asdangerous asRC lipo batteries can be, aBMSwould dowonders for helpingmonitor and protect such batteries. However, RC vehicles, and RC aircraftespecially,allrequireacombinationofveryhighpowerandlowweight.ABMSbothaddsweightandreducestheamountofpowerthatthebatterycanprovide.

Interestingly,somepeoplethatuseRClipobatteriesoutsideoftheRCindustry,oftenduetotheirwideavailabilityandlowprice,willaddaBMSunittothesebatteries to increase their safety.When the high power andminimizedweightrequirements are relaxed a bit, a BMS becomes a welcome addition to suchpotentiallydangerousbatteries.

ConnectingaBMS

ProperwiringbetweenaBMSandabatterywillbecoveredindetailinChapter11,sofornowI’lljustgiveyouanoverviewsoyouunderstandhowtheprocessworks.

Firstofall,itisimportanttomatchtherighttypeofBMSforyourbatterycells.A BMS for a lithium battery will be designed for either li-ion cells or forLiFePO4cells,butnotboth.ItisextremelyimportantthatyouusetherightBMSforyourcells.MismatchingtheBMSandcelltypescouldresultinoverchargingthebattery.Overcharged lithiumbatteriesareneveragood thing.Theyhaveatendencytogoall‘fireworksshow’onyou.SomakesureyouchoosethecorrectBMS!

Next,aBMSisgenerallythelastthingaddedtoabatteryduringtheconstructionphase.Iusuallyputmineonrightbeforesealingupmybattery.Thereasonforthis is that the BMS is usually connected onto the inter-cell series or parallelconnections. This avoids having to connect it directly to the cells themselves,which if you’re using a soldering method, prevents unnecessary heat fromreachingthecells.Andaswealreadylearned,excessheatisthemortalenemyoflithiumbatteries.

TheBMSitselfwillhavesmallwires,oftenreferredtoas“sense”or“balance”wires,andthickerwires(orpadstosolderonthickerwires)whichareusedforactual charging and discharging of the battery. Those thick wires will getconnectedtothebattery’smainpositiveandnegativeterminals(oroftenjustthe

negativeterminal)whilethethinbalancewireswillbeconnectedbetweeneverysetofparallelgroups.

SomeBMSshaveanequalnumberofbalancewirestothenumberofcellstheyaremeantfor(e.g.a10sBMSwith10thinbalancewires),butitisalsocommontoseeBMSswithonemorebalancewirethanthenumberofcellstheyareratedfor(e.g.a10sBMSwith11thinbalancewires).Ifyouhaveanequalnumberofcells and wires, then the balance wires each get connected to the positiveterminalofeachparallelgroup.Sofora10sbatteryand10sBMSwith10wires,eachwirewouldgetconnectedto1+,2+,3+,etc.,allthewayupto10+.Fora10sBMSwith11balancewires,thatfirstwirewillgetconnectedtothenegativeterminal of the first parallel group,which is also the negative terminal of theentirebattery.Thentherestwouldcontinueonlikeabove,withthesecondwireconnectionto1+,thethirdwireconnectingto2+,etc.,allthewayupthroughthe11thwireconnectingto10+.

Butyoudon’thavetoworryaboutthattoomuchnow.That’sallstilltheoryforus.We’llcirclebackaround inChapter11and talkaboutBMSconnections inmuchmoredetailwhenwediscusstheactualassemblingofbatteries.

Chapter9:Constructionmethods

Alright,sonowweknowallaboutlithiumbatterycells,howtheywork,whattodowith them, and how to keep them safe.But now you are probably asking,“howdowephysicallycombinethemintoalargerbattery?”Andifyouweren’t,wellthennowyou’reatleastthinkingaboutit.Wellbuckleupandhangontight,becausewe’vegotawhirlwindofoptionsandinformationhere.

Physicallyholdingcellstogether

Thefirstthingweneedtodiscussishowtoevenholdcellstogetherinapack.Forgettheelectricalconnections-we’llgettothatinanothercouplepages.Buthowdoweevenkeepthecellstogether?Theanswerlargelydependsonthetypeofcellsyou’reusing.

Cylindrical cells are both themost common types of cells for custom batterybuildingandalsosomeoftheeasiesttosecure,solet’sstartthere.Allstandardcylindrical lithium battery cells have commercials cell holders that have beenproducedtohelppeoplelikeyouandmecombinethemintolargerbatterypacks.Forsmallercellslike18650s,theseoftencomeintheformofplasticblockswithcircularholesinthemiddle.TheysnaptogetherlikeLegosandmakeiteasytobuild two matching plastic racks that can hold each end of the cells. Theseblocksareusuallyblack,butsomelargercylindricalcellssuchasHeadwaycells

likethe38120shavelargerorangecellholders.

Regardlessofthecolororwhatthey’remadefrom,they’reallprettymuchthesame.Theygenerallycomeineithersinglecellblocksorlargerrowsormatricesthatsaveyoutimewhenbuildinglargerbatteries.

Ifyouhaveaccesstoa3DprinterandaCADprogram(orcandownloadotherpeople’s CAD files from the internet), you can even 3D print your own cellholdersinanycustomshapeyouwant.

The advantages of these cell holders are mostly the convenience that theyprovide. They hold your cells nice and firm, making it easy to create yourconnections. They can also increase the safety of your pack-building processbecausetheygenerallyhavesmallpostsontheendthathelpkeeptheterminalsof the batteries lifted above your work surface. This helps prevent accidentalshort circuits that could be caused by laying your partially completed batterydownonametalobject.

Dependingonthetypeofcellholders, theycanalsoaddsomegoodrigiditytoyourbattery.Thecheaponesare,well,cheap- theydon’t reallyadd toomuchstrengthbecause their tolerancesare lowand they snap together fairly loosely.The blocks that come in largermatrixes are usually stronger since they don’thaveindividualjointsbetweeneverycell.

Ifyouarebuildingabatterythatwillseealotofmovementandstress,suchasforaskateboardorelectricbicycle,strongcellholderscanhelpstrengthenyourbattery.Unlessyou’regoingto3Dprintacustomshapetofityourapplication(or unless a rectangular shapeworks for you), then snap together cell holderswilllimityourbatterydesignoptions.

Anotherbenefitofcellholderslikethisisthattheyallowforairflowbetweenthecells,whichcanhelpcoolcellsduringuse.However,thisonlyhelpsifyouleaveyourbatteryunsealed.Ifyouheatshrinkyourbatteryorkeepit inacase,thenthe cooling effect of these cells holderswill be almost zero. The issue is thateventhoughthereisanairgapbetweenthecells,ifthebatteryissealedinaboxor in heat shrink, that air simply heats up and then stays between the cells.You’rebasicallycreatingalow-poweroven.Thisisnotnecessarilyabadthingifyour battery stays within normal operating temperature ranges. Batteriesnaturallyheatupduringuse,and theycanwithstand thosenormal temperatureincreases. Justdon’t try to leavegapsbetweenyour cells and then sealup thebatterywhilepattingyourselfonthebackthinkingyou’vegotgreatcooling.

For applications where waterproofing is not an issue, like for home energystorage batteries or other batteries that are kept indoors, the added coolingbenefitofabatterybuiltwithairgapsbetweenthecellscanbeaniceadvantage.Butyou’llalsowanttomakesurethatyourbatteriesaresafeandprotectedfromeverything,notjustwater.Accidentsoftenhappeninunforeseeableways.Apetclimbingonyourbatteryshelfcaneasilystartafirebyknockingoverunsealedbatteries.Justkeepthatinmind.

Anotheroptionforcylindricalcellsistosimplyhotgluethecellstogetherinto

whatever shape fits your application. Hot glue might seem low tech, but it’sactually a very good option for many applications. Hot gluing cells togethergivesyouthemostefficientuseofspace,sinceyouaren’tputtingplasticblocksorconnectorsbetweeneachcell.Ifyouaretryingtominimizesizeandweight,hotglueisprobablythebestoption.

Italsoallowsyoutobuildanyshapeyouwant,asyoucansimplygluethecellstogetherfollowinganytemplateyoudesigntofityourapplication.

Hot gluing can be fairly rigid, but it isn’t the strongest method to join cellstogether.It’slimitedbythestrengthofboththeglueandtheheatshrinkonthecells. Because most cylindrical cells have a layer of heat shrink around theoutsideoftheirshell,thehotglueactuallybondstotheheatshrinkinsteadofthecell itself. Thatmeans that if you glue two cells together and then try to pullthemapart,ifthegluedoesn’tfailfirstthentheheatshrinkwilleventuallytear.Ideallyyouwon’thavethis levelofstressonyourpack,but it issomethingtokeepinmind.

Anotherdisadvantageofhotgluingcellsisthatitcreatesadenserthermalmass.Thecellsinthecentercan’tdissipatetheirheattotheairbetweencellsbecausethere is very little air available. As we discussed before, air gaps don’t helpmuchwithcooling if thebattery is sealed,but if thebattery isn’tbeing sealedandpassiveoractiveaircoolingisavailable,thenlimitingtheairgapsbetweencellsbygluingthemtogetherwillhaveanegativeimpactonthecoolingrateofthebattery.

Formostbatteries, the lackofairspacearoundcellscausedbyhotgluing justwon’tbeanissue.Batteriesthatareusedforlowtomediumpowerapplicationssimplywon’tcreateenoughheattocauseaproblem.Verylargeandhighpowerbatterypackswillcreatemoreheatthough,sothisissomethingtoconsider.

Hot glue shouldn’t loosen up during normal battery use conditions. Hot gluemeltsataround120°Cor250°F,whichishotter thanyouwantyourbatterytooperate.However, if for some reasonyoudoplan foryourbattery tobe in anenvironment with abnormally high temperatures, remember that you mightsoftenorloseyourhotgluebetweenyourcells.

Somecompanieshaveexperimentedwithwaxblocks toholdcylindrical cells.AllCellTechnologies,acompanybasedinChicago,usesaproprietarywaxblendwith cylindrical holes to hold 18650 cells. Thiswax is designed to undergo aphasechange,shiftingfromsolidtoliquidform,asthebatterycellsbegintoheatup.Thishelpspulltheexcessheatoutofthecells.Youcanbuypre-builtpacksfrom AllCell, or you can find a number of homemade “phase change wax”recipesonlinetotryandmakeyourownversion.Ablockofwaxandadrillbitcanresultinanicehomemadebatteryholder!

Ifyou’reusingpouchcellsinsteadofcylindricalcells,you’llneedamuchbettersolution forholdingyour cells.There aren’tmanycommercially available cellholdersavailableforpouchcells,mostlybecausetherearen’tanystandardpouchcellssizes.

Theamountofprotectionforyourcellswilldependonyourrequirements.RC

lipo batteries are a great example of minimalist design. The pouch cells thatcompriseRClipopacksaresimplyheldtogetherbyasinglepieceofheatshrink.Occasionallythereisathinlayerofplasticorfiberglassaroundtheoutsideoftheshells,butoftenasinglelayerofheatshrinkisall that isusedtocombineandholdthecells.

The advantage of only using heat shrink is that you’ll get the lightest andsmallest battery possible. The disadvantage is that you’ve got very littleprotection.Anybumpsor impactsare transferreddirectly to theactual skinofthelithiumpouchcells.Anythingsharpcaneasilypuncturethecellsandcauseadevastatingfire.Thismethodalsoworksbetterforsmallerpacks.Aspacksgetlarger, the increasedweight and increased amount of contained energy requirestrongerandsaferbatteryenclosures.

Ahardplasticcaseisabetteroptionforhousingpouchcells.Youcangetfancyand build a custombox out of plastic or acrylic sheet, or you can use off theshelfproducts likeOEMcases.Toolcases, lunchboxesandstorageboxesthataremade fromhard plastic are great options. They aremass produced,whichoften keeps the cost down, and they are also already designed specifically forprotection.A layerof foamon the insideofahardcase isagood idea tohelpcushionthecellsandkeepthemfromimpactingthehardplasticdirectly.

Somepeoplebuildwoodenclosuresforbatteries.Thisisalsoanoption,thoughI’drecommendusingasoftfoamlayerbetweenthewoodandthecells,justlikein a hard plastic case. The problem with wood is that not only is it moreflammable,butif it isleftunsealedthenitcanalsoabsorbwaterif itevergetswet.Thiscouldcreateahumidenvironmentaroundyourcells,whichislessthanideal.

Metal cases are not usually a good idea for DIY batteries unless you haveensured that the battery and all connectors are 100% sealed and insulated, oryou’vepaintedorsealed thecase torender itnon-conductive. It’sveryeasy toaccidentally create a short circuit on a metal case if you aren’t careful. Heatshrinkwrappingtheentirebatteryandusinggood,enclosedconnectorscanhelpmakeametalcasemoreviable.

Prismaticcellsoftencomeintheirownhardplasticcases,whichmakesbuildingapackmucheasierprocess.Largeprismaticcellsinthe20Ah-100Ahrangeareusuallyusedinelectricvehicleconstructionandcanoftenbeusedas-is,noexternal cases required. They can be simply loaded into the trunk or enginecompartmentofacar,thoughacustomboxtoenclosethebatteriesandsupportthem is a good idea. Such a box can also help take physical strain off of theelectrical connections, busbars and wiring used to electrically connect thebattery.

Prismaticcellsareoftenheld inwoodencases,usuallybecause their largesizemakesithardtofindOEMplasticcasesthatcanholdsuchlargebatteries.Thefact thatprismaticcellsaresealedalsomakes it lessofan issue if thewoodenboxweretoabsorbalittlemoisture.SmallerprismaticcellscanbeheldinOEMplastic cases though, or just left exposed if they are built into their own rigidplasticcases.

Joiningcellsinseriesandparallel

There are a number of different ways to make electrical connections to joinlithiumcellstogether.Therightmethodoftendependsonthetypeofcellandtheloadrequirementsonthebattery.

Some lithiumcells comewith threaded terminals thatmake it easy tobolt thecells together. Many prismatic cells, especially the larger ones, have thesethreaded connectors. Sometimes they are threaded studs that extend from thecells and require a nut to be threaded onto the battery’s stud terminals, andsometimes they are threaded holes that require bolts to be inserted into thebatterycaseitself.

Headway cells are some of the only cylindrical cells that have threaded postsavailable for easy bolt-together pack assembly. Cylindrical cells usually havesmooth,flatterminalsthataredesignedtoeitherbeheldbyspringloadedbatterycontactsorspotwelded.

SpringloadedbatterycontactsarethecommonkindsusedforsmallerbatterieslikeAA’s. They’re the battery holders you often see in a remote control, andusually consist of a spring on one end and a button or rounded dome on theother.The spring ensures that the two endsof thebattery are held in constantcontactwiththecaseterminals.

Thismethodisprobablytheeasiestwaytojoinbatteriesintolargerpacksasitrequiresnotoolsandverylittleskill.Justlineupthe+’sand-’sandpopinthebatteries! The problem with this method is that it only works for low powerapplications,which iswhy it isused inmostconsumerelectronics.Thespringcontactshaverelativelyhighresistanceandsoifyou’redrawinganysignificantamountofpoweroutof thecell,you’regoing towindupwitha largevoltagedrop and a lot ofwasted heat coming from the spring terminals. If you try todraw toomuch power, you can even overheat the spring contacts,whichwillbegintoglowbrightred.Asyoucanimagine,thisisbad.Youwanttoavoidthis.

Oneof themain advantages of lithiumbatteries is that they canprovidequitehigh power. Taking advantage of this power means you can’t use thoseconvenientspring-contactholders.

Instead,theproperwaytojoinlithiumbatteriesthatdon’thavescrewterminalsis by spot welding. Spot welding is a type of simple welding that works bypassingaveryhighcurrentthroughtwopiecesofclampedmetal.Anelectrodeon either side of the clampedmetal (or inmany batterywelders, on the sameside) press the metal together and applies a short pulse of high current. Thecurrent flows from one electrode to the first piece ofmetal, then through thesecond piece of metal and finally into the second electrode. The highestresistanceisfoundbetweenthetwopiecesofsandwichedmetal.Becauseofthehigherresistancebetweenthemetal,thatjointquicklyheatsupfirstandcausesthemetaltoinstantlymelt,joiningthetwopiecesofmetalintooneatthatpoint.

Thekeytospotweldingisthatthepulseofcurrentisveryshort,oftenjustafewmilliseconds. Thatmeans that the only place that heats up substantially is the

jointbetweenthetwometalsbeingwelded.

Infact,mostbatteryspotweldersuseadoublepulse,wheretwoextremelyshortpulsesofcurrentfireinrapidsuccession.Thefirstpulseoftenhelpstosoftenthemetal and burn off any impurities, while a second and usually stronger pulseperformstheactualweld.

Aswe’vetalkedaboutrepeatedly,heatistheenemyoflithiumbatterycells.Byusing spotwelding to join cells together, only a very small amount of heat iscreated foravery short time,and that small amountofheatquicklydissipatesintotheambientair.Thisiscrucial,becausethemoreheatthatistransferredintothelithiumbatterycell,themoredamagewillbecausedtothecell.

Spotweldingisthemostcommonmethodusedforjoininglithiumbatterycellsthat don’t have screw terminals, and its the method used by nearly everyprofessionalbatterybuilder.

Professionalbatteryspotweldersusuallycostmanythousandsofdollarsandareoutside of the reach of DIY battery builders that only plan to build a fewbatteries. The professional spot welders often have CNC capabilities toautomatically weld a preprogrammed pattern and usually use capacitivedischarge. However, hobby level battery spot welders that use transformersinstead of capacitors are fairly common and can be found for a few hundreddollarsonsiteslikeAliExpressandeBay.

ThecheapesthobbylevelspotwelderscostaroundUS$100,butusuallyleavealottobedesiredinthequalitydepartment.Eventhe$200-$300spotweldersareknownforbeingabitofagamblesometimes.Theyusuallyeitherworkgreatorarrive broken. And because they are often only available by purchasing themdirectly from Asian vendors online, it can be often be difficult to return adefectiveorbrokenunit.

There has been something of a renaissance in DIY spot welders based onmicrocomputersliketheArduinoorRaspberryPi.Thesespotweldersoftenusea car battery as the current source and use microcontrollers to control theelectricalpulse.Whilethisoptionusuallyrequiresahandyoperatortobuildtheirownspotwelder,itoftenultimatelyresultsinabetterqualitywelder.

Themost commonmaterial used for spotwelding onto battery cells is nickel.Nickelisanexcellentoptionforbuildingelectricalconnectionsforbatterypacksbecause of its high conductivity and relatively low resistance.Copper is oftenused for electrical connections as it has even higher conductivity and lowerresistance than nickel, but that same low resistancemakes itmore difficult tospot weld. The problem is that most spot welding electrodes are made fromcopper and if you rememberhow the spotweldingprocessworks,weneed tocreate a higher resistance between the two pieces ofmetal being spotweldedthan between that samemetal and the electrodes. If all of the parts aremadefromcopper,itisdifficulttoensurethattheresistancebetweenthetwoweldedsurfaceswillbesubstantiallyhigherthanbetweentheelectrodes.Nickel,ontheother hand, is quite easy to spotweldwith copper electrodes yet still has lowenough resistance tomake it a good choice for electrical connectionsbetweenbatterycells.

Rolls of nickel strips are commonly available specifically for battery packwelding.Nickel strips are produced inmany differentwidths and thicknesses.Themaximumthicknessofnickelthatcanbeweldedbymosthobby-levelspotweldersisapproximately0.15mm.For18650cells,yougenerallyshouldn’tuseanythingnarrowerthan7mm,otherwiseyou’llbereducingtheconductivityofyourcellconnections.Iliketouse8mmwidestripsof0.15mmthickness,andwilloftenusemultiplestripsweldedontopofeachotherwhenahighercurrentbatteryisrequired.

Damian Rene, a custom battery builder in Madrid, calculated and released ahandytablefordeterminingtheamountofcurrentthatcansafelybecarriedbynickelstripofacertainsize.Theoptimalrangeiswherethenickelbarelyrisesinheat above the ambient temperature.The acceptable range iswhere the nickelbegins to heat up but does not excessively overheat. The poor range iswhereoverheatingoccursandcurrentsabovethislevelshouldbeavoided.

BecauseIliketouse8mmwide,0.15mmthicknickelstrip,mygeneralruleofthumbis thatfor thissize,onestripcancarry5A.If Ineedmore than5Aofcurrentcarryingcapacityperstrip,thenIweldanotherstripontopofthefirst.IalwaysweldonestripatatimetoensurethatIgetthebestpossiblewelds.Asyoucansee from the table, theacceptable range fornickel stripof that size is

somewhathigherthan5A,butIusethisasaruleofthumbtostayonthesafeside.

Itwouldalsobepossibletousethickernickelstripsinsteadofweldingmultiplelayers,butmosthobbygradespotwelderscan’thandlenickelstripsofmorethanaround0.15mminthickness.Weldingtwolayersofnickelhasthesameeffectasusingonelayerofnickelthatistwiceasthick.

Nickel strip is usually available in two varieties, pure nickel strip and nickelplated steel strips. Unless you have very low current requirements, you wantpurenickelstrip (which isoftenadvertisedas99.9%purenickelstrip).Nickelplated steel strips are cheaper toproducebuthavemuchhigher resistanceandthusareonlysuitableforlowpowerbatteries.

Becarefulwhenyoupurchasepurenickelstrip toensure that it is infactpurenickel.When purchasing nickel strip from international vendors, I have oftenseennickelplatedsteelpassedoffaspurenickel.Thetwolookidenticalandarehard to distinguish, even when given a sample of each. Both have the sameappearance and nearly identical densities (or close enough that you needlaboratory grade equipment tomeasure them accurately enough to distinguishthembymass).Together,DamianReneandIdevelopedtwoteststodeterminewhethernickelstripsarepureorsteelplated.

Thesaltwater test:Nickeldoesnotcorrodeeasily,unlikesteelwhich loves torust. Take a piece of nickel strip and scratch it up aggressively with strongsandpaper or a metal file. If you don’t have either, use a coin or a nail or

anythingreally-justmakesureyoureallyattackthesurface.Wewanttoscrapeoffsomeof thenickel layer, if it’s there.Thenmixupacupofsaltwater (theexact amount doesn’t really matter). Drop your scratched nickel strips in thewaterandwaita fewdays. Ifyoudon’t seeanyrust forming,you’vegotpurenickel!Ifyouseebrownororangerust,you’vegotnickelplatedsteelstrips.

The sanding/grinding wheel test: I developed this test on accident whileexperimentingwiththefirsttest.IusedaDremelsandingwheeltoscuffupthesurfaceofapurenickelstripandanickelplatedsteelstripbeforedoingthesaltwatertest.Thepurenickelstripwasfairlyuneventful,butwhenIusedasandingorgrindingwheelonthenickelplatedsteelstrip,Igotbrilliantsparksshootingfrom the strip as soon as the wheel ate through the thin nickel layer andcontacted the steel. Makes sense - steel creates spark showers. Apparentlythough,nickeldoesnot,andthatmakesthisanothergreattest.It’salsosuperiorto the saltwater test becauseyouget the result instantly.But if you’re still indoubt,goaheadandtossthepieceinsaltwaterafteryou’redonegrindingonit,andthatwillverifytheresultsofthegrindingwheeltest.

In addition to nickel strips, some people use nickel sheets for joining cells,especiallywhenbuildinglargerbatterypacks.Theadvantageofnickelsheetsisthat you can cut them to any shape you want. Nickel strips require makingstraight line connections, but nickel sheets can be cut to abstract shapes andreacharoundtodifferentpartsofabattery.Theyalsomakeiteasiertouseonelayer of nickel as they are wider and can carry more current than individualnickelstrips.We’lltalkfurtheraboutusingmultiplenickelstripstocarrymorecurrentinChapter10.

Nickel is agreatmaterial for spotweldingbetweencylindrical cells,butothertypes of cells don’t require any extra material, depending on your battery

configuration. Pouch cells have large battery terminals that can be joined in anumberofdifferentways.

Somepeopleusealuminumblockstoclampontothebatterytabs,andthenwirescanbesolderedorbolteddirectly to thealuminumblocks.Othershavedrilledholesinthetabsandboltedthroughthetabstoconnectwiresdirectlytothetabsthemselves.Becarefulwiththismethod,asyouwanttobesurethatyou’reonlydrillingthroughthetab.Clampingthetabtoajigwithpre-drilledholescanbeagoodmethodtoensurethatyourdrillbitdoesn’twalkandaccidentallypuncturethepouch.

The tabscansometimesbespotwelded,but theyareoftenmadeofamaterialwithlowenoughresistancethatspotweldingcanbedifficult.Pouchcellsoftenusealuminumforonetabandcoatedcopperfortheother.Bothmaterialscanbedifficulttospotweld.

Youmustbeincrediblycarefulwhenconnectingpouchcellsbecausethemethodof alternately stacking pouch cells results in many cell terminals in closeproximity. If twoadjacent cell terminals touchwhen their partners are alreadywelded in series, a short circuitwill occur resulting in a flash of light, a loudbang,andprobablythedisintegrationof theterminals.Andthat’s thebestcasescenario.Theworstcasescenariowouldbeifthetabsfusetogetherandcauseashortcircuitthatcan’tbebrokeneasily,resultinginthecellsquicklyoverheatingandpotentiallyexploding.Sowhenyou’reworkingwithstackedpouchcells,beincredibly careful to avoid short circuits. This usually means that aftercompletingaspotweldbetweentwobattery terminals, the terminalsshouldbeinsulatedsomehow,usuallywithheatshrink,electricaltapeorkaptontape.

Now we need to also talk about soldering as a method for joining cells.Generallyspeaking,lithiumcellsshouldnotbesolderedtogether.Ihatetobeatadead horse here, but aswe’ve discussed, heat is the enemy of lithium batterycells.Solderingironstypicallyoperateataround250°C.Whatwasthatthermalrunawaytemperatureofmanyli-ioncellsthatwelearnedaboutbackinChapter7?Oh right, 150°C.Lithiumcells risk entering thermal runawayat 150°C.Soyoucanseewhyputtinga250°Csolderingironontheendofalithiumbatterycellmightbeabitofaproblem,right?

Tobe realistic, unlessyou’re soldering for a really long time,you’reprobablynot going to heat the cell up to its thermal runaway temperature, but you aregoing to do damage to the cell, and that damage is permanent. Irreparabledamage is causedwhen the cell is heated tohigh temperatures, andwhenyousolder thecells,you’ll cause thatdamage in small areas that areclosest to thecellterminal.Itdoesn’tmatterhowquicklyyoucansolderthemandhowshortofaperiodthehotsolderingirontipstaysincontactwiththecells,it’sstillgoingtopassalotofheattothecell.

Therearebasicallytwoissueshere:Canyousolderonlithiumcells,andshouldyou?Technicallyspeaking,yes,youcansolderonlithiumcells.Itwillwork.Butshouldyou?Again,no.

No,no,no.

If you’ve spent any amount of time researching DIY batteries, then you’veinevitablyseenpeoplesolderingon lithiumbatterycells. Ifyousearcharound,you’llalsohearpeoplesayingthatit’soktosolderonlithiumcells,thatyoucandoitinacertainway,orthatit’sfineifyoudoitquickly,etc.Thesepeoplearesimplymisinformed,andaremistakingthefactthatwhileitispossibletosoldertolithiumcells,itissimplynotadvisable.

The worst case scenario for soldering is on cylindrical cells such as 18650s,specifically thenegative(anode) terminal.AswesawinChapter2,cylindricalcells have terminals at either end, and the positive (cathode) terminal issomewhat separated from the contents of the cells by the cap, vent and othercomponentsunder thepositive terminal.However, thenegative terminal is theshellofthecellitselfandispresseddirectlyupagainsttheelectrolytefillingthecenterofthecell.Thismeansthatwhenyouheatthatverythincellwallatthebottom of the cell, you’re directly transferring that heat immediately to thecontents of the cell. On the positive end of the cell you’ve at least got someseparation between the terminal and the electrolyte material below, but thenegative terminal shows no mercy. Heating the negative terminal with asolderingironissimplybeggingforcelldamageandprematurecelldemise.

Pouchcells,withtheirlongercelltabs,areslightlymoreresilientwhenitcomesto heating with a soldering iron, but I mean it when I say slightly. Thoseterminalsareveryclosetothecell’selectrolyteandbyheatingit,you’reriskingcell damage.Also,many pouch cell tabs can be difficult to solder due to thematerial used in the tabs. You might have to use special solder and fluxdependingonthematerialused.Occasionallyyouwillfindpouchcellsthathavenickel tabs thatwere spotwelded onto themain terminals at the factory. It ismucheasier(andsafer)tosolderontotheseadditionalnickeltabs.

Ifsolderingonlithiumcellsissobadforthem,thenwhydosomanypeopledoit?Well,foronething,it’sasupercheapwaytocombinecells.Youdon’tneedanyfancytoolsorspotweldersorcustommountingbrackets.Allyouneedisasolderingiron,somefluxandsomegoodsolder.

Also,manypeoplebuildlithiumbatteriesoutofsalvagedcellsthattheyrippedoutofoldlaptopbatteriesandotherdevices.TheDIYpowerwallcommunityisnotorious for this. They are building very large home-energy storage batteriesand seek to minimize costs as much as possible. Using salvaged cells andsolderingthecellstogethersavesalotofmoney.Furthermore,becausetheyareusingcellsthatwerebasicallyfreeorcheap,andbecausethosecellsarealreadyof unknown quality and remaining lifespans, they likely consider any celldamage to be inconsequential. What’s the problem with losing 10% of yourcells’capacitieswhentheywerefreetobeginwith?

Iseethisasbothpoorcraftsmanshipandofquestionableresponsibilityinterms

ofsafety.However,peoplearegoingtodowhattheyaregoingtodoandmyjobis simply togiveyou the facts and informationyouneed so thatyoucanplanyourownDIYbatterybuildtofityourneeds.

Cell-levelfuses

Individual fusesoneachcell, alsocalledcell-level fuses,arecommononverylargebatteriesandbatteriesthatpresentarisktohumanlifeandsafety.Electricvehiclebatteriesareagoodexample.Theimagebelowshowsthetypeoffusesused for each cell in a Tesla electric vehicle. The fuses themselves are smallwiresattachedtotheterminalofthebatterycell.

The idea behind cell-level fuses is that if any cell in a parallel group shouldsufferashortcircuit,itwouldoverheatandburnthefuse,thuscuttingitselfofffrom the circuit.Without a cell-level fuse, a single short circuiting cell couldeitherleadtoothershortcircuitsinachainreactionaroundit,orslowlydraintherestofthecellsdowntozerovoltage,ruiningtherestofthecellsintheparallelgroup.

For the vast majority of hobby-level DIY batteries, cell-level fuses aren’tnecessary.Thechanceofashortcircuitareverysmallwhenusingthebatteryasintended,especiallywhenusinggoodqualitycells.However,asthesizeandthuscost of the battery increases, and as ramifications of failure (such as injuryordeath) increase, theconsequencesofa singlebadcellbringingdownanentireparallelgrouporpackalsoincrease.

The larger the battery, and the more individual small cells that are in eachparallelgroup,themoreusefulindividualcell-levelfusescanbe.Theyareoftenused in theDIYpowerwall community, especiallywhen thosepacksaremadewith salvaged cells of unknown histories and qualities. In that case, DIY cell

level fuses are often made by cutting the wire legs off of 1/4 W or 1/8 Wresistors and soldering them between the cell terminal and the busbar. Aswetalked about though, soldering is not a greatway to connect anything tomostlithiumbatterycells.

Electric vehicle makers such as Tesla that use many thousands of cells pervehiclehaveemployedsophisticatedweldingmethodstocreatecell-levelfuses.Theseareoftenhardtoreproduceonhobby-levelbatteries.Onemethodthathasbeenemployedistocutdowntheareaofnickelstripactuallyweldedtobatteryterminalstoaverysmallwidth.Thisiseasierwithlasercuttingtechniquesbutcanbedonebyhandaswell.Thiscreatesashortandthinsectionofnickelthatservesasacell-levelfuse.

Suffice it to say thatmostprojectswon’t requirecell-level fusesunlessyou’replanning on driving in it, flying in it, or you think you could stand to losethousandsofdollarsifalowqualitycellshortcircuitedinit.

Chapter10:Batterylayoutanddesign

Congratulations,we’vecoveredeverythingweneedsofartobeginplanningourspecificbattery!Thisiswherethefunstarts,becausenowwecanstoptalkingintheoryandstartworkinginrealitytoplanabattery.

Therearemanyparametersthatgointobatterylayoutanddesign.Someoftheseparameters are interconnected and changing one will affect the others. It isimportanttoplanforallofourparametersbeforewecanbeginchoosingcellsoreven thinkaboutstartingassembly.Let’s tackle thedesignparametersoneatatime.

Voltage

We’llstartwithvoltage,whichisoneofthemostcrucialaspectsofyourbattery.Thevoltage isameasureof theelectrical force inyourbattery.Highervoltagemeanshigherelectricalforce.RememberwhatwelearnedaboutserieswiringinChapter6?Toincreasethevoltageofabattery,we’llneedmorecellsinseries.Inahighvoltagebatterywithmanycellsinseries,thoseelectronsarechompingat thebit, rearing togo!That’sdue to thehigherelectron force trying todrawthoseelectronsthroughthecircuit.ThatalsoexplainswhyaAAbattery(whichis1.5V)onlymakesatinysparkifyoushortit,buta9Vbatterywillproduceamuchlargerspark!

Generally speaking, higher voltage batteries are more efficient, as a highervoltagebatterycanprovidethesameamountofpowerasalowervoltagebatterywhilesupplyinglesscurrent.Fewerampsresultsinlesspowerbeinglostasheat.

Higher voltage isn’t always better though.You’ve got tomatch the voltage towhateverdeviceyouarepowering.AsdiscussedinChapter3,youalsoneedtoconsiderboth thevoltagedropand thevoltage rangeover theentiredischargecurveofyourbattery.

Thevoltagedropistheinstantaneousreductioninvoltagewhenaloadisappliedtothebattery.Thelarger theload, thelowerthevoltagewilldrop.This isalsoknownasvoltagesag.Inadditiontovoltagesagunderaninstantaneousload,thevoltagewillalsoslowlydecreaseover thedischargecurveofabattery. In thatcase, the voltagewill show a larger decrease in the beginning and end of the

discharge, and a relatively flatter voltage level in themiddle of the dischargecurve.

Li-ioncells range fromabout3.0V to4.2V,whileLiFePO4 cells range fromabout2.5Vto3.65V.Multiplyingthosevaluesbythenumberofcellsinseriesinyourbatterywillgiveyouthevoltagerangeofyourbatterybetweenits fullandemptystates.

Ifyourdevice requiresaconstantvoltage,youmightneed toconsiderusingavoltageconverterorvoltageregulator.ADC-DCconvertercantakethevoltagefromyourbatteryandstepitupordowntoanyvoltageyouspecify,assumingitis within the range of that specific converter. DC-DC converters can also beusefulifyouneedahighervoltagebutdon’twanttobuildabiggerbatterywithsomanycellsinseries.

For example, if youneed12V for a device but only have space (or aweightallowance)fortwocells,youcanbuilda2s1pbatteryconsistingofthetwocellsinseries,andthenuseasmallDC-DCconvertertostepupthevoltageto12V.Keep in mind though that using a step-up (or boost) converter will requiredrawinghighercurrentfromthebatterytomakeupfortheincreasedvoltage.

Now let’s try an exercise to practice designing the right voltage battery for aparticularproject.

For this example, we’ll use an electric bicycle as our project. Let’s say theelectricbicyclerequiresa36Vbattery.Ifwecheckthecontroller,we’llfindthatit actually has a lowvoltage cutoff (LVC) of 29V. (Thiswill be different foreachelectricbicycleandcontroller;I’mjustthrowingoutacommonexample.)

Thatmeansthatweneedtomakesurethatweuseenoughcellsinseriestoreachthepropervoltage.Inthiscasewecanuse10cellsinseries.A10sbatterywouldhaveanominalvoltageofabout37V(3.7Vpercell×10cellsinseries=37Vnominal). But we should also make sure that the battery can discharge to areasonable voltage to meet the low voltage cutoff (LVC). The LVC is 29 V,whichwouldmean2.9Vpercellonour10sbattery.That’sjustfine,aslithiumioncellscandischarge to2.5V, though2.9Visofcoursehealthierbecause itkeepsthemfromreachingtheirabsolutelowestlimit.Thisisalsowhynearlyall36Vbatteriesforelectricbicyclesusea10sconfiguration.

Whatwouldhappenifwetriedtousea9sconfiguration?Thatwouldgiveusa33.3Vnominalvoltage.Abatteryof33.3Vwouldstillworkforthisapplicationfor a littlewhile, but as it discharges, thebatterywould reach theLVCof thecontrollerat29V.Fora9sbattery,29Vwouldmeaneachcellisat3.2Vwhenthecontrollerreaches theLVC.Thatstill leavesanother20%orsoofcapacityleft ineachcell,making this is abitof awaste.Technically itwouldbequitehealthy for the cells since they wouldn’t be discharging very close to theirminimum,butitstillseemslikeawasteofgoodpotential,assumingwewanttousetheentirecapacityofourcells.

Andwhat ifwemadean11sbattery?We’dwant tochecktomakesure that itdoesn’texceedtheallowablevoltageofourdevice,whichisanebikecontrollerin this case. Let’s say that our ebike controller has a type of built-in safetyknownasahighvoltagecutoff(HVC).Inthiscase,ifthecontrollerisconnectedtoabatterywithavoltageaboveitsHVC,itwillshutitselfofftoprotectitself.Let’ssaythatourcontrollerhasa45VHVC.

An11sbatterywouldhaveafullychargedvoltageof46.2V(11cellsinseries×4.2Vpercell).That46.2Vishigher thanour45VHVC.Thatmeansan11sbatterywouldhavetoohighofavoltageforourapplicationinthisexample.Buta10sbatteryhasafullychargedvoltageofjust42V.That’sperfectforthiscase,soagainour10sbatteryhastherightconfiguration.

Rememberthatdifferentlithiumcellshavedifferentvoltagesthough.Theaboveexampleusedli-ioncells,whichhaveanominalvoltageofaround3.7Vandafullychargedvoltageof4.2V.LiFePO4cellshavelowervoltages.LiFePO4cellsarenominally3.2V,fullychargeto3.65Vandcanbedraineddownto2.5V.Thatchangesourcalculations.

Ifwe reworkour example abovewith the 36V electric bicycle, a 10s batteryusing LiFePO4 cells will no longer work. Fully charged, a 10s battery usingLiFePO4cellswouldonlyreach32V.Considering thevoltagesag, thebatterywouldlikelyreachthelowvoltagecutoff(LVC)of29Vinthisexamplealmostimmediately.Notgood.Weneedmorecellsinseries.

Whydon’tyoutrytoworkoutwhatthepropernumberofLiFePO4cellswouldbeforthisexamplewitha36Vbattery,assuminga29VLVCanda45VHVC.ThenI’llworkitoutwithyouandwe’llseeifwegetthesameanswer.

…

Ok,haveyousolvedityet?Ihopeso,becausenowit’smyturn.Let’ssee,weneedtomakesureourlowestvoltageisaroundtheLVCof29V.Ifwechoosea12sconfiguration,thenat29Vwe’dsee2.42Vineachcell.That’slowerthanthe2.5Vwecandischargeeachcell to.But ifwehaveaBMSprotectingourbattery then thatwouldbe just fine,because theBMSwouldcutpower to thebattery as soon as the first cell hit 2.5 V. Now what about the fully chargedvoltage.Atfullcharge,our12sbatterywouldgiveus43.8V,calculatedas:

Fullchargevoltage=12cellsinseries×3.65VperLiFePO4cell=43.8V

That’s below ourHVC. So this looks pretty good, since our 12s battery staysbetweentheLVCandHVC.

Whatabout an11sconfiguration?Thatwouldmean thatwehit theLVCabitearlyatabout2.64Vpercell,andthatweleavesomechargestillinourcells.Ifyourgoalismaximumrangeonthisebike,thatwouldbeaproblem.Butifyouareokwithsacrificingsomerange togiveyourcellsahigherdischargecutoff(and thus a longer life expectancy) then that could be a good thing. At fullcharge,our11sbatterywouldbeat40.15V,whichiswellunderour45VHVC.Soitlookslike11scouldalsowork.

In thiscase thereweretwopossibleanswers.An11sconfigurationwouldbeabitmoreconservative,whilea12sconfigurationwouldensurethatyouusethefullcapacityofthecells.You’ddefinitelyneedaBMSonthe12sconfigurationthoughtomakesureyoudidn’tover-dischargethecells,oruseanothermethodlikeavoltagealarmtoalertyouwhenthecellsarenearlyempty.

Thisisthemethodyou’llusetodeterminethevoltageofyourbatteryinmanycases where a fixed range of voltage is required. However, sometimes thevoltageisn’tcritical.Inthesecases,youwillhavemorefreedom.

Forexample,let’ssaythatwe’reworkingwithaheatingcoil,suchasapieceofnichromewire.Maybewewanttomakeaheatedjacket.Inthisapplication,wecansupplyanyvoltagewewant,and itwillsimplychange theamountofheat

thatwegeneratefromthewire.

Here’sanexample.Let’ssayourwirehasaresistanceof1ohmperfootandourlengthofwireis10feet,givingus10ohmsofresistance.Wecanuseohm’slawof I =V÷R (or current equals voltage divided by resistance) to determine ourcurrentrequirementandthenourheatingpower.Ifweusedasingle3.7Vli-ioncell,we’dhaveacurrentdrawof0.37A,givenbyohm’slaw:

Current=3.7volts÷10ohms=0.37amps

Nextwecancalculatetheheatingpowerinwatts.Theequationforpoweris:

Watts=volts×amps

Inthisexample,ourheatingpowerequals:

3.7V×0.37A=1.37W

Ifyouweren’taware,1.37wattsisprettylow.That’snotgoingtoheatourjacketverymuch.Ifwewanttoincreasethepower,wecansimplyincreasethevoltage.Let’s saywe used a 10s li-ion battery instead of that 1s battery (whichwas asinglecell).The10sbatterywillgiveus37voltsnominal.Ohm’slawgivesusourcurrentof3.7Awhichiscalculatedas:

37volts÷10ohms=3.7amps

Ournewpowerisabout137W,calculatedas:

37volts×3.7amps=137watts

Nowwe’reheatingthingsup!That’sprobablymoreheatthanwewantfromourjacket,butyougettheidea.

Soasyoucansee,whenyourvoltagerangeisflexible,yourpowercanchangesimplybychangingyourvoltage.

Let’s do one last quick example and then I promise I’m done talking aboutvoltagefornow.Whenitcomestoelectricmotors,highervoltagemeanshigherspeed. DC motors are often rated in terms of KV, which is the number of

revolutionsperminute(RPM)pervolt.Soa100KVmotorwouldspin100RPMat1voltor2,000RPMat20volts.

Thatsamemotorwouldalsohavealotmorepowerathighervoltage,butyou’dhave to be careful not to overvolt it and destroy it, either by burning out itswindingsorspinningitsofastthatitselfdestructs.

Ok,Ithinkyougetit.Enoughaboutvoltage.Let’smoveontoplanningforyourbattery’scapacity.

Capacity

Thecapacityofabatteryishighlyrelatedtothemaximumcontinuousdischargecurrentratingofthebattery.Butwe’llstartwithcapacityasit’sabitsimpler,andthenwe’llworkourwayovertocurrent.

Thecapacityofa lithiumbatteryiskindof likethesizeofagas tankinacar.Thelargerthegastankinacar,thelongeritwilldrive.Andthemorecapacityinyourbattery,thelongeritwillwork.

Thecapacityofalithiumbatteryisusuallymeasuredinamphours(Ah)orwatthours (Wh).Amphours are ameasureof thenumberof amps thebattery cansupply foronehour.Watthoursaresimilar,butmeasures thenumberofwattsthebatterycansupplyforonehour.

Between the two,amphoursaremorecommon in the industry.However,watthours actually give a better indication of the total energy in a battery. That’sbecausewatthoursarecalculatedbymultiplying thebattery’snominalvoltageby theAh ratingof thepack.Soa36V10Ahbatterywouldbeapproximately360Wh.

Whiletheamphourratingisaspecificmeasurementforeachbattery,watthoursalsoconsider thevoltageof thebatteryand is thusameasurementof the total

energy in a battery. That makes watt hours better for comparing betweenbatteriesofdifferentvoltages.A12V10Ahbatteryanda24V10Ahbatterybothhave the same amp hour rating, but the 24V battery has twice thewatt hours(240Whvs120Wh)becauseitstorestwicetheamountofenergy.Forbatteriesorbatterycellsofthesamevoltage,Ahisusuallyusedforcapacitycomparison.

Aswe learned in Chapter 6, the capacity of a lithium battery is increased byaddingmorecellsinparallel(orusinglargercapacitycells).Soifweneededa10Ahbatteryandwewereusing2.5Ahcells,wewouldwanttoputfourcellsinparallel.Thatgivesusatotalcapacityof10Ahby:

2.5Ah×4cells=10Ah

Wecouldhavealsojustusedasingle10Ahcell,butthoselargecellscomeinfeweroptionsandareusuallylimitedtopouchandprismaticcells.Usingsmallercellsinparallelgivesusmoreoptions.

The exact capacity of your battery will of course depend on your specificapplications. If you’re building a small drone, then youmight be fine with a4Ahbattery.Anelectricbicycleusuallyusesabatteryofbetween10Ah-20Ah.Anelectricvehicle’sbatteryisusuallyover100Ahandsometimescanbemanyhundredsofamphours!

Tocalculatehowmuchcapacityyourbatterywillneed,itishelpfultofirstthinkintermsofwatthours.Let’suseasimpleexamplefirst.Let’ssayyourloadisalightbulbwhichtakes12V.Let’salsosaythatourexamplelightbulbdraws1Acontinuously during use. If your application requires that your battery shouldpowerthelightbulbfor8hours,thenyoucancalculatehowmuchenergyyou’dneed.

First,calculatethewattsusedbythelightbulb.Remember:watts=volts×amps

Thatmeansthatthewattsusedbythelightbulbareequalto12watts,calculatedas:

12volts×1amp=12watts

This gives us 12 watts of continuous power being used by the light bulb.Remember,wattsareaunitofpower,notenergy.

Nowweneedtodeterminetheamountofenergyusedbythelightbulb,whichiswherewatthoursareused.Awatthour (Wh) isameasurementofenergy,notpower.Todeterminetheamountofenergyourlightbulbuses,wejustmultiplythepowerituses,inwatts,bytheamountoftimeitusesthatpower,inhours.

Watthoursconsumed=continuouspower(inwatts)×time(inhours)

Inourcasewewantthelightbulbtoruncontinuouslyfor8hours.Tocalculatethetotalenergyneededfor8hours,wemultiply12wattsby8hours.Thisgivesus96watthours.

Ok,nowwe’regettingsomewhere.Ourlightbulbneeds96watthourstooperatefor8hours.Ifwe’reusinga12Vbattery,wejustneedtofindtherightnumberofamphoursthatwillgiveus96watthoursofenergyinthebattery.Thewatthoursofabatteryarecalculatedbymultiplyingthevoltageofthebatterybytheamphours.

Watthoursofabattery=voltage×amphours

Thatmeansthattogettheamphoursofabattery,wewouldtakethewatthoursofthebatteryanddivideitbythevoltage.

Amphoursofabattery=watthours÷voltage

Inourcase,thatmeanswedividethewatthoursneeded(96Wh)bythevoltageofourbattery(12V).Thisgivesus8Ah.

96Wh÷12V=8Ah

Thatmeansthatforourexampleofa12Vlightbulbthatrequires1Aofcurrentandoperatesfor8hours,weneeda12V8Ahbattery.Thisshouldmakesense,sincewealreadyknewthatthebulbdraws1A,whichmeansthateveryhouritdraws1Aperhour,or1Ah.Sofor8hoursofuse,we’dneedatotalof8Ah.

Let’s try another example. Let’s saywe have a 100W heater that operates at24V.Andlet’ssaywewanttorunthatheaterfor20hours.We’dwanta24Vbatterythatcanprovide2,000watthoursofenergy,calculatedas:

100W×20hours=2,000watthours

Todetermine theAhof thebatterywe’dneed,wesimplydivide2,000Whby24V.

2,000Wh÷24V=83.33Ah.

Thatmeanswe’dneedanapproximately24V83Ahbattery to run that100Wheaterfor20hours.

NowIwantyoutotryacoupleofexamples,justtomakesureyou’retotallyonboardwiththeprocess.Trythesethreeproblems:

1) Wehave a load that requires 36V and draws 5.5A.We need it to runcontinuouslyfor5hours.Whatsizebatterydoweneed?

2) Wehavea48V10Ahbattery.Ifweconnectittoaloadthatdraws25Acontinuously,howlongwillthebatterylast?

3) Alright, thisoneisabit trickier,butI thinkyoucanhandleit.Takethatsame48V10Ahbatteryfromquestion2above.Let’sfirstconnectittoa50Wloadfor2hours.Thenwe’lldisconnectitfromthe50Wloadandconnectittoa20Wloadinstead.Howlongwillitlastonthe20Wloaduntilthebatteryisdepleted?

Answers:

1)Thisloadis36V×5.5A=198watts.

At5hours,ituses198W×5hours=990Wh.

990Wh÷36V=27.5Ah

Soweneeda36V27.5Ahbattery.

Alternatively, you could have gotten the same answer with a short cut. Justmultiplythe5.5Aitusesby5hours,givingyou27.5Ah.

2)Weknowthatourloadislargerthanourcapacityrating(25Aislargerthanthe 10 Ah rating) so we already know this battery will last less than 1 hour.Dividingthe10Ahcapacitybythe25Aloadgivesus10Ah÷25A=0.4hours,or24minutes.

3)Firstlet’scalculatethetotalenergyofthepackinwatthours.48V×10Ah=480Wh.Ifweconnecta50Wloadfor2hours,thatisatotalof100Whthatwewill drain from the pack (50 W × 2 hours = 100 Wh). So after 2 hours ofsupplying50Wcontinuously,our48Vbattery isdownto380Wh(480Wh-100Wh=380Wh).Nowweconnectittoa20Wload.Dividingtheremaining380Whby20Wgivesus19hours(380Wh÷20W=19hours).

Ifyougotallthoseanswerscorrect,great!Ifyoumissedoneormore,makesure

yougooverthesolutionsuntilyouunderstandwhereyoumadeamistake.Themathisn’ttoohard,butit’sveryimportanttogoslowandmakesureyougetthisright.Ifyoumakeanerrorinplanningyourbattery,youcouldendupwithnotenough capacity to fulfill your needs, or toomuch capacity. (And that wouldgiveyouabatterythatislargerandmoreexpensivethanyouneeded!)

Onelastnote:rememberthatcellsoftendon’tprovidetheirentirecapacityevenwhen new, and that their capacity will decrease as they age. If your projectrequires 10Ah of capacity and you use cells that add up to exactly 10Ah, itwouldn’tbesurprising foryourbattery toonlyprovide9.7Ahwhennew,andcloserto9.0Ahafterafewhundredchargecycles.Soifyourprojectallowsforit,considerbuildingabatterythathasslightlymorecapacitythanyouneed.

Maximumcontinuouscurrent

When introducing capacity in the previous subsection, I mentioned that thecapacityishighlyrelatedtothemaximumcontinuousdischargecurrent.Thisisbecausethelargerthecapacityoftheassembledbattery,thelargerthemaximumcontinuous discharge current of the assembled battery will be. That is why IspentsomuchtimeinChapter5discussinghowtocalculateCrates.(Andthat’swhyIforcedyoutotakeaminitestonCratestoo!)

EvenifyouusetheexactsamebatterycellstobuildtwobatteriesofdifferentAhcapacities, the larger capacity battery will be capable of a higher maximumdischarge rating. A higher maximum discharge rating is the same thing as ahighercontinuouscurrentrating.

This is where C rates come into play. Let’s take a common 18650 cell forexample:thePanasonic/SanyoNCR18650GA.Thiscellhasacapacityof3.5Ahand is rated for10Amaximumcontinuousdischarge rate.Todetermine theCrateofthiscell,wedividethemaximumcontinuouscurrentthatitcansupplybythecapacityinamphours,givingus:

10A÷3.5Ah=2.86C

That means that not only is the cell rated for 2.86 C maximum continuous

discharge, but any battery pack built out of these cells will also be rated for2.86Cmaximumcontinuousdischarge,nomatterhowbig thebatterypack is.However,alargerbatterypackwillbeabletosupplyalargercurrent.

Let’stakealookattwopotentialbatterypacksbuiltfromthesecells.We’llmakea hypothetical 3s3p pack and a 3s6p pack.Now let’s compare them.The firstbatterypackwillbe11.1Vnominal(3cellsinseries×3.7Vnominalpercell)and10.5Ah(3cellsinparallel×3.5Ahpercell).Thesecondbatterypackwillalsobe11.1Vnominal,but itwillbe21Ah,or twice thecapacityof thefirstbatterypack.Bothofthesebatterypackswillbecapableofa2.86Cdischargerate.However, that2.86Cdischarge rateequalsdifferentabsolutecurrents forthetwodifferentbatterypacks.Thefirstbatterycansupply30Acontinuously,whilethesecondbatterycansupply60Acontinuously.

Howdowedeterminethemaximumcontinuousdischargecurrentthatthepackcan supply? There are two methods. The first method is to multiply themaximum continuous discharge current of the cells by the number of cells inparallel.Forthefirstbatterythatwouldgiveus:

10Amaximumcontinuousdischarge×3cellsinparallel=30Adischargerate

ThesecondmethodistomultiplytheCrateofthebatterypack(orofthecells,sinceit’sthesameforboth)bythecapacityofthebatterypackinamphours.Forthefirstbatterythatwouldgiveus:

2.86C×10.5Ah=30Adischargerate

It is important todesignyourbatteryso that it canprovideenoughcurrent foryourapplication.Ideallyyouwantittobeabletoprovidemorecurrentthanyourapplicationrequires.Inengineeringterms,thisisknownasafactorofsafety.

Factorofsafety=maximumallowableload÷actualload

If I’m trying to power a light bulb that draws 5 A continuously, the lowestmaximum continuous discharge rate my battery can safely have is 5 A. Thatwouldgivemejustenoughcurrenttopowerthe5Alightbulb.Ifmybatteryisrated for10Amaximumcontinuousdischarge insteadof5A, then I’llhaveafactorofsafetyof2,becausemybatterycanprovidetwicethecurrent that thelight bulb requires. If my battery is rated for 20 A maximum continuousdischarge,thenI’llhaveafactorofsafetyof4,becausemybatterycanprovide4timesthecurrentneededbytheload.

Generallyspeaking,youwant tohavesomeleveloffactorofsafety.Itdoesn’tneedtobeashighas2or4,butitdefinitelyneedstobehigherthan1.Afactorofsafetyof1meansthatyourbatteryisworkingatitsabsolutelimit,providingtheexactmaximumamountofpowerthatitisratedfor.Afactorofsafetyoflessthan1meansyourbatteryisn’tevenstrongenoughfortheloaditispowering.That’sobviouslyunacceptable.

Thereasonafactorofsafetyisagoodideaisbecausemostlithiumbatterycellssufferadecreaseinperformanceasyouapproachtheirmaximumratedvalues.The18650GAcellsweusedintheexampleaboveareratedat10A,buttheygetveryhotataconstant10Adischargeanddon’tprovidetheirfull3.5Ahatthatlevelofdischarge.Instead,theygivecloserto3.2Ahwhenpushedtotheirmax.Iliketokeepthecurrentin18650GAcellsbelow7A.Thefactorofsafetyfora10Acapablecelloperatingatjust7Acanbecalculatedas:

Factorofsafety=10A÷7A=1.4

MostmediumtolowpowerprojectscanbehandledbyLiFePO4andli-ioncells.Ifyourprojectrequiresareallyhighmaximumdischargecurrentthough,you’lleither need to placemany cells in parallel to supply enough current, or you’llneedtouseRClipocells.Remember,RClipocellsarethedangerousonesthatyou need to be extremely careful with. Sometimes though, they are the onlyoption.A prime example iswhen the highest levels of power are needed andweightorspaceisanissue.

Ifyouhaveaprojectthatneeds100Aandyouonlyhaveroomforafew3Ahcells,you’renotgoingtofindastandardli-ionoraLiFePO4cellthatwillwork.You’llhavetousesomehighpowerRClipocells.However,ifyouhadthespaceandweightallowances,youcouldjustparallelmanyli-ionorLiFePO4cellsandreach that 100 A discharge capacity. Assuming those cells were rated at 5 Amaximumcontinuousdischarge,you’dneed20cellsinparallel.Forcellswitha10Amaximumcontinuousdischargerating,you’donlyneed10cellsinparallel.

As you can see, there are nearly an infinite number of ways to design mostbattery packs for different needs.Voltage, capacity and current all affect eachotheraswellasthefinalspecificationsofabattery.Bymakingcompromisesorchanging the balance between those three parameters, a myriad of options ispossible.

ChoosingappropriatecellsIt’snearlyimpossibletochoosetherightcellsforyourprojectwithoutknowingyourfinalvoltage,capacityandcurrentneeds.Onceyouknowthose,youcangoaheadandfindtherightcellstofityourneeds.

Currentrating

The first aspect of choosing the right cell formost projectswill likely be themaximumcurrentrating.Ifyouhavea40Aload,thenyou’llneedenoughcellsinparalleltosupply40A.Ifyouknowthatyouonlyhaveroom(orthebudget)for4cellsinparallel,thenyoubettermakesureeachofthosecellscanprovideatleast10Acontinuously.

Using that same example with a 40 A load, if your space and weightrequirementsallowit,youcouldalsobuildan8pbatteryinsteadofa4pbattery.Inthatcase,yourcellswouldneedtobecapableofsupplyingatleast5Aeach.

Nowwhatifyouneedthat40Aloadtorunfor30minutes?It’sprettysimpletocalculate thecapacityyou’llneed. Just remember toconvertyourminutes intohoursoryou’llbecalculatingthenumberofampminutesthatyou’llneed.

Requiredcapacity=40A×0.5hours=20Ah

Yourbatteryshouldhaveacapacityofatleast20Ahtorunthat40Aloadforahalfhour.Thatmeansthathowevermanycellsyouuseinparallel,theyneedtoaddup20Ah.

Let’s say we’re using 3.5 Ah cells that can provide 10 A continuously. A 4pbatterywillgivesusthe40Acontinuousdischargeratethatweneed,butitwillonlygiveus14Ahofcapacity.That’snotenough to runour load for thehalfhour we used in the previous example. In fact, it will only give us about 21minutesofruntime.

Runtime=14Ah÷40A=0.35hoursor21minutes

Since our capacity doesn’t give us enough run time,we’ll needmore cells inparallel.A6pconfigurationwillgiveus21Ahifwe’reusingthosesame3.5Ahcells. That is above our requirement of 20Ah, perfect! It will also give us amaximumdischargecurrentof60A,whenweneedatleast40Aofcontinuouscurrent.Thatwillgiveusafactorofsafetyof1.5forthemaximumcurrentdraw,whichisgreat!

Nowwe’veworkedoutanumberofexamplesandIhopeyouseehowcapacityandmaximumcurrent draw are integrally relatedwhen it comes to building abatterypack and choosing cells.Capacity andmaximumdischarge current aretwoofthemostimportantfactors,butthereareafewotherfactorsthatmayberelevanttoyourproject.Thesecanalsoaffectwhichcellsyouchoose.

Cyclelife

Cyclelifevariesfromonetypeofcelltothenext.Somecellscanonlyperform200cycles.Otherscanlastformorethan2,000cycles.Dependingontheneedsofyourproject,cyclelifecouldbealimitingfactor.

If youneed thehighest cycle life foryourproject, you’reprobably stuckwithLiFePO4 cells.Theyare themostwidelyavailable cells that canperformover1,000chargecycles.

Ifyoucanreplacethebatterywheneveryouwant, thencyclelifemightnotbeveryimportantforyou.

WeightWeightisanotherissue.InChapter3wetalkedabouthowLiFePO4cellsaretheheaviest cells and li-ion cells are the lightest. Keep that in mind if weight isimportant.

Size

Just like weight, physical size can be a limiting factor for many projects. Tocramthemostenergyintothesmallestspace,theNCAchemistryofli-ioncellsarewhat you need.Remember though that they have somewhat lower power,whichisthetradeoffforbeingsoenergydense.

LiFePO4cellsarethelargestcellsyoucanuse.Ifyouhaveverylimitedspace,you’re unlikely to use LiFePO4 unless its benefits including safety and longcyclelifearecriticalforyourproject.

Safety

SpeakingofthebenefitsofLiFePO4,ifyourprojectrequiresthehighestlevelofsafety standards, LiFePO4 is a great option. As we learned, LiFePO4 is thesafestchemistryduetoitsincrediblyhightemperatureneededtocausethermalrunawayaswellasitsslowanddifficultcombustion.

Li-cobalt and RC lipo, on the other hand, are the most dangerous lithiumchemistriesavailable.Ifsafetyisaconcern,thoseareprobablynotthecellsyouwanttouse.Ifyou’restuckwiththemduetootherlimitations,makesuretouseaBMS to protect the cells during both charging and discharging.ABMScanmakethosecellsmuchsafer.Usedproperly, li-cobaltandRClipocanbequitesafe.Butincomparison,theyarestillmoredangerousthanotherchemistriesoflithiumbatteriesandaccidentswiththosecellsaregenerallymorecatastrophic.

Groupingcellsinabattery

Onceyouknowtherequiredvoltage,capacityandmaximumdischargecurrentneededbyyourbattery,andyou’vechosenyourcells,youcanbegindesigningthelayoutofyourbattery.Themethodyouusewilllargelydependsonthetypeofcellsyouchoose.

Ifyou’reusingprismaticorpouchcells,you’remorethanlikelygoingtouseastackedsetup.Thiswillmeanthatyou’replacingthecellsinrowsorstackswithalternative directions to align the positive terminals of each group with thenegativeterminalsofthenextgroup.

Ifyou’reusing1pparallelgroups,meaningjustonecell,thisisverysimple.Juststack or line up your cellswith alternating cathodes and anodes, then connecteachonetothenextinseries.

If you’re using multiple cells in parallel, you may want to first connect theparallelgroups.Afteryou’vestackedandconnectedtheparallelgroups,youcanstackthecombinedparallelgroupsandmakeyourseriesconnections.Theorder(parallelgroupsfirstversusallconnectionsatonce)will largelydependon thetypeofconnectionsyou’remakingandtheshapeofthebattery.Sometimesitiseasiertoworkwithparallelconnectionsfirst,butothertimesitdoesn’tmakeadifference.With busbars on linear battery cells, it might just be easier to laydownthebusbarsatonce,assumingtheyarelongenoughtodotheparallelandseriesconnectionsatonce.

It’s hard to get too creative with pouch cells or prismatic cells. Their shapessimplydon’tallowformanyvariationsinpacking.That’swhythetypicalstacksorrowsmethodisalmostalwaysusedwithpouchandprismaticcells.

Cylindrical cells will give you much more freedom and creativity in batteryshapeandlayout.Withcylindricalcells,there’salmostnolimittothewaysyoucanbuild a pack.As long as you canhold the cells together using anyof themethodswediscussedinChapter9,youcanbuildapackinthatshape.

Withcylindricalcells,therearetwomainstackingorientationsthatcanbeused,linear packing and offset packing. Linear packing is less efficient because itleaveslargervoidsinthecenterofthebatterypack.Offsetpackingisthemostefficientmethod.

Linearpackingiscommonwhenusingsnap-togetherbatteryholderblocks.Mostof theseblocksareonlydesigned to snap togetherusing linearpacking.Somemolded plastic battery holders use offset packing, but they are generally lessmodular and come inpre-made sizes.Thisgivesyou less freedom than singlesnap-togetherblocks.

Offsetpacking ismorecommonwhenusing thehotgluemethod toholdcellstogether.Notonlydoesittakeadvantageofthespacesavingsofhotglueversusrigidcellholdersbutitalsogivestwopointsofcontactoneverycellonwhichthehotgluecanaffixitself.

Regardlessof themethodofalignmentyouuseinyourcylindricalcellbattery,you’llwanttopaycloseattentiontowhereyou’llmakeyourseriesandparallelconnections.RememberthatinChapter9wediscussedhowmuchcurrenteachnickel strip can carry. Whatever conductor you’re using to make your seriesconnections, if it can’tcarry theentirecurrent that thebatterycansupply thenyou’llneedtousemultipleseriesconnections.

For example, look at this diagram of joining two 4p parallel groups that areconnectedinseries.

Ifweusea singlepieceof0.15mm thickand8mmwidenickel strip to jointhese in series at one point, we can only draw approximately 5 A from thisbattery.However,ifweusedfourstripsofnickel,wecoulddraw20Afromthebattery(assumingthecellsareratedforatleast5Aeach).

Ifweputalloftheseeightcellsinastraightline,we’dneedtostackournickelstripsupon topof eachother.Wecoulduse fournickel strips longenough tospan all eight cells, but that would be a bit of a waste. The current flowingthroughthenickelbetweenthefirstandlastcellsintheeightcelllinedon’tseeasmuchcurrentasthenickelinthemiddleoftheeightcells,whichcarriesthefull20A.Thatmeanswecanactuallysaveonmaterialbybuildingapyramidoffournickelstrips thatareeachprogressivelyshorterbyonecellwidthoneachside.

Thatpyramidstructureistheproperwaytojoincellsinserieswithnickelstripwhen the cellsmustbe in a single straight line.Butwhat if the cells in seriesaren’t required to be a single straight line? In that casewe can be evenmoreefficient. Ifwe align the same cells in two parallel straight lines of four cellseach,wecanuseasinglelayerofnickelconnectedbetweeneachparallelgroupofcellsinthe4pgroups.

Now we’re creating four individual series connections instead of four seriesconnectionsthatareallstackedontopofeachother.Thisstillgivesusa20A

dischargecapabilitybecause thecurrenthas fourpiecesofnickel strip to flowthrough,witheachpieceofnickelcapableofsupporting5A.Thismethodusesless nickel and also requires fewer welds at each location than the pyramidstackingmethod.

Whenpossible,it’salwaysbettertoalignseriesconnectionssothattherecanbeas many individual cell-to-cell connections as possible. This is better thanstackingmultiple nickel strips on topof eachother tobeef up the connection.Bothmethodsarevalidandwillproducethesameequivalentelectriccircuit,butincreasingthecell-to-cellconnectionsissimplybetterform.

Let’slookatanexampleofan8s4ppack.Wecouldbuildasimplerectangleofcellsthatiseightcellslongandfourcellswide.Thiswouldgiveustwooptionsfor designing our connections. The first option would be with two parallelgroupsperrow.Thesecondoptionwouldbewithoneparallelgrouppercolumn.Bothwouldgiveus the samecircuit, but the secondoptionwouldallowus tomakemoreefficientseriesconnections.

This same concept also applies when producing other geometric or abstractshapes. You always want to maximize the number of cell-to-cell seriesconnectionswhenpossible.

Themorecellsyouhaveinyourbattery,themorecomplicateditwillbetoplanthe connections. One of the most confusing parts of planning the inter-cellconnectionswhenusingcylindricalcellsisthattheconnectionsaredifferentoneachsideof thebattery.Andit’s important that they’redifferent - ifyoumakethesameconnectionbetweentwocellsonbothsidesofapackthenyou’vejust

shortedthecellstoeachother!

One trick I often use is to drawmybattery cells out on a thin piece of paperusing a heavypencil ormarker.Thismakes it easy to seemarksmadeon theothersideofthepaper,especiallywhenhelduptothelight.IfIdrawbothsidesofmybatteryonopposites sidesof thepaper so that theyalign, I can flip thepaperbackand forthas if I’mphysically flippingmybatteryover.Then Icanholdthepaperupand“see”theconnectionsontheothersideofthebattery.IfIhaveconnectionsbetweenthesametwocellsonbothsidesofthepaper,IknowI’veroyallymessedup.Thatwouldbeadisastrousshortcircuit.

Usingpaper todesignyourbatteryhelpsyouavoid theseconnectionmistakes.You can also layout your battery using a computer program. I generally use aPhotoshopknockoff,buteventhesimplestdrawingprogramswillwork.Youcandrawjustonesideofyourbatteryandusetwodifferentcolorstorepresent theconnections onopposite sides of the battery.Callmeold fashioned, but I stillprefertoflipmypaperovertodraweachside.

Ifyoureallywanttouseacomputertodesignyourcelllayoutbutstillliketheidea of the physical paper diagram (it really helps during the actualwelding/connecting phase!) then you can always design both sides on thecomputer,print themoutandtapethemtogetherback-to-back.Thenwhenyouaremakingyourconnections,youcaneasilyflipthepaperovertoeachsideandseearepresentationofhowyourbatteryshouldlook.It’sa littlemoreartsandcraftsy,butitgetsthejobdone.

Makingdifferentbatteryshapes

Nomatterwhat type of cell you’re using, the easiest shape of battery pack tobuild is a rectangle. Easiest by far. That doesn’tmean rectangles are the onlyshapeyoucanworkwith,butwe’llatleaststartfromthere.

Asmentionedpreviously,pouchcellsandprismaticcellsareprettydifficult tobuild abstract shapeswith, but they lend themselves reallywell to rectangularshapes. And fortunately for us, rectangular shapes work quite well in manyapplications.Everythingfromelectricskateboardstohomeenergybatteriesandelectricvehiclebatteriesusuallycomeinrectangularprisms.

Why? Because it’s easy, mostly.Why go to the trouble of designing a fancyshaped home battery when a square easily mounts in a closet or on a wall.Homesarebuiltwith90degreeangles,andsoaretheboxesthatwe’llprobablyusetoencloseourbatteries.

Same things with cars. Things are usually relatively flat and angular in mostareas of the trunk and undercarriage. Why? It’s cheaper and simpler. Fancycurvesbelongontheoutsidewherepeopleseethem.Storageplaces-theexactplacesyou’regoingtostickyourbatteries-thoseareusuallyflatwithcorners.Samethingwithelectricskateboards.They’vegotaflatdeck,sowhywouldn’ttheywantaflatbattery?

The fancy shapes usually come about when you’re trying to put batteriessomewheretheyweren’tnormallyintendedtogo,oryou’retryingtomaximizeavailablespace.Orboth.

Electricbicyclesareagreat example.Triangle shapesareoftenused inebikesbecausetheyfitintothetriangularshapeofthefrontofastandardrigidbicycleframe. Bicycles with suspension have even weirder shaped frames and oftenrequireevenweirdershapedbatteries.

If you’re trying to conceal batteries inside odd shaped enclosures, then you’ll

alsorunintoweirdshapesthatcanbeabitmoredifficulttoconstruct.

In these cases, if you already have the physical thing that will contain yourbattery(likeabicycleframeoranelectronicsenclosure,thenthebestplanistomakeatemplate.Measureyourshapeandthendrawittoscaleina2Dcomputerprogram like Paint or Photoshop. Now you can play around with all sorts ofinterestingshapesofcells.Whenyouwanttotestyourdesign,simplyprintyourdrawing,cutitoutwithscissorsandtestfititintoyourdevice.

Ioncebuiltanelectricbicyclebatteryforacustomer’soddshapedbicycleframeusing this exact method. From halfway around the world he emailed me themeasurementsofhisframe.Ithendrewupatemplateforthecellsandemaileditbacktohim.Hetest-fitthepapertemplateintohisframetoensureitwouldfit.Ithenbuiltthebatterytofitthattemplateandshippedit7,000milestohim.Itfitinthebikeframelikeaglove.

Once you have your shape all planned out, the hard part isn’t quite over.Extremelyoddshapedbatteriescanbedifficulttoconnect,especiallyiftherearelargegapsorvoidsinthebattery.Ifyou’reusingpouchorprismaticcells,youcanuseflexiblewirestospanlonggapsbetweencells.Justmakesuretouseathick enough wire gauge to account for the amount of current that will beflowingoverthedistance.Ifyou’reusingspotweldedcells,spanninggapscanbetrickier,butit’sstillpossible.

For really abstract shapes, a spot welder with handheld welding probes willmake the battery construction much easier. The stubby electrodes on most

hobby-levelwelderscanbeverylimiting.Mostonlyreachabouttwocylindricalcellsdeep.Bybending thecopper electrodesout and tilting thebatteryduringwelding,youmightbeabletogetasdeepasfourcells.Butifyouuseawelderwithhandheldprobes,youcanreachanywhereinthebatteryandnotbelimitedbythereachofthestubbyarmsonthefrontofthewelder.

Remember what we talked about in the last section regarding making suresufficient current can flow between series-connected cells? In abstract shapedbatteries, you’ll need to be extra careful that you’re using enough layers ofnickelstrip tocarrysufficientcurrentbetweengroupswelded inseries.This iseasier in rectangular batteries because series connections can often be madebetween every cell in a group. If you’ve only got enough room to fit a singleconnection between two cells in two adjacent parallel groups, you’ll have tostackenoughnickeltohaveasufficientcurrentcapacityinthatconnection.

Chapter11:MakingbatteryconnectionsHerewego,thetimehascomeandwe’rereadytobuildanactualbattery!Butbeforewe get too carried away,we need to get a few things ready. First andforemost,weneedtoensurethatwe’vepreparedasafeworkingenvironment.

Safetyconsiderations

We discussed the importance of safety in general whenworkingwith lithiumbatteriesback inChapter7, so Iwon’tharpon it too longhere. It’s importantenoughforaquickreviewthough.Theseare themainsafety tips torememberwhenyou’rereadytoactuallystartbuildingabattery.

Always remove jewelry andweargloves.Awedding ringmakes agreat shortcircuit,asdosweatypalms.

Cleantheworkareathoroughlybeforelayingoutyourbatteries.Ifpossible,layoutsomelargesheetsofpaperonyourworksurface.They’llcoveranyexistingmetal debris youmissed andmake it easier to see new bits and pieces layingaround.Avoidnewspaperas theblackandwhiteprintcanmake ithard to seesmallobjects. I likewhitebutcherpaper.Evenafewsheetsofcomputerpapercanworkwelltoo-justtapethemtogetherattheedgessoyoudon’tlosethingsunderthem.

Wear safety glasses if you’ll be spot welding or soldering. Spot welders canthrow sparks. Soldering on anything springy like a wire or nickel strips canaccidentally flingmolten solderaround if it springsbackwhile soldering. Inacompetitionbetweenyoureyeballsandsparksormoltenmetal,youreyes loseevery time. You’ve only got two of them. And if you value your depthperception,havingonlytwoeyesleavesnoredundancy.

Makesureyoudon’thavedirectairflowonyourbattery.Ifyou’resoldering,youprobably want a fan to help remove the fumes, but point it up above whereyou’reworking.Thelastthingyouwantisforanythingtocatchinthemovingair and knock some metal onto your exposed battery contacts. It sounds farfetched,butyou’llprobablyhaveapileofwiresornickelstripslayingoutonthebigsheetofpaperIjusttoldyoutouse.Fan,meetsail.

Lastly, use common sense about everything you do.Double check before youmake a connection to ensure that it is the correct connection to make. Don’tjugglemetaltoolsoveryourexposedbattery.Don’tleaveyourexposedbatterysittingaround inahighly traffickedarea.On thatnote, ifyoustop for thedaybeforeyoufinishabatterybuild,coveritwithaplasticbagorsheetsofpapertoensure nothing falls on your battery and shorts it. When I have to leave apartiallycompletedbatterylayingoutunattended,Iplaceabig“danger”signonittomakesurenoonemesseswithit.

Basically,useyourheadwhen itcomes tosafetyduringyourpreparationsandyoushouldbefine.

Onesafetynotespecificallyaboutcylindricalcells-Mostcylindricalcellssuchas18650cellsareconstructedwiththeentiretyofthebottomandthesidesofthecellconnectedtothenegativeterminalofthebattery.Thesidesoftheshellevenwraparoundthetop,meaningthenegativeandpositiveterminalsofthecellareseparatedbyjustafewmillimetersatthetopofthecell.

The cells have heat shrink around their outer shell to protect the cells fromshorting against each other. If that heat shrinkwas ever damaged, such as byconstantvibrationorchafing,therecouldbethepotentialforashortcircuit.Thisismostplausibleonthetopofthecellwhereconnectionsbetweenthecellcouldrub against the heat shrink. This isn’t particularly common, but cases wherenickel stripwelded to the top of the cell has vibrated and subsequentlywornaway the heat shrink have been observed. Such cases can often lead tocatastrophicfailurebyshortingonecell,leadingtothermalrunawayoftheentirepack.

Mostreputablecellmanufacturersincludeaninsulatingpaperorplasticdiskorwasher under the heat shrink on the top of the cell to add an extra layer ofprotectionagainstshortcircuitsoccurringonthetopofthecell.Thisisusuallyawhite circle that can be seen at the top of the cell, just under the heat shrink.However,cheapcellsoftenlackthissecondlayerofprotection.

Insulating paper washersmade from sticker paper are commercially availableandcansolvethisproblem.Theyaresometimesreferredtoas“fishpaper”.Thepapergasketscanbeappliedonthetopofacylindricalcelltoprovideanextralayerofshortcircuitprotectionandwon’tinterferewithconnections.Forgoodquality cells that already come with an extra insulating gasket, adding thisinsulating stickerwould be a second redundancy against shorts, but not a badidea. For cellswithout an insulating gasket included by themanufacturer, thisinsulatingstickerwouldbeagoodsecondlineofdefense.

Onelastthingtomention.Youprobablyalreadydidthiswhenyoucheckedyourcells,butyou’llwant tomakesureallcellsareat thesamevoltagebeforeyou

beginyourconnections.It’smoreimportantforparallelcells,astheinstantyoumaketheparallelconnection,cellsofdifferentvoltageswilltrytobalanceeachother.Ifthevoltagedifferenceislarge,ahugeamountofcurrentwillflowveryquicklythroughyourparallelconnections.

This is also important for series connected cells because any large voltageimbalance will have to get balanced eventually by either a BMS or balancecharger. The larger the initial voltage imbalance, the longer itwill take to getyourcellsbalancedduringtheirfirstcharge.

Cellmatching

Aspreviouslymentioned,whenyoucombineyourcellsintolargerbatterypacks,youwant tomake sure thatyourcells allhave the samevoltageand the samecapacity.Ifyouareusingnewcells,thisiseasyastestingthemwithavoltmetertoensuretheyareatthesamevoltage.Ifyou’reusingrecycledorsalvagedcells,thisisamuchmorecomplicatedprocess.

Salvagedcellsarenotonlygoingtobeatdifferentchargelevels,butthey’llalsohavedifferentcapacities.Justbecausetwocellsarebothat3.5Vdoesn’tmeantheyhavethesamecapacity.Oneofthemcouldhavebeena2.5Ahcellandtheothera3.5Ahcell.Ormaybetheywerebothoriginally3.0Ahcells,butonehasbeenusedformanymorecyclesandisnowdownto2.8Ah.

When you’re using salvaged battery cells, you’ll want to test each cellindividually todetermine itscapacity.Therearemanydifferent lithiumbatterycelltestersoutthere.Mostarefor18650’sbuttherearesomebareboardtestersthatcanbeusedonmostlithiumcells.

Aftertestingeachcell,itishelpfultowritethecapacityoneachonewithafelttipmarkersoyoudon’tmixthemup.You’llwanttomakesurethatyougrouptogetherlike-capacitycellswhenyoucreateparallelgroups.

For example, let’s say you aremaking a 7.4V 20Ah battery pack.Youmighthaveatotalof18salvagedcells,witheightcellsthatareallaround2.5Ahandten cells that are all around2Ah. In this case, insteadofputtingnice cells ineachparallelgroup,itwouldbebettertogrouptheeight2.5Ahcellstogetherinone group and the ten 2Ah cells together in the other group. Thatway eachparallelgroupisapproximately20Ah.Justrememberthatyoursmallestgroup(bynumberofcells)willdeterminehowmuchcurrentyoucandraw from thebattery. If you want to limit your cells to 2 A each, then you could draw amaximum of 16A from this battery because your smallest parallel group hasonly8cells.

Youcanseehowthiscangetcomplicatedasthenumberofbatteriesincreases.That’sanothergreatreasonwhyit’sbettertostartwithbrandnewcellsinsteadof usingmystery cells or salvaged cells.With brand new cells youwill neverhavetoplaythismatchinggame.Allyouhavetodoisdoublecheckthatallthevoltagesare the sameand then startbuildinggroupswith the samenumberofcellsineachgroup.

Cellalignmentandcontainment

Before you can begin to connect your cells, you need to have them alreadyalignedandcontainedinwhichevermethodbestsuitsyourproject.WediscussedmanydifferenttypesofcellholdersandenclosuresinChapter9.

Once you begin connecting your battery cells, the amount of potential energygrowsquickly.The last thingyouwant tobedoing isbemovingaround largeconnected cell groupsunnecessarily.Alwaysorganize and layout your cells intheproperorientationbeforeyoubeginmakingyourelectricalconnections.

Once you’ve begun your connections, you’llwant tominimize the amount ofbatterymovement. This is for both safety and quality reasons. Themore youmoveyourbattery, themorechancesyouhave for accidents.Additionally, themorerigidyourconnectionsare,themorelikelytheyaretofatigueorloosenasyourmoveyourbatteryaroundandstressthoseconnections.

Occasionally youwon’t be able to lay out your battery completely before theassemblyprocessbegins.Thisisoftentrueforverylargebatteriesthatmustbefirstbuiltintosubmodulesandthenmodulesthatareultimatelycombinedintoasinglelargebatterypack.Othertimesthebatterywillnothaveaselfcontainingenclosure, such as drone batteries that are simply enclosed in a layer of heatshrink wrap after the connections are made. In these cases, use your bestjudgment to ensure that your battery is handled safely during the constructionprocess.

Boltedconnections

Boltedconnectionsaregenerallytheeasiestmethodtojoinyourcells together,but pretty much limit you to prismatic cells or Headway’s unique cylindricalcells.

Forboltedconnections,youcanuseavarietyofconductivematerialstobridgethebatteryterminalsandmakeyourconnections.Thetwomostcommonoptionsarebusbarsandflexiblewire.

Busbarsareusuallymadeoutofeitheraluminumorcopper.Bothmaterialshavesimilarresistancesandworkwellforelectricalconnections.Copperisaslightlybetterconductor,butit’salsomoreexpensiveandcorrodesmoreeasily.

It’s best to try and match the thermal expansion of whatever material you’reusingascloselyaspossible.As theconnectionsheatup,dissimilarmetalswillexpand at different rateswhich can loosen the connections overmany heatingandcoolingcycles.Lockwashers,especiallysplitwashers,canhelppreventthislooseningphenomenon.

For wire connections, you’ll probably end up crimping or soldering a ringconnectorontotheendofyourwires.Youcansimplywrapthewirearoundtheboltorthreadedaxleandclampitbetweenwashersaswell,butthismethodhas

afewdisadvantages.Itwilllikelyresultinonlypartofthewiremakingcontactwith the cell terminal and will reduce the amount of current you can carrythroughtheconnection.Ifyourbatterywillexperiencevibrationormovement,thismethodcanalsoresultinthewireslowlyworkingitselffree.

Ringconnectorsareabetterwaytomountwirestothreadedaxlesorbolts.Itisbesttocrimpthemasopposedtosoldering,asthesoldercanweakenthewire.Thisismoreimportantforbatteriesthatwillexperiencemotionandvibration.Ifyourbatterywillbestationary,solderingyourconnectorsisprobablyfine.

If you’re usingwire tomake the connections between cells, be sure to use asufficiently large gauge wire to carry the current you need. The appropriategauge wire is different for every project, and you’ll need to determine yourspecificrequirements.Differentwirematerials,whethercopperoraluminum,arerated fordifferent levelsof current atdifferent thicknesses andunderdifferentatmospheric conditions (i.e. in a vacuum, in ambient air or in flowing air).Consulttheappropriatechartsforyourspecificwiretypetocalculatetheproperwiresizeforyourproject.Ahelpfultableonchoosingwiresizescanbefoundathttp://www.engineeringtoolbox.com/wire-gauges-d_419.html.

This is of course also true for the wires that will handle the charging anddischarging of your battery. Any wire that will carry the full current of yourbatteryshouldbeappropriatelysizedbasedontheconditionsinwhichitwillbeused.Whenindoubt(andwhenyourbudgetandweightrestrictionspermit it),usea largergaugewire.Your resistancewilldecreaseandyourefficiencywillincrease.

Becarefulwhentighteningboltsornutsontheterminalsofyourbatterycells.Too much force can easily strip the threads. Check with the manufacturer orvendorof thecells toseehowmuch torquecanbeapplied to the terminals. Ifthisisasmallbattery,atorquewrenchmightbeoverkill.Ifthisisamuchlargerbattery,a torquewrench isagood idea toensureyouputenough forceon theterminalconnectionswithoutover-torqueing them. It shouldgowithout sayingthat if thebattery is foravehicle, suchasacaroraircraft, thisstepshouldbecrucial.Alossofpowerfromalooseconnectioncanhavefatalconsequencesinsuchcircumstances.

Atorquewrenchisafairlyprecisetool.Ifyouwereassemblingjetengines,I’drecommend using a pretty darn good one. For tightening connections on abattery’sscrewterminal,acheap“eBayspecial”shippeddirectlyfromChinaisjustfineformostprojects.A$20torquewrenchmightsaveyoufromstrippingouttheterminalona$500electricvehiclebatterycell.Thinkaboutit.

Theadvantageofwireconnectionsoverbusbarsorspotweldedconnections isthatyouretainsomeflexibilityandvibrationresistance inyourconnections. Ifyourbatterywillseeanylevelofvibration,youshouldalwaysusestrandedwire.Themorestrands thebetter.Higherstrandcounts result inmore flexiblewiresthatwithstandincreasedmotionandvibrationwithoutfailing.

When possible, it is often helpful to begin with parallel connections beforeperforming series connections. This helps simplify the battery constructionprocess and reduces the chance for careless mistakes that can have largeconsequences. Ifyoucan,avoidgoingbackandforthbetweenmakingparalleland seriesconnections. Instead, try toconnectallofyourparallelgroups first.

Then when you have all of your parallel modules assembled and connected,moveontoconnectingthosegroupsinseries.

Dependingonthedesignofyourbatteryandtheshapeofitsenclosure,itmightnot always be possible to construct all parallel groups first. “Parallel groupsfirst”isnotahardandfastrule.It’sonlyasuggestionthatcanoftensimplifythebuildingprocessandhelpavoidmistakes.

Wireconnectorconnections

Some batteries, mostly RC lipo batteries, will come prepackaged withcharge/discharge wires and connectors already installed. This makes batterybuildingmucheasier.

Instead of having to directly connect the individual cells of each battery, yousimply need to use wire and matching connectors to join together individualpacks.Thenumberofbatterieswiredinseriesandparallelwillofcoursedependon the requirements of your project. RC lipo batteries arewidely available inbatterypacksup to approximately30Vand8Ah.Highervoltageor capacitypacksalsoexist,butaren’t ascommon.AnRC lipobrandknownasMultistarproduceshighercapacitybatterypacksofupto20Ah.

Formostsmallerbatteriesunderapproximately8Ah,youwon’thave tomakeany parallel connections at all. A simple chain of series connections will beenoughtobuildabatteryforyoursystem.Forexample,ifyouneeda63Vand6Ahbattery,youcouldconnecttwo6sandone5sRClipopacks,eachof6Ah,togetherinseries.Thatwouldcreateasingle6Ahbatterywith17seriescellsforanominalvoltageof62.9V.

To create this battery, you’d only need to add wire connections between thedischarge wires of each pack. They could either be plugged directly to eachother, or you could fashion a new wiring harness to create cleaner lookingwiring.

Remember of course that RC lipo batteries must be balance charged at leastsemi-regularlyifyouarenotincludingaBMS.Theyalsorequirevoltagealarmsorothervoltagemeterdevicestoensurethattheyaren’tdischargedbelowasafevoltage. If you don’t require the super high current available from RC lipobatteriesthough,youshouldconsiderinstallingaBMStotakeadvantageoftheBMS’scharginganddischargingprotection.ThebalancewiresonRClipopackscanbeconnecteddirectly toaBMS,whichwe’lldescribenear theendof thischapter.

Spotweldedconnections

Dependingonyourcells,spotweldingmaybetheonlyoptionavailabletoyou.This includes many cylindrical cells like 18650s. Spot welding has manyadvantagesoverotherconnectionsmethodsincludingitspermanenceandrobustelectricalconnections.Goodspotweldsshouldneverloosenup,unlikewiresorbusbarswhichcanloosenovertimeundercertaincircumstances.

As previously discussed, there are twomain types of spotwelding electrodes:flexible handheld electrodes and rigidmounted electrodes. The first type willgive youmore freedom in the build process,while the second type can oftenresult in more uniform welds as they tend to apply the same spring loadedpressuretoeachweld.Bothworkwell,andthechoicewilloftenbedecidedbythesizeorshapeofthebatteryyouarebuilding.Squarepackslendthemselvesnicely to rigid mounted electrode arms, while odd shaped packs sometimesrequireflexiblehandheldelectrodes.

Themajordifferenceintheprocessbetweenusingthetwodifferenttypesofspotwelding electrodes is the order of operations. Depending on the shape of thebattery,performingallparallelgroupweldsfirst issometimesmoredifficultorimpossiblewhenusingspotwelderswithstubbyelectrodearms.Thisisbecausetheylimitthereachoftheelectrodesandtheparallelgroupsmaybetoothicktoallowtheseriesconnectionstobemade.Insuchacase,thebatterymustbebuiltrow by row, performing both series and parallel connections together. This isperfectlyacceptable,butmorecaremustbetakentonotgetconfusedandmakeanincorrectconnection.

Anotherbigadvantageofhandheldweldingprobesisthatthattheyallowyoutonot only glue or otherwise arrange your battery entirely before you beginwelding,but theyalsoallowyou to leave itonyourworksurface.Thestubbyarmelectrodeweldersrequireyoutophysicallyliftyourbatteryuptothewelderandthenraisethearmsupbyliftingtheentirebatterytoengagethewelder.Thearmshavea switch that fire thecurrentpulseswhen thearmsare lifted to thecorrectheight.

Ifyou’rebuildingasmallbattery,liftingitrepeatedlycanbemanageable.Whenyou’re working with hundreds of cells in a single battery, it can becomeunwieldy.Atacertainpoint,youendupmanipulatingabatterythatisbiggerandheavier than thewelder itself, whichwould be almost comical if it wasn’t sotiringandfrustrating.

Insomecheaperhobby-stylewelders,itcanbehelpfultoaddsomethingheavyontopofthewelder.Thespringtensionofthearmsisoftenadjustable,butitcansometimes be close to the weight of the welder itself. That means the entirewelder can occasionally lift upwhen you use the battery to raise theweldingarms.Afewadditionalpoundsorkilogramsontopofthewelderhelpstosteadyit.

Mostwelderswithstubbyarmelectrodescanworkwitheitherafootpedalorbyraising theelectrodearms.Personally, Iprefer to raise theelectrodearms. I’malreadyliftingthebatteryuptothearmstobeginwith,soIliketousethesamemotiontocontrolthetimingoftheweldingpulses.Usingthearmstoengagetheweldingpulsesalsoensuresanequalpressureateachweld.Otherpeopleprefertousethefootpedalswitchtoengagethepulse.It’smoreofacomfortthing,andyoushouldtryoutbothoptionstoseewhatfeelsbestforyou.

Forhandheldweldingprobes, thefootpedal isusually theonlyoption, thoughsomeprobescomewithabuttonmountedontheprobesthemselves.Thebuttonis oftenmore convenient.We’re notmonkeys,we don’t have the samemotorskillswithourfeetaswedowithourhands.

Ifyouhavemanycellsthatarebeingweldinginparallelgroupsinstraightlines,acellholding jigcanbequitehandy, especially ifyouaren’tusing theplasticcellholders.Thisisgreatwaytoholdyourcellswhileyouweldparallelgroups,

andthenyoucanhotgluetheparallelgroupstogethertoweldtheminseries.

Acellholdingjigusuallyhasaseriesofhalfcylindricalcutoutsandaspacetolayanickelstripontopofthecells.MinecamewithmagnetsthathelptoholdthecellsinplacewhileI’mweldingthem.TheyareavailableonlinefromsiteslikeeBayandAliExpress,andmanywelderswillcomebundledwithaccessoriesincludingajig.

Ifyoucan’tfindajig,youcanalwaysmakeoneyourselfwithalittleingenuity.Drillsomeappropriatelysizedholesinapieceofwoodorplasticblock,thencutitinhalf.Youcanevendrillandcountersinksomelittleneodymiummagnetsinthereforanevenmoreeffectivejig.

Whenspotweldinglithiumcells,thereisanoptimumnumberofwelds.Toofewwillresultinincreasedresistancewhiletoomanycanintroduceunnecessaryheattothecells.Forcylindricalcellslike18650s,Igenerallyuse6-8weldingpoints,which is 3-4 welds when using two side-by-side welding probes. This hasproventobeanacceptablenumberofwelds.

Somedevicessuchascomputersandotherelectronicswilloftenemployonlyasinglesetofweldsintheirbatterypacks.However,thesedevicesgenerallyuseverylowpower.Ifyourdeviceisdesignedforlowpower,suchaslessthan1Aper cell, then a single weld may be enough. For devices that require higherpower, an additionalweld or two can give you two to three times the currentcarryingcapacity.That’simportantforhighpowerdevices.

Whenyouperformmultipleweldsonasinglecell,itsbesttogivethecellafewsecondstorestbetweeneachweld.Asingleweldcreatesverylittleheatonthecell.Youcanusuallytouchthepointimmediatelyaftertheweldanditwillfeelslightly warm. If you perform three or four welds in the span of a coupleseconds,you’llnotice thepointbecomesmuchhotter.This isbecause theheatbuildsupfasterthanitcandissipateintotheair.

When possible,wait at least a couple seconds before repeating aweld on thesamecell.Thiscanbeaseasyasmovingon to thenextcell in the linebeforecomingbackandgivinganycell “seconds”or“thirds”, so to speak.Thatwayeachcellonlygetsoneweldatatimeandtheyallcoolveryquicklywhileyoumoveonthenextcelldowntheline.

Wediscussedtheimportanceofdesigningyournickelstriplayoutsothatthereisenoughcurrentcarryingcapacityineveryseriesconnection.This iswherethattheory plays out in reality. Ideally you’ll have multiple connections betweeneachcell.Ifnot,andyouhadtogowiththestackingpyramidmethod,makesurethatyou’redoingthesestacksoneatatime.

Start with the longest piece first, which will form the base of the pyramid.Perform all of yourwelds on that base piece, then add the next shorter piece.Weldthatpiececompletelyandcontinueaddingprogressivelyshorterpiecesinthiswayuntilyouhavecompletedyourweldingpyramid.

One last little quick tip for spotweldingwithnickel strips.You alreadyknowyou should bewearing gloves.Consider carefully the type of gloves you use.Fabricgloveslikemechanics’glovescanoftencatchthesharpedgeofapieceofnickel,especiallywhenthenickeliscutwithapairofscissorsthatcancurltheedgeabit.Whenyoulaydownapieceofnickelonyourbattery,itcanbeeasytocatchtheedgeonyourgloveandaccidentallydragitacrossyourbattery.Thisisagreatwaytocreateashortcircuit.IprefertouselatexornitrilegloveswhenIdobatterywork.Theyarelesslikelytocatchonthenickelandtheygivemebetterdexterity.Theycanmakeyourhandsget sweaty though, soconsider allyouroptions.

Seriesvsparallelconnections

Both series and parallel connections are important, but they don’t necessarilyneed to be treated the same. In fact, they don’t even need the same style ofconnections.

Seriesconnectionsarewhereall thecurrent flows,so theyare inaway“morecritical”thanparallelconnections.Bothtypesofconnectionsareimportant,butseriesconnectionsmustbedesignedtocarrythehighestamountcurrentthatthebattery will see. Every series connection must be as strong (or preferablystronger) than theminimumrequirement tocarry thecurrent that thepackwillsupply.Justliketheweakestlinkinachaindeterminesthechain’sstrength,thelowestcurrentcarryingcapacityseriesconnectiondecideshowmuchcurrentcanflowthroughthebatterywithoutoverheating.

In circuits,Kirchhoff’s current law states that the current at every point in anelectrical loop is equal. That means the current at every series connection isequal. If one series connection is narrower, weaker, or uses less conductivematerial,itwillhaveahigherresistancethantherestoftheconnections.That’sthepointthatwillheatupfirstinthebatteryandbegintolimititspotential.

For example, imagine an 8s1p battery (shown in the diagram) made fromcylindrical cells,whichwouldbe eight cells connected in series. If every spotweldwasmadewithtwolayersof8mmwide0.15mmthicknickel,theseriesconnectionswouldbeabletosupportaround10Aofcurrentcontinuously.Butwhat if one connection used only a single layer of nickelwhile the others all

usedtwolayers?Thatseriesconnectionwouldonlybeabletosupportaround5A of current. If a 10 A load was connected to the battery, all the seriesconnectionswould experience the same 10A of current, but that single layerconnectionwouldheatupquicklyduetoitshigherresistance.

Thatwasabitofanextremeexample,whereoneseriesconnectionwashalfasstrongastheothers.Aslongasallofyourseriesconnectionsarestrongenough,itdoesn’tmatterifonehasabithigherresistancethantheothers.Butiftheyallhavefourlayersofnickelexceptforasingleconnectionthatyouforgotandonlygaveone layer of nickel, your batterymight aswell have beenbuiltwith onelayerofnickeleverywhere.

Parallel connections, on the other hand, will see much lower current. Likelyhundreds of times lower current.Why? Because all of the current is flowing“downstream”,sotospeak,throughtheseriesconnections.Theonlywaycurrentwillflow“acrossstream”,whichwouldbebetweenindividualcellsinthesameparallelgroup,wouldbeifonecellbecomesslightlyunbalanced.

During discharge, the cells in a parallel group will be working more or lessequally hard. That is to say, they’ll be providing the same current. But smalldifferencesintheirinternalresistancesmightmeanthatonecellprovidesatinybitmore current than the others.During discharge, a small amount of currentwill therefore automatically flow from the stronger cells to the weaker cellsinsideofasingleparallelgroup.Thisislikelyontheorderofmilliamps.

Think of it like three really strong men and one medium strength man allcarryingarefrigerator.They’llallbesupportingtheircorner,butthestrongguysmightneedtoliftjustalittlemoreonthesmallerguy’ssidetohelphimliftitthesameamount.

Sobecausethereisonlyaverysmallamountofcurrentflowingthroughparallelconnections(i.e.betweenthecellsinaparallelgroup),parallelconnectionscanhave very low current carrying connections. Cylindrical cells are often spotweldedwith the samenickel strip inparallel as theyare in series.However, asinglethinpieceofnickelwouldbemorethanenoughforparallelconnections,assumingthecellsintheparallelgroupareconnectedone-to-oneinserieswiththecellsinthenextparallelgroup.Alloftheheavycurrentwouldflowthroughthestrongseriesconnections,andathinwiresolderedtothenickelstripwould

beenoughtocreateparallelgroupsthatcanselfbalanceeachother.

There are two main reasons why cylindrical cells are often spot welded inparallelgroupswiththesameheaviernickelasusedfortheseriesconnections.Whentheparallelgroupsarecreatedfirst,strongweldedconnectionshelpholdthe cells together. Secondly, stocking and swapping different connectionmaterialsaddsmorecomplexity toa largebatterybuildingoperation.Thecostdifferenceisusuallyinconsequentialforsmallbatteries,meaningit’sjusteasiertousethesamematerialforallconnections.

When you’re working with bigger cells, and especially cells that are spacedfarther away, the cost savings can begin to add up. For packs with manyhundreds of kilowatt hours made using prismatic cells for electric vehicles,smallerparallelconnectionscansavealotofheavydutywire.Youcanusethelarge gauge wire for making strong series connections and save the smaller

gaugewireforparallelconnections.

Todo the samewith cylindrical spotwelded cells, you’llwant toweldup theseriesconnectionsfirst, thengobackandeitherspotweldorsolderonsmallerconnectionsusingthinnickelstriporwire.Remembertocheckthatallofyourcellsareatequalvoltagesbeforeconnectingthecellsinparallel,butyoushouldbecheckingforthatanywaysregardlessoftheorderoftheconnections.

ConnectingtheBMS

MostlithiumbatteriesusesometypeofBatteryManagementSystem(BMS)toprotectthebatteryduringdischargingandtobalancethecellsduringcharging.Ifyouaren’tusingaBMS,you’llneedtouseabalancecharger,whichwe’llcoverinmoredetailsooninChapter13.

TherearethousandsofdesignsoutthereforcommerciallyavailableBMSunits.Many of them are fairly unique, whichmakes it hard towrite one absolutelydefinitiveguide for correctlywiring every singleBMS in existence.However,mostBMSsare similar and follow the samegeneralwiring structure, so theseinstructions shouldguideyouwell for thevastmajorityofBMSs. If for somereasonyouencounter aBMS thathasoddmarkingsordoesn’t seem tomatchwhatyou’velearnedhere,justcontactthevendorormanufacturer.BMSsusuallycomewithwiringdiagramsforexactlythisreason.

Your BMS will have a few thicker wires (or pads to solder on the wiresyourself). These are the discharge and chargewires. They are thicker becausethesearethewiresthatwillcarrythelargestdischargecurrentandthesomewhatsmallerbutstillnotinconsequentialchargecurrent.YourBMSwillalsohaveanumber of thin wires. These are the balance wires. I like to start by firstconnectingthemainchargeanddischargewires.

Connectingchargeanddischargewires

Most BMS units generally have B-, P-and C-wires (or pads to solder thosewires). Occasionally there will be a B+ wire as well, but that is rarer. I say“generally” because, again, every BMS can be different and standards oftenaren’trespectedintheBMSindustry.ItisimportanttodoublecheckthewiringdiagramforyourspecificBMStoconfirmthateverythinginthischapteristrueforyourBMS.

TheB-wireisgenerallyforthenegativeterminaloftheentirebattery.That’salsothe negative terminal of your first parallel cell group. The B-wire should bewireddirectlytothenegativeterminalofthatfirstcellgroup.Itisimportanttoconnect theB-wire in such away that all cells in the first parallel group cansupplycurrentequally.Connecting to justoneendof theparallelgroupwouldforcetheparallelconnectionsclosertothewiretocarrymorecurrent.Itisbettertoconnectthewiretoallcells,ifpossible.

Thiscanbedonebysolderingitinbetweenthenickelstripofeverycellonspotwelded batteries, or connecting it to each terminal on bolted connectionprismatic cell batteries. The only exception to this would be if the batterydischargeratewillbe lowenoughthat theparallelconnectionscansufficientlycarry the current.Generally though thatwould be a very lowpower, long lifebattery.

TheP-wireisgenerallythenegativedischargewireforthepack.Thatmeansthatit will be plugged intowhatever device the battery is powering, such as your

lights,motorspeedcontroller,voltageconverter,etc.Thiswiredoesn’tconnecttoyourbatterycellsatall.ItgoesstraightfromtheBMStothenegativeterminalofwhateverdeviceyou’repowering.

ThenegativedischargewireontheBMSisalmostalwaysmarkedP-.However,I have seen the negative dischargewire on theBMSmarked as PD-on a fewBMSunits.Thatisveryrarethough.It’salmostalwaysmarkedP-onmostBMSunits.

The C-wire is generally the negative charger wire. It won’t connect to yourbatteryatall,butwillinsteadgostraightfromyourBMStothenegativesideofyourcharger’soutput.OntheveryfewBMSunitsI’veseenwithaPD-wireforthe negative discharge, the negative charger wire was marked as P-. That’sconfusing,Iknow,sinceP-isnormallythenegativedischargewire.WhatcanIsay?Gocomplain to theChineseboarddesignersand tell them to stick to thestandardconventions.

IftheBMShasaB+wire,itisgenerallythepositivedischargewire.Itwouldbeconnected directly to the main positive terminal of the entire battery, whichwould be the positive terminal of your last parallel group in series. If yourbatteryisa13sbattery,thiswouldbethe13thparallelgroup.

WhenIconnectwirestotheBMSpadsorlengthentheexistingBMSwiresbysoldering on longer wires, I always like to put connectors on the ends of thewires first. This is so that there aren’t exposed, bare ends of wires danglingaroundwhile I’mworking on the battery. Thiswould apply to your P-andC-

wires.YourB-wire(andB+wireifyourBMShasit)willconnectdirectlytothebattery andwon’t need a connector.You can of course include a connector ifyou’dliketobeabletodisconnecttheBMSfromthebattery,butthatispurelyoptional.

One casewhereyoumightwant aBMS that canbedisconnectedwouldbe ifyouwantedtousethebatteryatadischargeratethatwashigherthantheBMSisrated tohandle.Removing theBMSwouldallowyou todischargedirectlyviathe battery’smain positive and negative terminals. There are two things to becarefulofinthisscenariothough.First,makesureyourbatterycellscanhandlewhateverhighcurrentyou’retryingtodrawfromthem.Andsecond,makesurethatyouusesometypeofmeteroralarmtoensureyoudon’toverdischargethecells,astheBMSwon’tbeabletocutoffthedischargewhenthecellsareemptyifitisn’tconnected.

AfteryourmainBMSchargeanddischargewiresareconnected,youstillhavetoconnectadditionalpositivechargeanddischargewiresdirectlytoyourbatteryaswell as connect the balancewires.We’ll startwith the positive charge anddischargewires.

The BMS usually only connects to the pack using thick gauge wires on thenegativeterminal.YourmainpacknegativedischargewirewillcomefromyourBMS,butyoustillneedtoaddthemainpackpositivedischargewire.Makesuretoaddaconnectortotheendofthiswireaswellbeforeyouconnectittoyourbattery.It’dbeashametocreateashortwhenyou’rethisclosetofinishingyourbatteryconstruction.

The main positive discharge wire should connect to each cell with sufficientcurrentcarryingcapacity,justliketheB-wire.Again,onaspotweldedpackthiscanbeachievedbysolderingthemainpositivedischargewiretothenickelstripinbetweeneachcellonthelastparallelgroup.Onaboltedconnectionpack,thewirecanbeconnectedtoeachboltedterminalofthelastparallelgroup.

Theonlytimeitwouldbeacceptabletoconnectthemainpositivedischargewiretoonlyonecellinaparallelgroupisifthedischargecurrentwillbelowenoughfor the parallel connections to handle it. In the case of a parallel connectionmade froma single piece of nickel strip rated for 5A, you could connect thenegative and positive discharge wires to only one place on the first and lastparallelgroupaslongaswon’tdrawmorethan5Afromthebattery.

Themainpositivechargewirewillconnecttothesameplaceonthebatteryasthemainpositivedischargewire.Thebatteryischargedanddischargedviathepositive terminal of the last parallel group. The positive charge and dischargewires could theoretically even be a single shared wire. But for practicalpurposes, a second wire is often used to avoid disconnecting the positivedischargewirefromthedeviceitpowers.Withseparatewires,youcouldchargethebatterywhileitwasstillconnectedtoanotherdevice.Thiswouldobviouslybe useful for large batteries such as those powering a car or a home energysystem.Itwouldbeannoyingtodisconnect thosebatterieseachtimetochargethem.

Thepositivechargewirecouldalsobesplicedintothemainpositivedischargewire to avoid connecting directly to the cells, if thiswas desirable. If you aresoldering your wires onto nickel strip, this method can help avoid adding

unnecessaryheat to thecells.Youshouldof course try to solderon thenickelbetween cells and not directly above them in order to keep the point of heatfurtherawayfromthecells.Evenso,reducingthenumberofwiressolderedtothenickelnearthebatteryterminalsispreferable.

Becausethechargecurrentisoftenmuchlowerthanthedischargecurrent,itcansometimesbeacceptabletoconnectthepositivechargewiretoonlyonecellinthelastparallelgroup.Forexample,electricbicyclebatteriesoftenonlychargeatlevelsofaround2-4A,butthenickelstripjoiningtheparallelgroupsisoftenrated for 5 A. In this case, it would be perfectly fine to connect the positivechargewire in only one place on the parallel group. If wewere charging thesamebatteryat10Athough,thechargewireshouldbeconnectedinatleasttwoplacestoavoidanysegmentofthenickelneedingtocarrymorethan5A.

Connectingthebalancewires

That concludes the thick wires from your BMS. Now we must connect thebalancewiresontheBMS.Thebalancewiresconnecttoeachparallelgroupandare used by the BMS to monitor the voltages of each parallel group duringdischargingandtobalanceeachparallelgroupduringcharging.

InnearlyeveryBMS,therewilleitherbethesamenumberofbalancewiresasparallelgroupsoronemorewire than thenumberofparallelgroups.Botharecommon,sobepreparedforeither.

Ifthereareanequalnumberofbalancewiresandparallelgroups,thewiringisverysimple. Justconnecteachwire to thepositiveendofeachparallelgroup.LookattheboardoftheBMStoseewhichwirebelongstowhichparallelgroup.Theyshouldgoinorder,butitcansometimesbeconfusingtounderstandwhichendtostartwith.Ifitisn’tlabeledontheBMSitself(whichwouldlooklikeB1,B2,B3...nearthewires),thenchecktheBMS’swiringdiagram.

Sometimesitwon’tbelabeled,butallthebalancewireswillbewhiteorblack,with a single red balance wire on one end of the group of balance wires.Generallyspeaking,thatredwirewillbethehighestnumberedparallelgroupinthe battery and each successivewirewill belong to each lower parallel groupnumber.Alwaysdoublecheckthiswiththewiringdiagramthough.

If there isonemorewire thanthenumberofparallelgroupsin thebattery, thesame rules apply, except that the first wire will actually start at the negativeterminalof thefirstparallelgroup.Then, therestof thewireswillcontinueasnormal on the positive terminals of each successive parallel group until youreachthehighestnumberedparallelgroup.Ina5sBMSwithsixbalancewires,thebalancewiresshouldbelabeledasB1-,B1+,B2+,B3+,B4+,andB5+withthesixthandfinalwireconnectingtothepositiveterminalofthefifthcellgroup.

You must use the appropriate BMS for your battery based on the number ofparallelgroupsinseries. Ifyouhavea10sbattery,youcannotusea13sBMSand just leave the last three balance wires disconnected. The BMS will beexpecting to the see thevoltageof thosemissingcells.When itdoesn’tdetectthem,itwon’tallowthebatterytoprovidecurrent.

Youalsocan’tuseaBMSforonefewerparallelgroupsinseries,evenifithasan extrabalancewire.For example, you can’t use a5sBMSwith sixbalancewiresona6sbattery.Itjustwon’twork.

YourBMSwillneedtobesecurelymountedtoyourbatteryortheenclosureofyour battery to keep it from moving and disconnecting any of the numerouswires.SmallerBMSunitsaretypicallyabareboard,andshouldbeinsulatedorcoverediftheywillbemountedtothebatterydirectly.AnaccidentalshortcouldoccuriftheBMSweretobridgetheterminalsoftwoadjacentparallelgroups.

IoftenusekaptontapetomountsmallerBMSunitstobatteries.Kaptontapeis

nonstatic,nonconductiveand isquite stickywithoutbeinggummy likesomeelectricaltapes.

Forbatteriesthatwillexperiencevibration,suchasforanelectricskateboardorelectricbicycle,IliketomountathinpieceoffoambetweenthebatteryandtheBMSboardtoreducetheshockloadingontheBMS.Batteriescellscanusuallyhandlethevibration,butthemorefragileprintedcircuitboardoftheBMScouldbedamagedunderextremeimpactsorshockloading.

LargerBMSunitsthatareratedforhigherpowerareoftenenclosedintheirownaluminumcase toprotect theBMSandaid inheatdissipation.Thecaseoftenhas bolt holes formounting the BMS.Make sure to keep the aluminum caseawayfromtheterminalsofyourbatterytopreventashortcircuit.

IfyouchoosetoskiptheBMSaltogether,yourmainchargeanddischargewireswillbemuchsimpler.Youobviouslywon’thaveanyBMStostandinthemiddleofthecircuit,soyou’lljustconnectthechargeanddischargewirestoyourfirstandlastparallelgroups.Thenegativechargeanddischargewiresconnecttothenegativeterminalofyourfirstparallelgroup.Thepositivechargeanddischargewiresconnecttothepositiveterminalofyourlastparallelgroup.

Youdon’tactuallyneedtwosetsofwires-thesamepairofpositiveandnegativewires canbeused for charginganddischarging.Butdependingon the typeofconnectors you want to use and whether you want to leave your batteryconnected to the device it is powering, it can be useful to have a separatecharginganddischargingport.

Addingabalanceconnector

If you don’t include aBMS in your battery, then youmust include a balanceconnector.ThebalanceconnectorisconnectedjustlikethebalanceconnectorontheBMS.Asingle thinwire isconnected toeachparallelgroupof thebattery.Thisallows thebattery tobebalancedduringcharging,whichwe’lldiscuss inmoredetailinChapter13.

Even if youdo choose touse aBMS inyourbattery, adding an extrabalanceconnectormightbeagoodidea.Byhavingthebalanceconnectorexternaltothebattery, you canmonitor the voltages of the parallel groupswithout having toopen up your battery. This can be helpful down the road when you want toensurethatthebatteryisstillhealthyaftermanycycles.Itcanalsobegoodforconfirming that your BMS is still working. If your BMS ever dies or thebalancing function were to fail, you might not realize it until your batterybecomes very unbalanced and starts having issues. An external balanceconnectorcanhelpyouverifythattheBMSisstillworkingfine.

Performingfinalqualityandperformancechecks

At this point your BMS is connected and your battery is finished! Well,electricallyspeakinganyways.We’vestillgottosealitupandmakeitpretty,butthat’s for the next chapter.Now it’s time to double check that everything hasbeenconnectedcorrectly.

Use a digital multimeter or a voltmeter to check the voltage of your batterythrough both the charge and discharge wires. You should get somethingreasonableintherangeofyourfinishedbattery.A36Vbatterymightnotreadexactly36Vif thecellsweren’tfullychargedtobeginwith.Manyli-ioncellsship from the factoryat3.3Vpercell, soa10s36Vbatterybuilt from thesecellsshouldread33Vatboththechargeanddischargeconnectors.Ifforsomereasonyougetanoddnumber,especiallyanumberthatistoolow,it’sprobablydue toan incorrect connection somewherewith theBMS.Doublecheckallofyourmainconnectionsandbalanceconnections.

Ifyougetareadingof0Voranopencircuitreading,itmaybeaBMSissue,butitmayalsobeacellconnectionissue.Todeterminewhich,measurethevoltagefrom the negative terminal of your first cell group to the positive terminal ofyour last cell group. If it’s still 0V or an open circuit, thatmeans you likelyforgottomakeaseriesconnectionsomewhere.IfyougettheappropriatevoltagethenyouhaveaproblemwithyourBMS, likelyan incorrectBMSconnectionsomewhere.

Ifyou’vecheckedeverythingandstillgetanincorrectvoltagereadingthatisn’t

withintherangeofyourfullychargedtodischargedvoltage,youmightjusthavea bad BMS unit. This happens occasionally, especially with cheaper “Chinaspecials”.Ifyou’regoingtobuyacheapBMS,youmightwanttobuytwojusttohaveareplacement.Butifyou’regoingtobuytwo,youmightaswellspendtwiceasmuchonagoodonetostartwith.

Checking the voltage is a good way to make sure everything is correctlyconnected,butitwon’ttellyouhowwelleverythingisconnected.Forthat,it’sbesttotestyourbatterywitharealload.Ifpossible,testingwiththeactualloadit was designed for would be optimal. For some applications though, it’s notpossibletoputthebarebatteryinthedevice,orthedeviceisn’treadyyet.

Ifyourdevice isavailableandyoucan test thebatterywith it, try running thebatteryunderloadforafewminutestomakesureyoudon’tfeelconnectionsonthebatteryheatingup.A little bit of heat is fine.Under different loads,manycellswillwarmuptothe35-50ºCrange.Unlessyou’rebuildingapackthatisdesigned tooperateunderveryhighpower,youdon’twant it toget toomuchhotterthanthat.

An infrared or laser thermometer is great for checking the temperature ofdifferentpartsofyourbattery.Theycanbeboughtforlessthan$5onsiteslikeAliExpress.Paycareful attention toyour seriesconnections tomake sure theyaren’tgettingtoohoteither.Abrightredglowingpieceofnickelstripwouldbeadeadgiveawaythatyou’vemessedup,butevenjustasingleseriesconnectionthatismuchhotterthantheotherscanbeanindicationthatthatconnectionisn’tsufficientlystrong(i.e.,itneedsmorecurrentcarryingcapacity).

Ifyoucan’tuseyourintendeddevice,thereareotherwaystotestabattery.12Vbatteriescanbepluggedintoawidearrayof12VDCdevices,suchaslightsorheaters.Othervoltagesdon’tlendthemselvesaseasilytopoweringthedevicesyou have laying around, but they can be plugged into a voltage converter tooutput12VDCforasimilareffect.

Youcanalsocreatea“dummyload”byusingaheatingcoil,powerresistorsorachainoflightbulbstocreatearesistiveloadforthebattery.Halogenlightbulbsand power resistors are often used to build homemade battery dischargingdevices. A quick google search for either will find many different sets ofinstructions to build a battery discharger. I even posted a video on theEbikeSchool.comYouTubechannelshowinghowtomakeanadjustablevoltagehalogenlightbulbdischarger.

Testing your battery under a load isn’t crucial, though it’s a nice extra step toensurequality.Butaslongasyourbatteryisprovidingthepropervoltagefromits charge anddischarge connectors, thenyou’re ready tomoveon to thenextstep.

Chapter12:Sealingthebattery

Onceyou’refinishedwithyourconnectionsandconfirmedthatyourbatteryisingoodworkingorder,youcangoaheadandsealitup.Inmyopinion,alllithiumbatteries should be sealed in some way. An unsealed lithium battery withexposedterminalsissimplyanaccidentalshortcircuitwaitingtohappen.

TheonlycasewhereIcommonlyseelithiumbatteriesleftunsealedisintheDIYpowerwall community. They generally use snap together plastic cell holderswhich create a nice battery case and keep the cell terminals lifted off of thesurfacetheyreston,buttheydon’tactuallycovertheexposedterminals.

I’mnotsurewhythesefolksdon’tsealtheirbatteries.Maybetheywanteaseofaccesstoreplacebadcells,sincetheyusuallybuildtheirbatterieswithsalvagedcells.Maybe they want the benefits of passive air cooling and don’t want todesigninanactiveairflowsystem.Maybetheyjustfigurethatthebatteriesareuponashelforhiddeninaclosetandthereforethey’reprobablysafe.

Either way, such batteries with exposed terminals represent a fire hazard ifanythingweretoevershortthem.Amouseintheclosetwouldturnprettycrispyif it stepped in thewrong place and shorted some high capacity home energystoragebatteries.

Themethodofsealingyourbatterywilldependonanumberoffactorsincludingthe type of cells you used, the size and shape of your battery, and theenvironmentinwhichyourbatterywillbeused.

Hardcases

Prismatic cells areusuallyvery easy to seal.Their only exposed terminals aresmallthreadedrodsornutsandoftencomewithspecialrubbercoverstoinsulatethem.Iftheywereconnectedwithinsulatedwires,thenthewiresdon’tneedtobecovered.Ifbusbarswereusedtoconnectprismaticcells,theexposedbusbarsshouldbecoveredsomehow.Oftenarigidcaseisbuiltaroundtheprismaticcellsto cover the cells and busbars. A hinged door allows easy access while stillkeepingeverythingprotectedandsafe.

Pouch cells are also good candidates for hard cases.The delicate cells can beeasily punctured.Rigid cases such as Pelican cases provide protection againstaccidental damage. Pelican cases are fairly expensive though, and cheaperplasticcasesorhardplasticlunchboxesalsoworkwellasbatterycases.

Eventhoughhardcasesarerecommendedforpouchcells,duetothelightweightand energydense nature of these cells, they are oftenused in instanceswhereweight and volume areminimized. In cases of drones and light aircraft, rigidcasesaddunwantedbulk.Whenusingpouchcellswithoutahardcase,extracaremustbetakentoprotectthecellsfromaccidentaldamage.

Heatshrink

Heatshrinkisagoodmethodforsealingmanydifferenttypesofbatteries.Itisoftenusedasafirststepinthesealingprocess,thenfollowedupbyahardcasetoaddextraprotection.

Batteriesmadefromcylindricalcellsrequirecarefulsealingtokeepthebatterysafefromshortcircuits.Theendsofthecellsarenearlycoveredinnickelstripconnections,makingshortcircuitsmucheasier,dependingonthebatterylayoutandorientation.

Cylindrical cell batteries lend themselves well to heat shrink wrapping,dependingon the shapeof thebattery.Largediameterheat shrinkwrapping iscommonlyavailableonlineinsizeslargeenoughtoheatshrinkwrapachild.Notthatyoushoulddothat.

For batteries that will be used in a high vibration environment like electric

skateboardandbicyclebatteries, it’sagoodideatowrapalayeroffoamsheetaroundthebatteryfirstbeforeheatshrinkwrappingit.Thiswillgivesomeextrapadding and protection from bumps and shock loading. Arts and crafts foamsheets between 1-3 mm thick work well for this purpose and are quiteinexpensive. Some people use cheap yoga mats, though that foam is likelythickerthannecessaryandwilltrapmoreheatthanthinnerfoam.Dependingonyourapplicationthough,thickerfoammightbeanadvantage.

Thin fiberglass boards are also used for surrounding a battery and providingrigidprotection.Fiberglassdoesn’tabsorbimpactsaswellasfoam,butithelpstospreadoutandsharpbumpsoveralargerareaofthebattery.

Heatshrinkwrappingiseasiestforsquareshapeswhereitcanwraparoundtwoopposingedges,effectivelylockingitselfinplace.Fortriangularbatteriesoranyshape with steep inclines, heat shrink can be a bit more difficult to use. Theproblemisthatasingleloopofheatshrinkwrapoftencan’tshrinksmallenoughtogrip the opposing edges, and itwill therefore slide down the incline.High-ratioheatshrinkisavailablethatcanhelpsolvethisproblem,butit’shardertofindinsuchlargediameters.

Onetrickforheatshrinkwrappingtriangleshapesistouseprogressivelylargersizesofheatshrinkasyougofurtheralongtheinclineofthebattery.Thelargersheetswillshrinkdownonthesmallersheetsthatarealreadyinplaceandhelptolockthemdown.

Using heat shrinkwrapping loops that are oriented 90º to each other can alsohelplocktheheatshrinkdownonshapeswithinclinedplanes.

Manypeoplefinditdifficult todeterminethecorrectsizeofheatshrinktousefor their battery. Large diameter heat shrink is measured by the half-circumferenceinsteadofbythediameter.It’salsousuallylistedinmetricsizes.So a piece of150mm heat shrink isn’t a circle that is 150mm (6 inches) indiameter.Rather,whenlaidflat, itmeasures150mmacross.Thatflatwidth isequal to half of the circumference of the heat shrinkwhen it is opened into acircle.

Calculatingthesizeisfairlyeasyonceyouknowthosepoints.Allyouneedtodoismeasureyourbatterytofindhalfofitsperimeter.Ifthebatteryisaregularsquare or rectangle shape, measure the top and one side, then add those twonumberstogether.Thatwillgiveyouhalfoftheperimeterofyourbatteryinthedirection youmeasured.Because heat shrink generally shrinks 2:1, the largestpiece you could usewould be twice that number you just calculated, since itwouldshrinktoexactlytheperimeterofyourbattery.Obviouslywedon’twanttocut it thatclose–we’d rather theheat shrink try to shrinksmaller thanourbatteryandthussqueezeourbatterytight.Soouroptimalsizewouldbebetweenabitundertwiceourhalfperimeterandabitoverourhalfperimeter(soitcan

stillslideoverthebattery).

Maximumheatshrinksize=2×(length+widthofbattery)–“alittlebit”

Minimumheatshrinksize=length+widthofbattery+“alittlebit”

Basicallyallthesetwoequationsaresayingisthatourheatshrinksizeshouldbesomethingbetweenourperimeterandhalfofourperimeter.

Let’s do an example. Let’s say our battery is the rectangular one from thediagramtwopagesback.Let’ssayitmeasures70mmx75mmx150mm.Tocalculatethesizeofheatshrinkwe’dneedtocoveritaroundthelongdimension,asshowninthediagram,we’daddthelengthandwidthgivingus:

Minimumheatshrinksize=75mm+150mm+“alittlebit”=225mm+“alittlebit”

Sowe know that our heat shrink needs to be a bit larger than 225mm to fitaroundthebattery.Tocalculatethemaximumsize,wewillsimplymultiplythatnumberby2,whichgivesus450mm.Soourheatshrinkcanbeanywherefrom225mm to450mm.Preferablywe’d like somethingon the lower endof that

scaletomakesurethefitisastightaspossible.A250mm–300mmsizewouldbegreat.Andthat’sallthereistocalculatinglargediameterheatshrinksizes.

Whenheatingtheheatshrinkwrap,becarefulnottoapplytoomuchheat.Manyheatgunsaredesignedforheavydutypurposessuchasremovingpaint.Thesehighheatsettingscanmeltyourheatshrink in less thanasecond.Startwithalow setting and slowly increase the heat until you find the appropriate level.High powered hair dryers can also be used for heat shrink. I usedmywife’s2,000WhairdryerforyearsuntilIgotadecentheatgun.

Coolingissues

Dependingonyourapplication,yourlithiumbatterymightnotneedanyformofcooling.MostcommerciallithiumbatteriesinthefewkWhrangehavenoformofcooling.The lithiumbatteriesused fordevices suchasdrones,power tools,electricskateboardsandelectricbicyclesaresealedineitherheatshrinkorhardcasesandareusedwithinpowerlevelsthatdon’trequireactivecooling.Passivecooling thatoccurswhen thecaseof thebatterydissipatesbuilt-upheat to thesurroundambientairisallthatisnecessaryinmostcases.

Somebatteries haveBMSs that cut their powerwhen a certain temperature isreached.Thisiscommoninpowertoolbatteriesthatareoftenused(orabused)athighpowerlevels.

Cooling becomesmore important at high power andwhen human lives are atstake. This is most commonly found in the automotive and aeronauticalindustries.Electricvehiclesusuallyhaveactivecoolingsystemsusingair,water,oilorotherfluidstodrawheatoutofthebattery.

The vastmajority of the people reading this bookwill never need to consideraddingactivecoolingtotheirbattery.Aslongasthebatteryisn’tabusedanditisusedinanenvironmentwithatleastsomeformofpassivecoolingwhereaircanpassoverthebatterycase,itwilllikelybejustfine.Thermalrunawayinlithiumcells becomes an issue at around 150 ºC, yet lithium batteries in normal useshouldn’texceed60ºC.

Generallyspeaking,aslongasthecaseisexposedtoairsomehow,thebatteriescansufficientlycoolontheirown.Powertoolbatteriesmightneedtositontheshelfforafewminuteswhentheyhittheirthermalcutofflimit.Electricbicycleand skateboard batteries usually cool passively just from the air that rushesaround them while riding. Drone batteries are generally showered in thedownwashofthepropellers,whichprovidesaformofpassive/activecooling.

If you do require actual active cooling in your battery for high powerapplications,airisusuallythebestmethod.Asealedbatterycanhaveacoolingfansuchasacomputercasefanplumbedintothecasewithanexhaustportontheothersideofthebattery.Careshouldbetakentoensurethatforeignobjectsdon’tenterthebattery,butthismethodwillprovidesufficientcoolingformosthighpowerscenarios.

Ifyouneedwateroroil cooling,yourneedsare likelyoutsideof the scopeofwhatonebookcanhandle.Youneedateamofengineersdesigningyourbatterybecauselivesareprobablyontheline.Applicationslikeelectriccarsandaircrafthaveverysophisticatedcoolingsystems.Buildingbatteriesforsuchapplicationsshouldnotbetakenlightly.

Remember,addingmorecellsinparalleltocreatealargercapacitybatterywillhelpreducetherelativepowerrequiredbythebattery.HighercapacitybatteriesresultinlowerCratedischarging.Thisinturnresultsinlowerheatgeneration.Ifyourbatteryisapproachingathermallimit,there’sagoodchancethatitsimplywasn’tdesignedwithenoughcapacityoritwasn’tdesignedwithcellsratedfora

sufficientlyhighCrate.

Chapter13:Charginglithiumbatteries

Charging lithium batteries isn’t difficult. However, if charging is not donecorrectly then it can becomedangerous.Many lithiumbattery house fires youhearaboutonthenewsoccurduringthechargingphase.Thisdoesn’tmeanyoushouldbeworried.Itjustmeansyoushouldpayattentiontotheproperchargingmethodsandemploythemwhenappropriateforyourbattery.

ThetypeofchargerusedforlithiumbatterieswilldependonwhetherthebatteryhasaBMSornot.However,themechanismofcharginglithiumbatterycellsisthesame,regardlessofthebatteryconstruction.

ConstantCurrent,ConstantVoltagecharging

Lithium battery cells charge by using what is known as a constant current,constantvoltage(CC-CV)chargingscheme.

ACC-CVchargingschememeansthatthefirstpartofthechargingperiodisaconstantcurrentphasewhilethesecondpartofthechargingperiodisaconstantvoltagephase.

Duringtheconstantcurrentphase,electricityissuppliedtoalithiumbatterycellwithaconstantcurrent.Thismeansthecurrentisunchanging,eventhoughthevoltagewillchangeduringthisperiod.Adischargedlithium-ioncellmightbeataround2.7Vwhenconnectedtoacharger.ACC-CVchargerratedfor1Awillsupply1Aofcurrenttothebatterycell,startingatavoltageof2.7Vtomatchthevoltageofthecell.

Whenthelithiumcellexperiencesthecurrentflowfromthecharger,thevoltagewillinstantlyincrease,likelybyaround5%orso.Thisisessentiallytheoppositeofvoltagesagduringdischarge.Whenthelithiumcellremainsconnectedtothecharger, the voltage of the cellwill slowly rise as the charge state of the cellincreases.

This is where the CC-CVmagic happens.When the voltage rises up to a set

preset limit in the charger (usually 4.2 V for a li-ion cell), the charger willautomatically switch from the constant current phase to the constant voltagephase.Intheconstantvoltagephase,thevoltagewillremainconstantatthefullychargedcellvoltage(4.2Vinthisexample)butthecurrentwillslowlydrop.Thecurrentwillcontinuetodecreaseasthelastbitofcapacityisoffloadedintothecell.Thechargerwillcutthecurrentwhenitreachesacertainminimumamount,oftenaround100to200mA.Atthispoint,thelithiumcellisfullycharged.

The sameprocessoccurs inmultiple cellswhen theyare connected inparalleland series. Because parallel cells automatically balance each other, cellsconnected in parallelwill charge together at the same rate.And just like howparallelgroupsconnectedinserieswilldischargeatapproximatelythesameratewhenconnectedtoaload,thesameparallelgroupsconnectedinserieswillalsochargeatapproximatelythesameratewhenconnectedtoapowersource,whichistheoppositeofaload.Thatistosay,asthebatterypackcharges,allparallelgroupswillchargeapproximatelyequally.

Thecorrectchargingvoltageisimportanttoensurethatcellschargecompletelybutdonotovercharge.AswelearnedinChapter3,mostLiFePO4cellschargetoaround 3.65 V while most li-ion cells charge to around 4.2 V, though a fewspecialtycellsshouldbechargedto4.3Vor4.4V.Tofindthepropervoltageofa charger for a battery built of many cells connected in series, you simplymultiplythenumberofcellsinseriesbythefullchargevoltageofthecells.

A14sbatterywithli-ioncellswillneedachargerwithavoltageof58.8V.

Totalchargevoltage=14cellsinseries×4.2V=58.8V

A10sbatterywithLiFePO4cellswillneedachargerwithavoltageof36.5V.

Totalchargevoltage=10cellsinseries×3.65V=36.5V

Thenumberofcells inparallelwillnotaffect thenecessarychargevoltage.A14sli-ionbatterywillrequireachargevoltageof58.8Vregardlessofwhetherithasacapacityof10Ahor50Ah.Whatthenumberofparallelcellswillaffectisthe current that the battery can be charged at. More parallel cells mean thebatterycanhandleahigherchargingcurrent, just like italsomeansthebatterycanhandleahigherdischargingcurrent.Ofcoursewithenoughcellsinparallel,youmightreachthechargingcurrentlimitofaBMSbeforethechargingcurrentlimitofthebatterycellsthemselves.

ChargingwithaBMS

Using a BMSmakes the charging processmuch simpler. To charge a batterywithaBMS,yousimplyplugthechargerintothebattery’schargerport.

That’sit.

Done.

TheBMS takes care of everything. It starts by allowing the battery to chargefromthechargerusingthebulkchargingmethod.You’llrememberfromChapter9thatbulkchargingiswheretheentirebatteryischargedasoneunitbyflowingcurrent through all of the parallel groups of cells connected in series. Eachparallelgroupseesanequalamountofcurrent.Oncethebulkchargingprocessbrings the first fewcellsup to fullcharge, theBMSthenbeginsbalancing thebatterybydrainingenergyfromtheparallelgroupsthatreachfullchargefirst(ifusing the top balancing method, which is most common). The BMS stopsdrainingwhenallthecellsreachfullcapacity.

But that’sall takencareofautomatically.Fromtheperspectiveof theuser, thecharger is simplyplugged into thebatteryand that’s it.Nothingelse todobutwaituntilthechargingiscomplete.

Therearea fewtypesofchargersavailable forbatterieswithaBMS.Theyallworkfairlysimilarly.Themaindifferenceisquality.

Thebottomof the listbeginswithplasticcasechargers.Theseareverycheap,usually in the $20-$40 range. They usually have no cooling fan,which limitsthemtofairlylowcurrent.Usuallyintherangeofjustacoupleamps.

Plasticcasechargersareproduced tobeaffordable.Therearesomegoodonesout there, but there are also many that cut corners to save on costs. This isusuallytoprovideachargerincludedwithabatteryorelectronicdevicewithouthavingtospendtoomuchonproducingthechargeritself.

Plasticcasechargerscanworkfine,butthecheapestonesshouldbewatchedtoensure they aren’t overheating. Unfortunately, this often happens quickly andwithout warning when something shorts inside the charger. Not only can thisdestroyyourbattery,butitcancauseeitherthebatteryorcharger-orboth-tocatchonfire.

Ifthatsoundsbad,thenyoumightlikethenextclassofchargers.Thesearethealuminumcase,coolingfanchargers.Asyoumighthavealreadyguessed,thesechargers have an aluminumcase and include cooling fans.They are generallymore expensive, but can also handle higher current and power than the cheap

plasticcasedchargers.

Thehighercostandability tohandlehighercurrentgenerallyequates tobetterbuildquality,thoughofcoursethisisn’talwaystrue.Generallyspeakingthough,chargers with an aluminum shell and a cooling fan are both safer and higherqualitychargers.

Thenextstepupwouldbeanadjustable,aluminumcasechargerwithacoolingfan. These chargers are usually based on the same structure as the chargers Ipreviously described, but include the option to adjust the charging voltage,currentorboth.Theadvantageoftheseisthatyoucanchoosealowercurrentforhealthierchargingorahighercurrentforfasterchargingwhenyou’reinhurry.Youcanalsoadjustthevoltagetoalowerleveltohelpincreasethelifespanofthebattery.We’lltalkabouthowthatworksinthenextchapter.

At this level there are multiple versions of good quality chargers. One Iparticularly like that can handle anything from 12-72 V is the Cycle SatiatormadebyGrinTechnologies inCanada.Thischarger isnotonlyadjustable,butit’s also programmable with a digital screen to allow you to add multiplechargingprofilesfordifferentbatteries.It’sevenwaterproof.Itactuallydoesn’thaveacoolingfan,butinthiscasethatisn’tasignoflowquality.Itwassimplydesignedtobeincrediblyefficient,thusnotneedingacoolingfan.Itdoesgetabit hotwhen used at its highest power levels though, so be aware of that andmakesuretogivethechargerroomtobreathe.

The charger you choose will ultimately be decided by your specific

requirements. Low power batteries are often fine with cheap plastic chargers.Essentiallyeverylaptopandcellphoneusesacheapplasticchargertochargeitslithium battery, so you shouldn’t be afraid of them. The bigger quality issuecomesintoplaywhenpowerlevelsincrease,meaninghighervoltagesandhighercurrents. For higher power batteries, a better charger is a form of insuranceagainstdamage.Foranextra$20or so, a chargerwithanaluminumcaseandcoolingfanislikelyagoodinvestment.

ChargingwithoutaBMS

ChargingwithoutaBMSiswhere thingsgetabitmorecomplicated.Batterieswithout aBMScan still bebulkchargedusing the samechargers listedabovethataremeantforbatterieswithBMSs.However,onlyusingabulkchargerovermanycharginganddischargingcyclescanresultintheparallelcellgroupsinabatterybecomingunbalancedfromonetoanother.

Goodquality lithiumcells canhandlemanymore chargeanddischarge cyclesbeforebecomingunbalanced,buteventuallyallparallelgroupswillunbalanceifgiven enough cycleswithout a BMS to correct for this problem.Onemethodusedtoavoidbalancechargingforaslongaspossibleistosetthechargevoltageslightly lower than the maximum rated charge voltage. For li-ion cells, thismightmean charging to around 4.17V per cell instead of 4.2V per cell. Byslightly undercharging the entire pack, the cells that eventually begin toovercharge will take longer to reach a voltage above their maximum ratedvoltage. Eventually though, only bulk chargingwith no balance chargingwillcausesomecellstochargehigherthantheirmaximumratedvoltage.

Inordertoavoidthiscase,abalancechargerisusedtochargethebatterywhilealso balancing the cell groups. Batterieswithout a BMSmust have a balanceconnectorwiredtoeachcellofthebattery.Abalancechargerrequiresthebatterytobeconnectedbyboththebalancewiresonthebatteryandthechargingwires(or the dischargingwires, as they connect to the same location on the batterywhenthereisnoBMSinvolved).

Thechargerstartsbybulkchargingthebatteryviathechargewires(ordischargewires) until the battery approaches full capacity. At that point the balancecharger works quite similarly to a BMS. The balance charger monitors thevoltageoftheindividualparallelgroupsandbleedsoffsomecapacityfromthecellsthatreachfullcapacityfirstwhilecontinuingtochargetherestofthecells.

Remember,batterieswithoutaBMSdon’talwayshavetobebalancedcharged.AsImentioned,goodqualitycellscanlastformanycycleswhilestayingcloselybalanced.Highercurrentchargesanddischargeswillcausecellstomorequicklylose their balance. Low quality cells will also lose their balance quicker. Thehigher the currentor lower thequalityof the cellsused in abatterywithout aBMS,themoreoftenitshouldbebalancedcharged.

Thebestmethod,shouldyouchoosetogowithoutaBMS,is totestyourownbatterytoseehowquicklyitbecomesunbalanced.Youcanalwaysuseadigitalmultimeter or voltmeter tomeasure the voltage of each parallel group via thebalanceconnectoronyourbattery.

Chargingtemperature

It is very important to respect the manufacturer’s ratings for chargingtemperature. Charging lithium cells above or below their rated chargingtemperaturecandamagethecellsandbepotentiallydangerous.

Charging lithiumbatterycells at low temperatures,below0ºC,willdamageordestroy the cell. At these low temperatures, lithium metal will permanentlycollectontheanode,reducingthecapacityofthecell.Notonlydoesthisrobthelithium cell of capacity, but it can also increase the risk of the battery cellcatching on fire if it is suffers an impact such as a drop or fall. Freezingtemperatures aren’t normally reached for most li-ion powered projects, butbatteries that are left outdoors overnight or even during the day in very coldclimatesmightexperiencethissituation.

Electric vehicles often include safetymechanisms to prevent charging at verylowtemperatures.Someelectricvehiclesevenemploybuilt inheaterstowarmthebatterytoasufficienttemperaturebeforechargingcansafelyoccur.

Forsmallerbatteries,bringingcoldbatteriesinsideandallowingthemtowarmuptoatleast10ºCforafewhourswillhelpavoidthisdamageduringcharging.Itisalwaysbettertousealowerchargecurrentwhenchargingcolderbatteries.

Charging lithium battery cells at high temperatures can both reduce theircapacityandpotentiallyleadtothermalrunaway.Chargingalithiumbatterycellproduces heat due to the internal resistance of the cell. If the cell is alreadyapproaching a dangerous heat level, additional heat from charging can push itovertheedgeandintotherealmofthermalrunaway.

Remember that lithium batterieswill have awider safe temperature range fordischargingthanforcharging.Alwayssticktothemanufacturer’sratedlimitsforchargingtemperatures.

Chapter14:Increasingcyclelife

Lithium batteries are expensive. Fortunately for us they normally last a longtime,butanythingthatcanbedonetoincreasetheirlifespanhelpsdelayacostlyreplacement.Thereareanumberofusefultricksthatcankeepyourbatterycellshealthier for longer and increase the number of cycles that your battery canprovide.

Chargetoalowermaximumvoltage

Most lithiumbattery cells don’t like spending a long period of time at highervoltages. In fact, it is recommended to store lithium battery cells at very lowstatesofcharge.TheonemajorexceptiontothisisLiFePO4,whichcantoleratestaying at full charge voltage for longer periods of time with less celldegradation.

Formost li-ion chemistries, keeping a cell at a high voltage causes damagingchemical reactions in the electrolyte of the cell. Over time, these chemicalreactions slowly decrease the capacity of the cell. This can result in theprematuredeathofthecell.

Thisdoesn’tmeanthatyouwon’tgetthenumberofchargecyclesstatedbythemanufacturerofthecellthough.Whatitdoesmeanisthatthere’sthepotentialtoactuallygetmorecyclesthanwereoriginallystated,ifyouplayyourcardsright.

Testshaveshownthatbychargingli-ioncellsto4.1Vinsteadof4.2V,thecellscould perform many more charge and discharge cycles. In some cases, cellchargednohigherthan4.1Vperformovertwiceasmanycycles.

Inordertochargeabatterytoalowervoltage,you’lleitherneedtomodifyyourcharger,oruseaspecialchargerthatisadjustable.Evennonadjustablechargers

oftenhavepotentiometersinsideforfinetuningthevoltage.Youshouldn’tmessaroundwiththeseifyoudon’tknowwhatyou’redoing.However,ifyoucangettheinfofromthevendoronwheretomaketheadjustment,youcanturndownthevoltageabit.Keepinmindthatthiscouldvoidyourchargerwarranty.

Adjustable chargers make this easier as they are designed to allow this veryfunction.Theonlyproblemwiththisunderchargingmethodis thataBMSthatbalances the cells at their full capacity likely won’t ever enter its balancingmode, as the cells never reach their full voltage. For this reason, it’s goodpractice to occasionally charge a battery to full voltage to allow the BMS tobalance thecells.Youmightbeable to find someBMSs that aredesigned forlowervoltagebalancing,buttheyarefairlyrare.

Themajordisadvantageofthisunderchargingmethodisthatyouwon’tgetthefullcapacityfromyourcells.Forexample,chargingali-ioncellto4.1Vinsteadof4.2Vmeansyou’llonlygetabout90%ofitsfullchargecapacity.Ifdoublingthe life of your battery is worth cutting 10% capacity off of each dischargethough,thenthismaystillbeagoodoption.Youcanalsoplaywiththeamountofunderchargingyouperformtofindtherightbalanceforyourneeds.

Interestingly, theoppositeof thismethodalsoapplies.Bycharging li-ioncellspast4.2V,suchasto4.3V,youcangetmorecapacitythanthecellsareratedfor.A3Ahcellmightdeliver3.2Ah.However,thisnotonlydrasticallyreducesthe lifespan of the cell, but it can also be very dangerous. You should neverchargecellshigherthantheirratedvoltage.

Dischargetoahigherminimumvoltage

Similarly,youcanalsouseahigher lowvoltagecutoff.Lithiumbatterieshavelongerlifespanswhentheyaren’tdischargedtolowvoltagelevelsunderheavyloads.Li-ioncells areusually rated fordischarging to2.5V,but stoppingat ahighervoltagewillresultinachievingmorecyclesfromthecells.

Thiscreatesasimilarproblemtounderchargingthough,inthatyoudon’tgetthefullratedcapacityofthecellsforeachchargecycle.

Manyelectricvehiclemanufacturersusethismethod.Everwonderhowtheycangivea10-yearwarrantyontheirbatterieswhenmostcellsareonlydesignedtolast 1,000 cycles at most? They are often using only the middle part of thevoltagerangefortheircells.Inthiscase,theychargetoalevelslightlyunderthemaximum rated voltage and stop discharging just above the minimum ratedvoltage. Not all electric vehiclemanufacturers do this, but some have used itquitesuccessfully.

Chargeanddischargeatlowercurrents

Thelowerthechargeanddischargecurrentofalithiumbatterycell,thehealthieritwillremain.Evencellsthatareratedforveryhighdischargecurrentsarestillhappieratlowercurrentlevels.

Fastchargingisconvenientbutlowersthenumberofusefulcyclesyou’llgetoutof your battery.When time isn’t an issue, you should charge at lower rates toincreaseyourbatterylife.

Itcanbedifficult todischargeat lowerrates ifaspecificapplicationordevicerequiresafixedcurrent.Insuchcases,increasingthecapacityofthebatterywillresult in relatively lower discharge rates. For example, a 2A load on a 2Ahbatterycellisa1Cdischargerate.Butifyoucanusea4Ahbatteryinstead,thatsame2Aloadresults in justa0.5Cdischargerate.The4Ahbatterywill lastlastformorecompletecharge/dischargecyclesinthisexample.

Keepcellscool

We’ve discussed this multiple times so I won’t spend too long on it. As wealreadyknow,heatistheenemyoflithiumbatterycells.Testshaveshownthatthehigherthetemperatureatwhichlithiumbatterycellsarestoredandused,thefewercyclestheyprovide.

The sweet spot seems tobe right around room temperatureat25 ºC.Batterieswillnaturallyheatupduringcharginganddischarging,butyoushoulddoyourbest to store them at room temperature when not in use. Doing so will helpprolongtheirusefullives.

While lithium batteries can generally provide good capacity at highertemperatures compared to other types of rechargeable batteries, the state ofchargecanhaveabigeffectontheirreductionincapacity.Performingcompletechargeanddischargecyclesatelevatedtemperaturescanquicklydestroythecellin just a fewchargecycles.Conductingonly50%dischargebefore rechargingcan greatly improve the number of cycles the battery can perform at hightemperatures.

Chapter15:Disposingofoldlithiumbatteries

Whenlithiumbatterieseventuallywearoutandendtheiruseful lifetimes, theygenerally shouldn’t be thrown away with normal household garbage. Lawsregarding disposal of lithium batteries vary all over theworld and even fromstate to state in the US. In many places, lithium batteries are consideredhazardouswaste.

In some locations it is permissible to dispose of small quantities of lithiumbatteries innormalmunicipalwaste. Inothers,anyamountof lithiumbatteriesmustbedisposedofseparatelyorsenttorecyclingcenters.Stillothershavenolawsatallregardingdisposingoflithiumbatteries.

Insomeareas,lithiumbatteriesarecollectedfordisposalbyincinerationundercontrolled conditions.This prevents the potentially dangerous case of landfillsfilling up with lithium cells and starting massive fires, but is an unfortunatewasteofsuchavaluableresource.

Evenifyourareapermitsdisposaloflithiumbatteries,recyclingisamuchmoresustainable option. Lithium is a limited resource. Itsmining is both dirty andexpensive. By recycling old lithium batteries, you’re helping to save thisresourceandpreventfurtherenvironmentaldamagecausedbyitsrawcollection.

Most areas have free recycling drop-off locations. These are often located atschools, universities, malls, post offices and other public places. It costs younothing but benefits us all. If you don’t knowwhere you can recycle lithiumbatteriesinyourarea,contactyourlocalmunicipalwastecompany.

Whether you are sending the batteries off for recycling or just throwing themawayinyourhouseholdgarbage,alwayspreparethecellsfirst.Lithiumbatterycells should be discharged completely down to or past their minimum ratedvoltage. The cell terminals should also bewrapped in plastic or coveredwithtapetopreventashortcircuit.

Chapter16:Examplebatterybuildingprojects

There are an endless number of projects out there that canmake use of DIYlithiumbatteries.Tohelp reinforcesomeof theconceptswe’ve learned in thisbookandtogetyoustarteddownthepathonyourownproject,I’mgoingtogooversomeexamplebatteriesforcommonprojects.

Onenote inadvance: thesearegoing tobeverybroadoverviews.Theyaren’tgoingtogetintothenitty-grittydetailsthatareuniquetobuildingeachproject.Each project could be a book in itself. I’m just going to focus on the batterybuildingandkeeptheoverviewatafairlybroadlevel.Ifyouwanttotakeadeepdive into these typesofprojects, thenyoushouldspendmore timeresearchingthespecificsthatgointotheentireprojectandpartsselection.

Ok,nowlet’sgetstarted.

5VbackupbatteryandUSBdevicecharger

There are thousands of different backup batteries out there. Some are evenavailableforcheaperthanit’dcost tobuildoneyourself.But justforfun, let’slookathowwecouldmakeoneourselves.

First I’ll choose a cell type. I couldmake a very small charger with a single18650cell.That’sthecellformatusedinthecommon“lipstickstyle”cellphonebatterychargers.Butlet’smakesomethingwithabitmorecapacity.I’llchooseali-ionpouchcell.Aquicksearchturnsupa4AhpouchcellonAliExpressfor$7.Itevencomeswithwiresalreadyconnectedtothetabs,perfect.

Mybatterycellis3.7Vnominal,butIneedtooutputaconstant5VtoaUSBport to chargeUSBdevices such as a phoneor e-reader.Thatmeans I need asmall5VDC-DCboostconverter.AnotherquicksearchonAliExpressandI’vefound a smallDC-DCboost converter rated for 1A discharge. It even comeswiththeUSBportalreadyattached.Ithasalowvoltagecutoff(LVC)of2.5Vwhichisalsoconvenientlytheminimumallowablevoltageformyli-ioncell.I’llprobablywant to test that tomakesure theLVCworks,but it soundsgoodsofar!

Lastly,Ineedachargingboard.BacktoAliExpress,whereI’vefoundaTP4056chargingmodulefor$0.37.Shippingisanother$0.35though.Ouch,that’swheretheygetyou.

TheTP4056modulehasamicro-USBporttoaccepta5Vinputandwillchargemyli-ioncellupto4.2V.ExactlywhatIneed!

BecauseI’musingapouchcell,Ineedsometypeofcasetoprotectit.Onelastsearchfor“plasticelectronicsenclosure”bringsupapileofoptions.I’llchooseonethatfitsmycellandhasalittlemoreroomforthetwoelectronicsboards.

NowI’vegotallofmypartsandIjustneedtocombineeverything.I’llstartbysoldering thewires frommypouch cell ontomyTP4056 chargingboard.Thepouchcell’sredwirefromthepositiveterminalgetssolderedtotheB+padonthechargingboardandtheblackwirefromthenegativeterminalonthepouchcellgetssolderedtotheB-padonthechargingboard.

Now I should test the charging circuit. I’ll plug in a micro-USB cord to theTP4056chargingboard,connecttheotherendtoany5Vcellphonechargerorevenmy laptop, then wait. And watch. I don’t see flames, so that’s good. AcouplehourslaterandthelittleredLEDswitchestogreen.Myvoltmetershowsmethatthecellisat4.20V.Excellent,it’sworkingsofar.

Now I’ll add the discharging board, which is myDC-DC converter. I simplysoldertheredpositivewirefromthebatterytothe“In+”padonthedischargingboardfollowedbythenegativeblackwirefromthebatterytothe“In-”padonthedischargingboard. Icanalso justaddawire fromthepadson theTP4056chargingboardtoo,sinceIconnectedtheminthepreviousstep.

Aquick testwithamultimeter showsme that theUSBport isoutputting5V,which means that it should be working. Now for the final test. I’ll plug myphone’sUSBchargingcableintotheUSBportonmyFranken-chargerandtheotherendintomyphone.Myphonelightsupandstartscharging.It’sALIVE!

NowIcanseal thewholethingup.Ishouldprobablywrapthepouchcell inathinsheetoffoamtoprotectitfromthehardcaseifIweretodropit,thenIcanjusthotglueitintothecase.Thetwocircuitboardscansitontopofthecellorintheendoftheplasticcase,andI’llcutalittlespacefortheirUSBportsaswell.Ishouldprobablycutaholeso thatIcansee thechargingLEDonthechargingboardtoo.

Andthat’sit.Done.Finito.Terminado.Gamur.

12sRClipoelectricskateboardbattery

Electricskateboardscanrunonmanydifferentvoltages.TheygenerallyuseRCairplanemotorsandcontrollers,sothat’swhatwe’llassume.Andbecausewe’realready using RC parts, we might as well go with an RC lipo battery forsimplicity.Itwillmakeiteasytobuildthebatterywithoutanyspecialtoolssuchasa spotwelder.Remember though, thesebatteries canbedangerous sowe’lltakeproperprecautionstoensurethatwebuildasafebattery.

12sisacommonconfigurationforanelectricskateboard.At3.7Vpercell,thatworksouttoanominalvoltageof44.4V.

Wearegoing tobuildafairly lightweightbatterybecausewearen’tgoing thatfaronourskateboard.Let’ssaywewant5miles(8kilometers)ofrange.Doinga little research showsme that 20Wh/mile (12.5Wh/km) is a fairly commonefficiency figure for electric skateboards. That means we’ll need 100 Wh totravel5miles.At44.4V,thatmeansweneeda2.25Ahbattery.

Requiredcapacityofthebatteryinamphours=100Wh÷44.4V=2.25Ah

CheckingontheonlineRCshopHobbyKing,Icanfind6sRClipobatteriesthatareratedfor2,200mAh(2.2Ah).That’scloseenoughformyneeds.IfIwantedto be safe though, I could always choose a battery pack with just a slightly

highercapacity.

I’llneedtwo6sRClipobatteriesinseriestomakea12sbattery.The6sbatteriesalreadycomewithbulletconnectors,soI’llgrabafewmoreofthosealongwithsome12awgsiliconewireformyconnections.

I’lltapethetwoRClipobatteriestogetherandmakeashortdouble-endedmalewirewithmybulletconnectorsbysolderingamalebulletconnectorontoeachendofashortlengthofwire.Iwillthenuseittoconnectthepositiveterminalofthefirst6sbatterytothenegativeterminalofthesecond6sbattery.NowI’vegota 12s battery. Technically it’s a 12s1p battery pack, though it could also bedescribedasa2s1ppackof6sRCbatteries.Samething.

The twodischargewires fromtheRClipopacks that Ididn’tusewillbecomethemaindischargewiresformy12sbattery.Thosewillplugintothecontrolleronmyelectricskateboard.

For charging, I’ll need a balance charger. An iMAX B6 is an inexpensivebalancecharger thancanhandleup to6sbatteries. Icouldchargeeachbatteryoneatatime,butthat’dbeannoying.Instead,I’llpurchaseanRClipo“balanceboard”whichallowsme tochargebothbatteries atonce. I justdisconnect thetwo6sbatteriesfrommyelectricskateboard’sspeedcontrollerandplugboththedischargewiresandtheirbalancewiresintothebalanceboard.ThenIplugthatbalance board into the iMAXB6 balance charger and it will charge bothmybatteries at once.When it’s done charging, I can rewiremybatteries in series(carefullysothatIdon’tmakeawiringmistake)andthenplugthembackinto

thecontrolleronmyelectricskateboard.

Aquicknote:Ishouldn’teverletthesebatterieschargeunsupervised.IfIleavethe house or office before they’ve finished charging, I’ll disconnect them andwaituntilIreturntofinishcharging.Safetyfirst.

Onelastsafetystep.It’scriticalthatIdon’tdischargemyRClipopackstoolow.Doing so canbedangerous anddamage thebatteries.Toprevent this, I’ll buytwoRC lipo lowvoltagealarms.When I ridemyelectric skateboard, I’llplugonedirectlyintoeachbalancebalanceconnectoronmyRClipobatteries.TheyeachhaveasmallscreenthattellsmethevoltageofeverycellandIcansetthevoltagethatthealarmwillsoundattoletmeknowthatI’vereachedmydesiredcutoff.

Technically,Icanletitdropdowntoabout3.0Vbutthat’dbedangerouslylow.There’sno reason to take that risk. I’ll setmycutoffat3.3V tobesafe.NowwhenIridemyelectricskateboard,ifIeverhearthealarm,Iknowit’stimetostopriding.Icanalsouseitasasimple‘fuelgage’bycheckingthevoltageofmybatteriestoknowhowclosetheyaretoempty.

Andthat’sit.I’vebuiltasimple12sRClipobatteryandI’vetakenafewextrastepstomakeitassafeaspossible.Onethingtonote:Iprobablywon’tgetthecomplete2.2AhoutofmybatteriessinceI’mnotdischargingthemdownto0%stateofcharge(asthat’dbedangerous).SoitmightbeagoodideatostartwithbatteriesthathaveaslightlyhighercapacitythanwhatIthinkI’llneedsothatIcanstillgothesamedistancewithoutcompletelydischargingthebatteries.

10s(36V)electricbicycletrianglebattery

Mostelectricbicyclesusebatteries in therangeof24-48V.I’llbuilda36VbatteryandI’llaimforacapacityof10Ah,asthat’salsoquitecommonintheebikeindustry.

Iplantoputmybatteryina trianglebagin thecenterofmybikeframe,butIwant touseasmall trianglebagtonot takeuptoomuchspace.SoI’llbuildatriangle shaped battery to most efficiently use the space. I could build arectangular battery, whichwould bemuch simpler, but then I’d need a largertrianglebaganditwouldwastealotofspaceinthecornersofthebag.

I’lluse18650li-ionbatterycells.IknowIhavea15AcontrollersoIneedmybattery tohaveacontinuousdischargecurrent ratingofat least15A.Tomeetthecurrentandcapacityrequirements,I’llchoosetheNCR18650GAcellmadebySanyo/Panasonic.Ithasacapacityof3,500mAhandamaximumcontinuousdischargecurrent ratingof10A. Icanuse threecells inparallel tomake10.5Ah, and three cells in parallel will also give me up to 30 A of continuousdischargecurrentability.That’s twicewhat Ineed,which isgreat.ThatmeansI’llbeusingthecellswellwithintheirratedlimits,whichismuchhealthierforthecells.

Thisresultsina10s3ppack,whichmeansI’mgoingtouse30cells.TimetofireupmyfavoritePhotoshopknock-offsoftwareanddesignabatterylayout.

IalsoneedaBMS.ThistimeI’llheadtomyfavoriteAliExpressBMSsource,which isavendorknownas“Greentime”.They’vegota10s li-ionBMSratedfor 30A continuous discharge.Again, that’s twice the 15A discharge currentthatI’llbeusingonmyebike,whichmeansIwillhaveasafetyfactorof2fortheBMSaswell.Excellent.

NowIcanbuildmybattery.I’lluseahobbylevelspotwelderandjointhecellsusingpurenickel stripsof0.15mm thickness and8mmwidth.Sinceeachofthesepiecesofnickelcancarryaround5Aeach,I’llneedthreepiecesofnickelforeachseriesconnectiontosupportmy15Aexpectedload.InsomeplacesIcanweldapieceofnickelbetweeneachof the three cells in adjacentparallelgroups(suchasthe+2to-3seriesconnection,whichisonthebacksideofthebattery in the diagram).However, there are a fewplaceswhere a single pointconnects twoparallelgroups (suchas the+1 to -2 seriesconnection). In thoseplaces,I’llneedtousethreelayersofnickel.

Tokeepthisbatterysmall,I’llusehotgluetojointhecellstogetherinsteadofsnap-togethercellconnectors.I’llstartbygluingupthefirsttworowsof6cells.ThenI’llweldasinglepieceofnickelacrossthenegativeterminalsofthethreecellsinthefirstparallelgrouponside1.Thatwillbemynegativeterminalforthefinishedpack.

ThenI’ll turnthebatteryover toside2andwelda longpieceofnickelacrossthepositive terminalsof thosesamethreecells in thefirstparallelgroupAND

thenegativeterminalsofthethreecellsinthesecondparallelgroup.Thatsinglepieceofnickelcoveringallsixcellsjustmadeaparallelconnectioninthefirstparallel group, a parallel connection in the secondparallel group, and a seriesconnectionbetweenthefirstandsecondparallelgroups.ButIneedmorenickelforthisseriesconnectionbetweengroups1and2becausethereisonlyasinglepointofconnection.SoI’lladdtwomorestripsofnickel:thefirstwillbelongenough to cover the inner four cells (two in each group) and the secondwillcoveronly themiddle cells (one in eachgroup).That givesme three stripsofnickelwhichisenoughnickelateverypoint in theseriesconnectiontohandlethemaximumamountofcurrentthatthebatterywillexperiencebasedonmy15Aload.

Now I’ll turn the battery back over to side 1. Here I’ll make the seriesconnectionsbetweenthepositiveterminalsofgroup2andthenegativeterminalsofgroup3.Theorderdoesn’t reallymatter. I’ll do the series connections firstsincetheyaremorecriticalandI’dliketohavethenickelinthoseconnectionsweldeddirectly to the cell terminals.Here I canweld a single piece of nickelbetweeneachof thethreepairsofcells.ThatmeansIwon’tneedtodoanyofthat pyramid layering business. Nice and simple. After I do the three seriesconnections,Icangobackandaddaparallelconnectiontothepositiveterminalsofgroup2andtothenegativeterminalsofgroup3.

I’llflipthebatterybackovertoside2andcontinuetheconnectionsinthesameway.Forthenextconnectionbetweenthepositiveterminalsofgroup3andthenegativeterminalsofgroup4I’llneedtodothepyramidlayeringagainsinceIhaveonlyonepointofconnection.ThenI’llglueononemorerowatatimeasIcontinuetomakeconnectionsallthewaytothetenthparallelgroup.

LastlyIneedtoaddmyBMS.I’llsolderthenegativedischargeandchargewires

totheBMSboardandthentothenickelstripsonmybattery’snegativeterminal(thenegativeterminalofparallelgroup1).I’llsolderthewiresonthenickelinbetweenthecellstomakesureIdon’theatupthosecellstoomuch.Icouldhavealsosolderedthewiretothenickelfirst,andthenspotweldedthenickel,butthatdoesn’tleaveyoualotofroomforspotweldsifyourwireisverythick.ThenI’llsolderonmypositivedischargeandchargewires to thenickelstripon thepositive terminals of the tenth parallel group.Lastly I’ll solder on the 10 thinbalancewires.

Tofinishmybattery,I’llwrapitinalayerof2mmEVAcraftfoamtoprovideasmall amountof shockabsorptionand thenusea fewsheetsof largediameterheatshrinktosealthebattery.Thetriangleshapemakesitabitodd,butplayingaroundwithafewsizesofheatshrinkwillallowmetosealthewholethingfromafewdifferentangles.

Lastly,I’llneedacharger.ThesamevendoronAliExpressthatIgotmyBMSfrom,Greentime, also sells chargers. I’ll select a 42.2V charger, as that’s theappropriatevoltage formy10s li-ionbattery.Analuminumshellcharger ratedfor2Achargingwilldonicely.Thatshouldgivemeabout5-6hoursforafullcharge.Plentyof timeformyneeds.If Iwantedafastercharger though,Icanchoose anything up to 4.5A, asmy cells are rated for around 1.5A each forchargingcurrent.Idon’twanttopushthemtoohardthough,so2Atotalisfineforme.

Andthat’sit!That’sallIneedtobuildatrianglebatteryforanelectricbicycle.

38s(120V)LiFePO4prismaticcellelectriccarbattery

Nowit’stimeforabigproject.Let’sbuildabatteryforanelectriccar!

Forcells,IthinkI’llgowithaprismaticformat.ThatwayIdon’tneedtoworrytoomuchaboutaprotectivecase-manybigprismaticcellsforEVusealreadycomeinrigidplasticcases.

ThereareanumberofgoodoptionsintheLiFePO4chemistry,andtheyarebothsaferandlongerlasting,soI’llchoosethatchemistry.

NowIneed tocalculatemyneeds.Let’ssayIknowI’llbeusinga120VDCmotoranda200Aelectronicspeedcontroller.Thatmeansmyvoltageisdecidedforme,Ineedaround120voltstopowermycontrollerandmotor.ForLiFePO4cellswithanominalratingof3.2Vpercell,thatmeansI’llneedaround38cells.

ButhowmuchcapacitypercellwillIneed?Well,let’ssaythatIwanttotravelaround 55 miles on a charge. And I’ve done my research, so I know thatcompared tootherswithsimilar setups,mysystemshouldgetanefficiencyofaround200Wh/mi.NowIcancalculatemycapacityneeds,firstinwatthours.

Requiredcapacityofbatteryinwatthours=200Wh/mi×55miles=11,000Wh

SoIneedbatterywithacapacityof11,000Wh.SinceIknowthevoltageI’llbeusing,IcannowcalculatetheAhI’llneedforeachcell.

Requiredcapacityofbatteryinamphours=110,000Wh÷120V=91.7Ah

Thatmeans I need to choosebattery cells that are at least 91.7Ah.Searchingaround,Ifindmultiplesourcesfor100AhLiFePO4prismaticcells.Ibuy38andI’mnearlyreadytogo.IjustneedaBMS.Isearcharoundandfinda38sBMSthatisspecificallymadeforLiFePO4cellsandcanhandleover200Aofcurrent.NowI’mreadytostartplanningmylayout.

Mybattery cells are 2.5 inches (6 cm)wide,meaning that if I linedup all 38cells,mybatterywouldbenearly8feet(240cm)wide!That’snotgoingtofitinmytrunk.Instead,I’lljustmakethreerowsofcells,whichwillhaveafootprintofabout2.5ft×1.5ft(76cm×42cm).That’smorereasonable.

My prismatic cells have convenient screw terminals for connections. I decidethatI’llusecopperbusbarstoassemblemybattery.Icaneitherbuythebusbarsonlineorjustbuysomecopperbarstockanddrilltheholesmyself.I’llwanttocheckwithabusbarcharttomakesurethatthesizeofmybusbars(andtypeof

copper)aresufficienttocarrythe200Amaximumcurrentthatmycontrollerisexpectedtodraw.

Because I’m using 100 Ah battery cells, I don’t need to make any parallelconnections.Instead,Icansimplywireallofmycellsinseries.

Lastly,I’llneedtoconnectmyBMS.Itconnectsjustlikeinmyelectricbicyclebatteryexampleabove,butbecauseI’musingcopperbusbarsIcanjustaddaringconnectedtothewiresandIdon’thavetosolderanything.

For a charger, I need around 140 V to fully charge my LiFePO4 cells.Unfortunately,achargerthatsizeisn’tgoingtobecheap,butacompanynamedZivanprovidesmewitha140Vand18Achargerthatshouldchargemybatteryfromemptytofullinaround7hours.

LiFePO4chargersareverycommonlyavailablefor incrementsoffourcellsonthe lowervoltageranges,suchas24V,36Vand48V(8s,12sand16s).OneneattricktosavemoneyonahighvoltageLiFePO4chargeristouseamultipleoffourcells,suchas36cells,andthenyoucouldjustusethree36VLiFePO4chargersconnect toyourbatteriesinthreesubgroups.It’salittlemoreworkasyou’llhavetousemultipleBMSstomakesub-batteries,butthoselowervoltagechargersaremucheasiertofind.

14s(52V)18650homeenergystoragebattery

This type of battery is mostly known as a DIY powerwall and is perfect forsupplementingyourhomeenergygenerationprojectorforbuildinganoff-gridsystem. If you have solar panels, awind turbine or any other form of energygenerationonsite,aDIYpowerwallcanstorethatenergytobeusedatamoreconvenient time, or sell it back to the grid duringpeak energydemand.Somepeopleevenusehomeenergystoragebatteriesto“buy”electricityatnightwhenitscheapandthenusethatelectricitystraightfromtheirbatteriesduringthedaywhenitwouldcostmoretogetitfromthegrid.

ThereisabigDIYpowerwallcommunityonlinethatspansanumberofwebsitesand forums.ThevastmajorityofDIYpowerwallbuildersuse salvaged18650cells,sothat’swhatwe’lldointhisexample.

I’llassumethatIwanttousea48Vinverterformyenergyuse,sothatmeansIneeda13sor14sli-ionbattery.Thebestoptionis14s,asthelowvoltagecutoffofaround42Vismoreinlinewithmostcommercial48Vinverters.

IcancalculatemybatteryneedsbydetermininghowmuchenergyIwanttouseeachday.Let’ssaythatIhaveagoalofstoringatleast5,000watthours.Fora52Vli-ionbattery,thatmeansI’llneedaround96Ah.Let’sjustrounduptoaneven100Ah.

Requiredcapacityofbatteryinamphours=5,000Wh÷52V=96.2Ah

I stated that I want to use salvaged cells. Assuming I can find a pile ofapproximately2Ah18650cells,thatmeansI’llneedaround50cellsperparallelgrouptoreachmygoalofabout100Ah.Noticethateverythingis“about”and“around”and“approximately”.Suchislifewhenyou’reusingmysterysalvagedcells.

Ok, time togodumpsterdiving.Ormore likely, time tovisit the localbatteryrecyclingcenter.Afterbuyingabout150lbs(70kg)ofrecycledbatterypacks,Icancrackthemallopenandpullapartthebatterycells.NowI’veprobablygotatleastathousandorso18650cells.Icanusea4-cell18650tester,orbetteryetanarmyofthosetesters,tocheckthecapacityofallofthosecells.ThiswilltakeafewweeksdependingonhowmanycellsIcantestatonce.

I ultimately end up with 700 cells that are all fairly close in capacity andsomewhatabove2Ah.Somemightbe2,050mAh,somemightbe2,110mAh,buttheyareallfairlyclose.Generallywewouldwantallcellsinaparallelgrouptobeexactlythesamecapacity,butwe’lltakethebestwecangetwhenworkingwithsalvagedcells.

I’ll prepare the cells by sanding their terminals smooth to remove any spotweldednickelshardsthattrytohangonthere.

NowI’lldividethemupintogroupsof50andtrytokeepthemasclosetothesamecapacityaspossible.ThenI’llconstructparallelmodulesof50cellsusingsnap-togetherblockconnectors.

Ideallythese50cellsineachmodulewouldbespotweldedtogether.However,theDIY powerwall community usually solders these, so that’swhat I’ll do inordertodemonstratethatmethod.Theyalsousuallyusecelllevelfuses,soI’lldothattoo.

Icutapileoflegsoffof¼Wresistorstouseasthecellfuses,thensolderthemtothecellterminalsusingathicksolderingirontiptoholdintheheat.Imighthavetousefluxaswell,dependingonthecellterminals.IcontactthesolderingirontiptotheendofeachcellforaslittletimeaspossiblesothatIdon’ttransfertoomuchheattothecell.

Theotherendsof thefusesaresolderedtooneormorecopperbusbars.Thosewillmakethepositiveandnegativeterminalsfortheentiremodule.

Oncemyparallelgroupsareassembled, I’llwire them inseriesbyconnectingeachof thebusbars to thenextparallelgroupuntil I’vebuilt anentire14s50pbattery.Copperwirewithringconnectorsonboltspassingthroughthebusbarswouldbeagoodwaytowiretheminseries.

Lastly,I’llneedtoaddabalanceconnectorwithabalancewirethatconnectstothepositiveterminalofall14parallelgroups.MostDIYpowerwallfolksdon’tuse a BMS but rather use a balance charger. A big 14s balance charger isn’tcheap, but it’s what I need for this battery to keep all of thosemystery cellshappyandworkingforaslongaspossible.

IdeallyI’llusethispackatalowCrate;0.5Cwouldbeagoodupperlimit.Thatmeans each cellwon’t be deliveringmore than 1A,which should keep themhappyandhealthy.At52V,amaximumcurrentof50Awouldgivemearound2,000 W of maximum continuous power. That’s enough for my off-griddoomsdaycabin.Besides,Iwouldn’twanttoruntoomanydevicesatonceandriskdrawingattentionfromthezombies.

4s(14.8V)18650RCdroneorFPVbattery

AgreatuseforDIYlithiumbatteriesisforRCdronesandaircraft.MostoftheseaircraftuseRClipobatteries,butwecanactuallybuildalighter18650batterythatwill givemore flight timewith lessweight.Forpeople in the firstpersonview(FPV)flyingcommunity,fighttimeiskey.

Let’ssayI’vegotanRCFPVairplanethatoperateson4s(14.8V).Andlet’ssaythatIgenerallyusean8AhRClipopackthatweighsaround415grams.Wecanbuildsomethingbetterthanthat!

Let’sstartwithourcurrentrequirement.Let’ssaymyairplanedrawsamaximumof30Abutgenerallycruisesataround10A.If Iuse thesameNCR18650GAcellsfromtheelectricbicyclebatteryexampleearlierinthischapter,Icouldusethreecellsinseriestogetamaximumcontinuouscurrentratingof30A.That’stheextremelimitofthecellsandexactlymatchesthepeakcurrentourairplaneuses.Thatwouldbeabitcloseforcomfortifthatwastheairplane’scontinuouscurrent,butmyairplaneusuallycruisesataround10A.Somybatterywillonlyseeburstsof30Awhen theplane is climbingaggressively.That’s finebymeandwithinthelimitsofthebatterycells.

Three cells in parallelwould giveme10.5Ah,whichwill provide evenmoreflighttimethanmyold8Ahbattery.I’llneeda4s3parrangementforatotalof12cells.

I’ll justhotglue thecells together to saveonweightandspace.Then I’ll spotweldthemusingthesamemethodIusedintheelectricbicyclebatteryexample.However, itwill bemuch easier this time as I’ve got straight rowsof parallelgroups.

IfI’musingnickelstripthatcanhandle5Aperstrip,thenI’llneedsixpiecestosupport the30Amaximumcurrent that I expect thebattery to supply.Since Ihave three cells in eachparallel group, I’ll use two layers of nickel per seriesconnection between each pair of cells. Thatwill giveme six pieces of nickelstripforeveryseriesconnection.

Iwon’tuseaBMSbecauseIwanttosaveonweight,soI’llsoldermydischargewiresdirectlytothefirstandlastcellgroupandadda4sbalanceconnector.ThatmeansI’llhavetouseabalancecharger,suchasaniMAXB6,tosafelychargeandbalancemybattery.

Lastly,I’lluseapieceofheatshrinktocoverthewholebatterysotheexposedterminals can’t be shorted on anything. This results in a battery that has 2moreAh thanmy originalRC lipo andweighs about 200 g less. Thatweightsavingsmeansevenmoreflighttime.Timetogocruisetheskies!

Conclusion

Ihopeyoufound thisbookboth informativeanduseful.Lithiumbatterieswilllikelybepoweringourdevicesfordecadesintothefuture.Learninghowtobuildyour own batteries opens the door to a whole newworld of custom powereddevices.Fromvehicles toon-siteenergystorage togamesandevenwearables,makingyourowncustomlithiumbatteriesenablesyoutobuildandcreateonawholenewlevel.

Withthatinmind,itisimportanttoalwaysmakesafetyaprioritywhenworkingwith lithium batteries. With the good comes the bad. Lithium batteries drivetechnologyforwardbuthavealsocauseddeathanddisasterswhenengineersanddesignerscutcornersorignorestandardsafetypractices.

AsBenParkersaid,“withgreatpowercomesgreatresponsibility”.You’vebeengiven the knowledge, and now youmust use it responsibly. Accidents due toimproperlithiumbatteryuseleavealastingmarkontheentireindustry.Pleasehelp by being a force of good moving the industry forward in a safe andresponsibleway.

Acknowledgements

The idea for this bookwas born out of a conversationwith a friend ofmine.Thankyou,ElieFuhrman,forsparkingmycreativity.Iofcourseneedtothankmywifeaswellforputtingupwithme,notjustwhileIwrotethisbook,butalsofor years of finding half completed batteries laying all around the housewith“danger - don’t touch!” signs on them. My parents taught me to follow mydreamsandnot stopuntil I could say that I’d either accomplishedmygoal orproudly gave it everything I had. I appreciate everything they’ve done formeand gratefully acknowledge that if I’m a decent person then it’s due to theirexcellentparenting.MydadstartedmeonmyjourneytobecomingaMaker,andwhenIwentofftocollege,mymentorAndyHolmespickeditupfromthere.Ithink I learnedmore fromworking inAndy’s shop than I did frommy entireengineeringdegree.AndIlearnedaLOTfrommyengineeringdegree.IoweabigthankstoDamianRenewhotraveleddownthelithiumbattery-buildingroadwithmeaswesimultaneouslylearnedmoreaboutbatterybuilding.Beingabletobouncenewideasanddiscoveriesoffofeachotherwasanincredibleresource.Lastly,I’dberemissifIdidn’tthankMaxPlessandThorinTobiassen,whowerethereduringapivotaltimeandhelpedmegetintoebikes.Thisultimatelystartedmeonawildride(literallyandfiguratively)thatinpartledtowhereIamtoday.Thanksguys.

Abouttheauthor

MicahToll is amechanical engineer andentrepreneurwithnearlyadecadeofexperienceintheelectricbicycleindustry.Micah’sbookTheUltimateDIYEbikeGuidehassoldthousandsofcopiesallovertheworld.Hehasbuiltnearlyeverytype of custom lithium battery for consignment and for his personal projects.Micahbelievesintheprincipleof“Don’tbuywhatyoucanmake”andpromotesaMaker lifestyle based on handiness, resourcefulness and skill collecting.HecurrentlylivesinTelAvivwithhisbeautifulwifeSapirandhisdogSeven.

References

TheinformationinthisbookcomesfromacombinationoftheknowledgeI’vegained over years of experience and diligent research from academic andindustry literature. Ihope these sourceswillbeofasmuchuse toyouas theyhavebeentome.

al,QingsongWanget."Thermalrunawaycausedfireandexplosionoflithiumionbattery."JournalofPowerSources208(2012):210-224.

al,RizaKizilelet."AnalternativecoolingsystemtoenhancethesafetyofLi-ionbatterypacks."JournalofPowerSources194.2(2009):1105-1112.

Buchmann,Isidor.BU-205:TypesofLithium-ion.30November2016.31January2017<http://batteryuniversity.com/learn/article/types_of_lithium_ion>.

—.BU-410:ChargingatHighandLowTemperatures.2April2016.22January2017<http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures>.

Dahn,Jeff."WhyDoLi-ionBatteriesDieandCanTheyBeImmortal?"WINSeminar.WaterlooInstituteforNanotechnology,Waterloo.30July2013.

Golubkov,AndreyW.etal."Thermal-runawayexperimentsonconsumerLi-ionbatterieswithmetal-oxideandolivin-typecathodes."RoyalSocietyofChemistry7(2013).

Marom,Rotem,etal."Areviewofadvancedandpracticallithiumbatterymaterials."JournalofMaterialsChemistry27(2011).

SearchfortheSuperBattery.Perf.DavidPogue.Prod.ChrisSchmidt.PublicBroadcastingService,2017.

Svoboda,JamesA.IntroductiontoElectricCircuits.Vol.9thEdition.NewYork:WileyPLUS,2013.

vanSchalkwijk,WalterAandBrunoScrosati.AdvancesinLithium-IonBatteries.NewYork:KluwerAcademicPublishers,2002.

VariousAuthors.2007-2017.2010-2017<www.EndlessSphere.com/forums>.

Wu,Yuping.Lithium-IonBatteries:FundamentalsandApplications.BocaRaton:CRCPress,2015.

Yuan,Xianxia,HansanLiuandJiujunZhang.Lithium-IonBatteries:AdvancedMaterialsandTechnologies.BocaRaton:CRCPress,2011.

Index

18650cells,15,17,25,46,56,61,91,135,136

21700cell,14

3Dprint,54,55

abstractshapes,85,87,89

Accidents,55,139

balanceboard,129

balanceconnector,108,121,130,136,138

balancewires,49,53,96,103,105,106,107,121,129,132

batteryshape,81

BMS,22,48,50,51,52,53,70,79,91,96,103,104,105,106,107,108,109,110,117,118,119,120,121,123,131,132,133,134,136,137

bulkcharging,49,118,120,121

busbars,57,80,93,95,96,112,134,136

Crate,31,32,75,76,116,137

cellholders,54,55,57,82,93,98,112

cell-levelfuses,65,66

charger,48,49,50,51,91,103,104,117,118,119,120,121,123,127,128,129,133,134,135,136,138

controller,69,104,129,130,133,134

cyclelife,16,18,19,23,33,36,45,79,123

cylindricalcells,14,15,46,47,54,55,56,58,62,64,81,86,89,91,93,96,98,100,101,113

DC-DCconverter,68

dischargecurve,67

DIYpowerwall,26,28,65,66,112,135,136

electricbicycles,8,22,35,69,115,130

electriccar,8,34,133

electricmotors,35,71

electricskateboard,34,108,113,128,129,130

electricvehicles,8,9,14,17,19,26,29,47,102,122

electrolyte,10,12,19,20,23,45,64,123

factorofsafety,76,77,78

fire,16,21,22,37,41,55,57,59,97,112,119,122,131

flux,64,65,136

foam,57,108,113,128,132

gloves,42,90,99

hardcase,57,113,128

Headwaycells,54,58

Heatshrink,57,113

highpower,17,18,19,20,21,23,24,52,56,59,77,99,110,115,116

highvoltagecutoff,69

Hotglue,55,56

Kirchoff'scurrentlaw,100

LG18650HB2,18

LiCoO2,18

LiFePO4cells,22,23,24,34,36,40,46,52,67,69,70,77,79,118,133,134

LiFePO4prismaticcells,133

li-ioncells,12,16,17,18,19,20,21,22,23,24,30,34,36,37,38,40,52,63,69,77,79,110,118,120,123,124

LiMn204,17,18

LiNiCoAlO2,19

LiNiMnCoO2,18

lithiumpolymer,19,20,21

lowpower,17,34,58,61,77,98,104

lowvoltagecutoff,69,70,124,127,135

maximumcontinuouscurrent,31,137

mistakes,45,86,95

modular,82

ohm’slaw,71

overheating,31,37,63,100,119

PanasonicNCR18650B,19

Panasonic/SanyoNCR18650GA,75

Parallelconnections,37,100

parallelgroup,27,37,48,49,50,53,65,66,85,92,97,101,104,105,106,107,108,118,121,131,132,135,136,137

plasticcases,57,58,112,133

porousseparator,10,12,20

Pouchcells,12,13,47,62,63,64,112

price,16,25,27,52

Prismaticcells,13,47,57,58,112

puff,13

RClipo,16,18,19,20,21,22,23,24,25,49,52,57,77,79,95,96,128,129,130,137,138

rectangle,85,87

recycling,27,126,135

research,8,9,19,129,133,142

restvoltage,23

Runtime,78

safety,7,16,18,22,23,36,37,52,54,65,69,76,77,79,90,91,93,122,130,131,139

salvagedcells,27,28,65,66,92,112,135,136

SamsungICR18650-26F,17,18

SamsungINR18650-25R,19

seriesconnections,34,37,38,41,42,48,80,83,84,85,89,95,96,97,100,101,102,103,110,132

shortcircuit,40,41,42,43,44,47,57,63,65,86,90,91,99,108,112,126

solarpanels,9,28,135

solder,42,43,53,64,65,90,94,103,104,106,128,132,134,136,137

soldering,44,52,63,64,65,66,90,94,104,105,106,128,129,136

SonyUS18650VTC5,17

spotwelded,15,58,60,63,64,88,95,101,103,104,105,132,136

Springloadedbatterycontacts,58

springterminals,58

tabterminals,13

tabs,38,47,63,64,127

thermalrunaway,18,23,45,46,63,64,79,91,122

threadedterminals,13,58

TP4056chargingboard,128

triangle,130,132,133

venting,46

voltagealarm,70

voltagedrop,58,67

voltagesag,23,33,36,67,70,117

windturbine,135

wiregauge,88

wiring,40,43,48,57,67,96,103,104,107,129

wood,57,98