nickel cad batteries
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INTRODUCTION OF Ni-Cd
BATTERIES Nickel-cadmium cells have an anode
(negative) in cadmium hydroxide and a
cathode (positive) in nickel hydroxide,immersed in alkaline solution
(electrolyte) comprising potassium,
sodium and lithium hydroxides. The
cells are rechargeable and deliver a
voltage of 1.2 V during discharge.
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Types of Ni-Cd batteries
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AMPERE- HOUR RATING FOR
BATTERIES An ampere-houroramp-
hour(symbol Ah , Ah, A h) is a unitofelectric charge, with sub units milli-
ampere-hour(mAh) and milli-amperesecond (mAs).
One ampere-hour is equal to
3,600coulombs (ampere-seconds), theelectric charge transferred by a steadycurrent of one ampere for one hour.
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The ampere-hour is frequently used in
measurements ofelectrochemical systems
such as electroplating and electricalbatteries.
The commonly seen milliampere-hour (mAh
or mAh) is one-thousandth of an ampere-
hour (3.6 coulombs).
A milliampere second (mAs) is a unit of
measure used in X-ray diagnostic imaging
and radiation therapy.
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This quantity is proportional to the total X-
ray energy produced by a given X-ray tube
operated at a particular voltage.
The same total dose can be delivered in
different time periods depending on the X-
ray tube current. The Faraday constant is the charge on
one mole of electrons; approximately
equal to 26.8 ampere-hours. It is used inelectrochemical calculations.
An ampere-hour is not a unit ofenergy.
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In a battery system, for example, accuratecalculation of the energy delivered
requires integration of the power delivered(product of instantaneous voltage andinstantaneous current) over the dischargeinterval.
Generally, the battery voltage variesduring discharge; an average value maybe used to approximate the integration ofpower.
In summary, the higher the mAh, thelonger the battery will last.
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CHARACTERSTICS OF Ni-Cd
BATTERIES 1.2 Volt secondary cells using an alkalinechemistry with energy density about doublethat of lead acid batteries.
They use nickel hydroxide Ni (OH) 2 for thepositive electrode (cathode), cadmium Cd asthe negative electrode (anode) and analkaline potassium hydroxide KOHelectrolyte.
The cells are sealed and utilize arecombinant system to prevent electrolyteloss and extend the useful life.
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They use nickel hydroxide Ni (OH) 2 for the
positive electrode (cathode), cadmium Cd as
the negative electrode (anode) and analkaline potassium hydroxide KOH
electrolyte.
Once the battery of choice for low power
portable products they have lost market
share to the newerNickel Metal Hydride and
Lithium batteries.
Open cells are prismatic in shape, withcasing in plastic (possibly flame retardant) or
stainless steel or nickel steel.
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APPLICATIONS OF Ni-Cd
BATTERIES Motorized equipment
Power tools
Two way radios
Electric razors
Commercial and industrial portableproducts
Medical instrumentation Emergency lighting
Toys
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ADVANTAGES OF Ni-Cd
BATTERIES Low internal resistance (less than half
the equivalent NiMH cells).
High rate charge and discharge ratespossible.
Up to 10C discharge rates for short
periods typical. Flat discharge characteristic (but falls
off rapidly at the end of the cycle) .
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Flat discharge characteristic (but falls
off rapidly at the end of the cycle).
Tolerates deep discharges - can be deep
cycled.
Wide temperature range (Up to 70C). Typical cycle life is over 500 cycles.
Charging process is strongly
endothermic-the battery cools during
charging.
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Rapid charge typically 2 hours, but can be
as low as 10 to 15 minutes.
The columbic efficiency of nickel cadmium
is over 80% for a fast charge but can drop
to below 50% for slow charging.
The sealed nickel-cadmium cell can be
stored in the charged or discharged state
without damage. It can be restored for
service by recharging severalcharge/discharge cycles.
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The electrolyte is commonly available,
low cost potassium hydroxide KOH.
Available in a large variety of sizes and
capacities.
This makes it possible to charge veryquickly, as the I2Rheating and
endothermic chemical reactions
counteract each other.
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SHORTCOMINGS OF Ni-Cd
BATTERIES A major drawback of this technology is its
susceptibility to memory effect.
Originally, the terms memory effect or memory
problem was coined to describe a cyclic memoryproblem where the NiCad battery would
"remember" the amount of discharge for previous
discharges and limit the recharge life of the
battery. The problem is less prevalent withmodern Ni-Cd batteries, which are designed to
avoid cyclic memory issues.
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The memory effect is caused by a change in
crystalline formation from the desirable
small size to a large size which occurs when
a NiCad battery is recharged before it is
fully discharged.
The growth of large crystals increases thecell impedance and can eventually prevent
the battery from discharging beyond that
point and/or cause rapid self-discharge ofthe battery.
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The growth of large crystals can be avoidedby either completely discharging it each
time it is used or by using a NiCad batterycharger which has a built-in dischargecircuit.
Memory effect can sometimes be reversed
by putting the battery through severalcomplete discharge and recharge cycleswhich helps to recover the smaller crystalformations. This is called reconditioning.
NiCad batteries are also prone to damageby overcharging.
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Low cell voltage of 1.2 Volts compared with
primary alkaline cells 1.5 Volts and only quarter
of the capacity of the alkaline cells. Self-re-sealing safety vents must be incorporated
to prevent damage due to overheating and
pressure build up.
Cadmium is a high cost heavy metal and its use inconsumer products is now deprecated on
environmental grounds.
The gradually being phased out in favor ofNickel
metal hydride and which superior energy density
characteristics and performance characteristics.
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TEMPERATURE
CHARACTERSTICS
"Eveready" sealed nickel-cadmium cellsexperience a relatively small change ofoutput capacity over a wide range of
operating temperature. Charging, however, must be done in a
much narrower range.
Temperature limits applicable to operationof the cells are listed in the specificationsheets for each battery.
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The capacities vs. temperature curves
which are on some individual specification
sheets represent cells discharged at thetemperatures shown after charging at
room temperature for 14 hours at the 10
hour rate . This characteristic is also generalized on
the following curve.
Contd./-
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OPERATION OF SEALED Ni-Cd
BATTERIES
Any secondary cell is a combination of activematerials which can be electrolytic oxidized andreduced repeatedly.
The oxidation of the negative electrode occurringsimultaneously with the reduction of the positivegenerates electric power.
In a rechargeable battery both electrode reactions
are reversible and the input of current in theproper direction from an outside source will drivethe primary or discharge reaction backwards andin effect recharge the electrodes.
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In the uncharged condition the positive
electrode of a nickel-cadmium cell is
nickelous hydroxide, the negative cadmiumhydroxide.
In the charged condition the positive
electrode is nickel hydroxide, the negativemetallic cadmium.
The electrolyte is potassium hydroxide. The
average operating voltage of the cell under
normal discharge conditions is about 1.2
volts.
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The over-all chemical reaction of the nickelcadmium system can be considered as:
(Charged) KOH (Discharged). During the latter part of a recommended
charge cycle and during overcharge, nickel-cadmium batteries generate gas.
Oxygen is generated at the positive (nickel)electrode after it becomes fully charged andhydrogen is formed at the negative(cadmium) electrode when it reaches fullcharge.
Cd + 2NiOOH + 2H2O =Cd (OH) 2 + 2Ni (OH) 2
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DISCHARGING OF BATTERY
HIGH CURRENT PULSE DISCHARGE:-
High rate nickel-cadmium cells will deliverexceedingly high currents.
If they are discharge continuously undershort circuit conditions, self-heating may doirreparable damage.
The heat problems vary somewhat from one
cell type to another, but in most casesinternal metal strip tab connectors overheator the electrolyte boils.
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General overheating is normally easy to
prevent because the outside
temperature of the battery can be usedto indicate when rest, for cooling, is
required.
In terms of cut off temperature duringdischarge, it is acceptable practice to
keep the battery always below
45degreeC (113F).
The overheated internal connectors are
difficult to detect.
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This form of overheating takes place in a few
seconds or less, and overall cell temperature may
hardly be affected. It is thus advisable to withdraw no more ampere
seconds per pulse, and to withdraw it at no greater
average current per complete discharge, than
recommended on the data sheet for the"Eveready" cell in question.
In special cases, where cooling of the cell or
battery is likely to be poor, or unusually good,
special tests should be run to check the importanttemperatures before any duty cycle adjustment is
made.
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Output capacity is any discharge composed ofpulses is difficult to predict accurately becausethere are infinite combinations of current, "on"
time, rest time, and end point voltage. Testing on a specific cycle is the simplest way to
get a positive answer.
The cell is designed so that the oxygen formed inthe positive electrode can reach the metalliccadmium surface of the negative electrode which itoxidizes directly.
Thus, in overcharge, the cadmium electrode isoxidized at a rate just sufficient to offset input
energy, keeping the cell in equilibriumindefinitely.
At this point of equilibrium the positive electrodeis fully charged.
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SELF-DISCHARGE Self-discharge characteristics of Energizer
nickel-cadmium cells are shown in the chart
below.
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CONTINUOUS-OVERCHARGE
The overcharge capability of Energizer cylindricalnickel-cadmium cells is outstanding.
The next chart illustrates initial and subsequent
discharge curves after 2 years continuousovercharge without periodic discharges.
The first discharge after the 2 year charge periodyields a slightly reduced voltage curve and 65%
capacity. The second cycle after 2 years continuous
overcharge provides essentially the samedischarge curve as the initial one.
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VARIOUS OTHER BATTERIES
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