Single-Cell Gauging 101

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What is Fuel Gauging Technology?

• Fuel Gauging is a technology used to predict battery capacity under all system active and inactive conditions.

• Battery capacity – Percentage– time to empty/full– milliamp-hours– Watt-hours– talk time, idle time, etc.

• Other data can be obtained for battery health and safety diagnostics.• State of Health• Full Charge Capacity

73%Run Time

6:23

3

Outline

• Battery chemistry fundamentals• Classic fuel gauging approaches

– voltage based– coulomb counting

• Impedance Track and its benefits

Single-Cell Gauging 101Part 3: Impedance Track Benefits

5

• Voltage based gas gauge: Accurate gauging under no load • Coulomb counting based gauging: Accurate gauging under load• Combines advantages of voltage and current based methods• Real-time impedance measurement• Calculate remaining run-time at given average load using both

open circuit voltage and impedance information.

Impedance TrackTM Gas Gauge

V = OCV(T,SOC) - I*R(T,SOC, Aging)

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• OCV profiles similar for all tested manufacturers

• Most voltage deviations < 5 mV

• Average SOC prediction error < 1.5%

• Same database can beused for batteries from different manufacturersfor the same chemistry

Comparison of OCV=f (SOC, T) profiles

100 50 0 SOC %

15

5

-5

-15Volta

ge D

evia

tion

(mV)

100 75 50 25 0 SOC Error %

4

1.33

-1.33

-4

100 75 50 25 0 SOC %

4.2

3.93

3.67

3.4

Open Circuit Voltage (OCV) Profile

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How to measure OCV ?

• OCV measurement allows SOC with 0.1% max error

• Self-discharge estimation is eliminated

Cel

l Vol

tage

(V)

0 0.5 1.0 1.5 2.0 2.5Time (hour)

4.2

4.1

4.0

3.9

3.8

3.7

System ON System ONSystem OFF

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How to measure impedance?

•Data flash contains a fixed table: OCV = f (SOC, T)•IT algorithm: Real-time measurements and calculations

during charge and discharge.

100 75 50 25 0 SOC %

4.2

3.93

3.67

3.4

Open Circuit Voltage Profile

OCV

VBAT

IRBAT

AVG

BATBAT I

V-OCVR =

V = OCV(T,SOC) - I*R(T,SOC, Aging)

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Issue for Traditional Battery Capacity Learning

+

Shipping

Fully Discharge

+

Fully Charge

+

Half Discharge

• Involve a lot of test equipment and time• User may never fully discharge battery to learn capacity• Gauging error increase 1% for every 10 cycles without learning

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• Charge passed is determined by exact coulomb counting• SOC1 and SOC2 measured by its OCV• Method works for both charge or discharge exposure

Learning Qmax without Full Discharge

Q

Cel

l Vol

tage

(V)

0 0.5 1.0 1.5 2.0 2.5 3.0Time (hour)

4.2

4.1

4.0

3.9

3.8

P1

P2

2SOC1SOC

QQ

max

Start of Charge

Measurement PointsOCV

0 0.5 1.0 1.5 2.0 2.5 3.0Time (hour)

Q

P2

P1Start of Discharge

Measurement PointsOCV

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.

current

........resistance updates

Total capacity updates

currentintegration

Cooperation between integration and correlation modes

discharge relaxation charge

currentintegration

voltage correlation

SOC updates

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Impedance Track Fuel Gauge

Advantages• Combine advantages from both voltage based and coulomb counting

• Accurate with both small current (OCV) and large load current

• Throw out inaccurate self-discharge models (use OCV reading)

• Very accurate with new and aged cells

• No need for full charge and discharge for capacity learning

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Fuel Gauging Benefits

• Accurate report of remaining run time• Better Power Management• Longer Run Time

– Power management– Accuracy: less guard band needed for

shutdown– Variable shutdown voltage with

temperature, discharge rate, and age based on impedance

• Orderly shutdown – Automatically save data to flash with

reserve energy when battery dies• Fuel gauging enables mobile applications

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Run Time Comparison Example Impedance TrackTM gauge shutdown vs. OCV shutdown point

• Systems without accurate gauges simply shutdown at a fixed voltage

• Smartphone, Tablets, Portable Medical, Digital Cameras etc… need reserve battery energy for shutdown tasks

• Many devices shutdown at 3.5 or 3.6 volts in order to cover worst case reserve capacity

• 3.5 volt shut down used in this comparison

• Gauge will compute remaining capacity and alter shutdown voltage until there is exactly the reserve capacity left under all conditions

• 10 mAH reserve capacity is used

• Temperature and age of battery are varied

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2000

2500

3000

3500

4000

4500

0 20 40 60 80 100 120 140 160 180

Fuel GaugingOCV vs. IT Use Case exp – NEW battery w/ variable load mix

Conditions:• New Battery

• Room temp (25°C)• 10 mAh reserve capacity for

shutdown

Run Time in Minutes

Vo

ltag

e

Impedance TrackTM Gauge Shutdown @ 3.295V168 minutes run time

Extended runtime with TI Gauge:

+40%

OCV

Shutdown @ 3.5V 120 minutes run time

4.2

3.6

3.0

2.4

Bat

tery

Vol

tage

(V

)

Open Circuit Voltage (OCV)

EDV

Qmax

I•RBAT

Quse

Cell voltage under load

16

2000

2500

3000

3500

4000

4500

0 20 40 60 80 100 120 140 160

Fuel Gauging OCV vs. IT Use Case Exp – OLD battery w/ variable load mix

Conditions• Room temp (25°C)• 10 mAh reserve

capacity for shutdown

Run Time in Minutes

Vo

ltag

e

Impedance TrackTM Gauge Shutdown @ 3.144V142 minutes run time

OCV

Shutdown @ 3.5V 90 minutes run time

Extended runtime with TI Gauge:

+58%

4.2

3.6

3.0

2.4

Bat

tery

Vol

tage

(V

)

Open Circuit Voltage (OCV)

EDV

Qmax

I•RBAT

Quse

Cell voltage under load

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2000

2500

3000

3500

4000

4500

0 20 40 60 80 100 120 140

Fuel GaugingOCV vs. IT Use Case Exp – NEW battery COLD w/ variable load mix

Run Time in Minutes

Vo

ltag

e

Impedance TrackTM Gauge Shutdown @ 3.020V117 minutes run time

OCV

Shutdown @ 3.5V 53 minutes run time

Conditions Batty• Cold (0°C)

• 10 mAh reserve capacity for shutdown

Extended runtime with TI Gauge:

+121%

4.2

3.6

3.0

2.4

Bat

tery

Vol

tage

(V

)

Open Circuit Voltage (OCV)

EDV

Qmax

I•RBAT

Quse

Cell voltage under load

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2000

2500

3000

3500

4000

4500

0 10 20 30 40 50 60 70 80 90

Conditions(0°C)• Cold (0°C)

• 10 mAh reserve capacity for shutdown

Run Time in Minutes

Vo

ltag

e

Gauge shutdown at 3.061 volts:

82 minutes run time

OCV

Shutdown @ 3.5V 21 minutes run time

Extended runtime with TI Gauge:

+290%

Fuel GaugingOCV vs. IT Use Case Exp – OLD battery COLD w/ variable load mix

4.2

3.6

3.0

2.4

Bat

tery

Vol

tage

(V

)

Open Circuit Voltage (OCV)

EDV

Qmax

I•RBAT

Quse

Cell voltage under load

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Fuel Gauging – Impedance TrackTM Advantages

• Dynamic (learning) Ability– Temperature variability in applications

• IT takes into account cell impedance changes due to increase/decrease in temperature

• IT incorporates thermal modeling to adjust for self-heating– Load variation

• IT will keep track of voltage drops due to high load spikes

• Aged Battery– IT has the ability to adjust for changes in useable capacity due to cell aging

• Increased Run Time– A lower terminate voltage can be utilized with an IT based gauge

• Flexibility– Cell Characterization– Host system does not need to perform any calculations or gauging algorithms

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Unused Battery Capacity implications

• Cell cost: avg $0.15 for every 100mAh• Lower Terminate Voltage (TV) = Larger Battery Capacity• 500mV lower of TV ~ 5% increase on capacity for new battery →

save ~$0.10 for 1500mAh battery• 500mV lower of TV ~ around 50% increase on capacity for aged

battery → save ~$1.00 for 1500mAh battery and extend run time!• Money saving opportunity for manufacturer while still extending

end-user runtime

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Cost of an inaccurate gauge

• Assuming customer discharge and charge once every day → three month use time = 90 day ~ 90 cycles → Battery internal impedance almost doubled → Aged battery scenario

• Non-Impedance Track based gauge → inaccurate gauging results due to battery aging→ much shorter run time or even system crash

• Battery warranty by operators could be one year or even two years.• Customer return entire units due to faulty gauging results → Returns

within warranty period cost company money• Impedance Track based gauge can extend the battery run time and

eliminate some costly return due to faulty gauging results

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Summary

• Accurate gauges for portable electronics are as critical to long run-time as reducing your design’s power and having a beefy battery.

• A variety of gauges are available with different approaches and different trade-offs.

Finish

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Back Up Slides:Impedance Track Reference

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Single Cell Impedance Track (IT) Basic Terminology and Relationships

• OCV – Open Circuit Voltage• Qmax – Maximum battery chemical capacity

(SOC1/SOC2 is correlated from OCV table after OCV1/OCV2 measurement)• SOC – State of Charge

• (* From Full Charge State)• RM – Remaining Capacity

• (SOC start is present SOC, SOC final is SOC at system terminate voltage)

|SOC|SOC

PassedQQ

21max -

=

max

*

Q

PassedQ1SOC -=

maxfinalstart Q)SOCSOC(RM ×= -

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• FCC – Full Charge Capacity is the amount of charge passed from a fully charged state until the system terminate voltage is reached at a given discharge rate

• FCC = Qstart + PassedQ + RM

• RSOC – Relative State of Charge

FCC

100RMRSOC

Single Cell Impedance Track (IT) Basic Terminology and Relationships

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Single Cell Impedance Track (IT) Fuel Gauge Introduction

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Gauging Error definition

• Reference points– at charge termination SOC =

100%– at EDV SOC=0– Charge integrated from fully

charged to EDV is FCCtrue

• From these reference points, true SOC can be defined as

SOCtrue= (FCCtrue-Q)/FCCtrue

• Reported SOC at all other points can be compared with true SOC.

• Difference between reported and true SOC is the error. It can be defined at different check points during discharge.

15%

3%

0%

Error check-points

0 1 2 3 4 5 6

3

3.5

4

4.5

Capacity, Ah

Vol

tage

, V

EDV

Qmax

Q

FCC

Check point at 0% is not meaningful – EDV is the voltage where system crashes!

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Single Cell Impedance Track (IT) Error Definition and Calculation

• Relative State of Charge (RSOC) Error

RSOC Error = RSOC calculated - RSOC reported

(RSOC reported is the RSOC reported by bq275xx Impedance Track TM algorithm)

100

FCC

PassedQQFCCRSOC start

calculated

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• Remaining Capacity (RM) Error

RM calculated = FCC - Qstart - PassedQ

(RM reported is the RM reported by bq275xx Impedance Track TM algorithm)

FCC

RMRMErrorRM reportedcalculated

Single Cell Impedance Track (IT) Error Definition and Calculation

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Example error plotsTrue vs reported RSOC RSOC error

0 20 40 60 80 1008.5

9

9.5

10

10.5

11

11.5

12

SMB RSOCtrue RSOC

RSOC

Vol

tage

0 10 20 30 40 50 60 70 80 90 1004

3

2

1

0

1

2

SMB RSOC

RSOC

RS

OC

err

or

Remaining capacity test Relative RemCap error

0 1 2 3 4 58.5

9

9.5

10

10.5

11

11.5

12

SMB remaining capacitytrue remaining capacity

Capacity, mAh

Vol

tage

0 12.5 25 37.5 50 62.5 75 87.5 1003

2.13

1.25

0.38

0.5

1.38

2.25

3.13

4

SMB RSOC

SOC

Rel

ativ

e R

emC

ap e

rror

, %