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

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• 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

Cell

Volta

ge (V

)

0 0.5 1.0 1.5 2.0 2.5Time (hour)

4.24.14.03.93.83.7

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

Cell

Volta

ge (V

)

0 0.5 1.0 1.5 2.0 2.5 3.0Time (hour)

4.24.14.03.93.8

P1

P2

2SOC1SOCQQ

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

Volta

ge

Impedance TrackTM Gauge Shutdown @ 3.295V168 minutes run time

Extended runtime with TI Gauge:

+40%

OCVShutdown @ 3.5V

120 minutes run time4.2

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 80 100 120 140 160

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

Volta

ge

Impedance TrackTM Gauge Shutdown @ 3.144V142 minutes run time

OCVShutdown @ 3.5V

90 minutes run time

Extended runtime with TI Gauge:

+58%

4.2

3.6

3.0

2.4

Batte

ry V

olta

ge (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

Volta

ge

Impedance TrackTM Gauge Shutdown @ 3.020V117 minutes run time

OCVShutdown @ 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

Batte

ry V

olta

ge (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

Volta

ge

Gauge shutdown at 3.061 volts:

82 minutes run time

OCVShutdown @ 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|SOCPassedQ

Q21

max -=

max

*

QPassedQ

1SOC -=

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

FCC100RMRSOC

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

Volta

ge, 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 startcalculated

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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)

FCCRMRM

ErrorRM 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

RSO

C e

rror

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,

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