1. Single-Cell Gauging 101

2. 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

4. Single-Cell Gauging 101 Part 2: Classic fuel gauging approaches

5. Goal: Full Use of Available Battery Capacity+
100%
Charging Voltage Tolerance
Actual Useful Capacity
80%
60%
40%
20%
Shutdown Uncertainty
due to inaccurate gauging 0%
Capacity
Only 80-90% of Available Capacity may actually be used!
High Accuracy Gas Gauge Increases the Battery Run-time

6. Traditional Battery Pack-Side Gas Gauge
Power
Management
I2C/HDQ
Gas
Gauge
GPIO
Host Processor
TI OMAP
PACK-
Protector TS
This is the block diagram of traditional battery pack for single cell handheld applications. The gas gauge is located in the pack side to continuously monitor the battery operation conditions and provide the remaining capacity or runtime to the host through I2C or single wire protocol.
However, when the battery cycle life is over, we have to throw away the pack even though the fuel gauge is still in good condition. We have to buy another pack with the fuel gauge, which increase the end user cost.
Could we put the fuel gauge in host side to minimize the cost?
Battery Pack 6

7. System-Side Impedance Track Fuel Gauge
PACK+
Power
Management
Gas
Gauge TS
GPIO
Host P
TI OMAP
Protector
PACK-
Portable Devices
This diagram shows that the fuel gauge is located in the host side to minimize the battery pack cost. So whenever the pack dies, we do not have buy another electronics in the battery pack. What is the main challenge?
Battery Pack 7

8. What does the Fuel Gauge do?
Communication between battery and user
Measurement:
Battery voltage
Charging or discharging current
Temperature
Provide:
Battery Run Time and Remaining Capacity
Battery health information
Overall battery power management (Operation mode)

9. How to Implement a Fuel Gauge?
Voltage Based: SOC = f (VBAT)
Coulomb Counting:
Impedance Track: Real time resistance measurement

10. Voltage Based Fuel Gauge
4.2
Open Circuit Voltage
(OCV)
I•RBAT
3.8
3.4
Battery Voltage (V)
EDV
3.0
Quse
Qmax
Battery Capacity
Applications: low end cellular phone, DSC,…
Pulsating load causes capacity bar up and down
Accurate ONLY at very low current ?

11. Battery Resistance ?
Impedance = f( Temperature, State of Charge, and Aging)
Resistance doubles after 100 cycles
10-15% cell-cell resistance variation
10-15% resistance variation from different manufacturers

12. Impedance Dependent on Temperature and DOD
Impedance is strongly dependent on temperature,
State of Charge and aging
Full Charged
Fully Discharged
DOD=1-SOC (State of Charge)
SOC=1 (Full charged battery)
SOC=0 (Full discharged battery)
SOC: State of Charge
DOD: Depth of Discharge

13. Impedance Differences for New Cells
RSER
Rhf R1 R2 C2
Manufacturer 1
Manufacturer 2
R(Z) - 
- Im (Z) - 
0.05
0.025
1 mHz
1 kHz
1 mHz
1 kHz
R(Z) - 
- Im (Z) - 
0.05
0.025
Low-frequency (1 mHz) impedance variation 15%
At 1C rate discharge, 40-mV difference, causes maximum SOC error of ±26%

14. Battery – Transient Response
3.830
3.855
3.880
3.905
Time (Second)
Battery Voltage (V)
Complete relaxation takes about 2000 seconds
Different voltage at different instants
Voltage difference between 20 and 3000 seconds is over 20 mV
Load Removal
*C/3 rate current used for both tests
Time (Second)
3.250
3.275
3.300
3.325
Battery Voltage (V)
Load Removal L C1
RSER
Rhf R1 R2 C2

15. Voltage Relaxation and State of Charge Error
100 50
-50
SOC %
Battery Cell Voltage (V)
SOC % Error
Battery Cell Voltage (V) 20 10
-10
-20
±20mV difference
Error depends on particular voltage at the moment of estimation
Maximum error reaches 15%, average error 5%

16. SOC Error of Voltage-Based Fuel Gauging
Error for a New Cell
Error Evolution with Aging 15
11.25
7.5
3.75
SOC %
SOC Error %
SOC %
100 75 50 25
SOC Error %
There are several contributions to error in voltage compensation
Transient error
15% cell-to-cell variation of impedance
Measurement error
For new cell overal error is in acceptable range, however it is rapidly exceeding acceptable range with aging (right side slide) ?
20-mV relaxation measurement error
15% cell-to-cell resistance tolerance
Battery resistance doubles every 100 cycles

17. Voltage-Based Fuel Gauge
Advantages
Learning can occur without full discharge
No correction needed for self-discharge
Very accurate with small load current
Disadvantages
Inaccurate due to internal battery impedance
Impedance is function of temperature, aging, and State of Charge

18. Coulomb Counting Based Gauging
Battery is fully charged
During discharge capacity is integrated
Qmax is updated every time full discharge occurs
Li-Ion Battery Cell Voltage
4.5
0.2C Discharge Rate
4.0 Q
3.5
EDV
3.0 1 2 3 4 5 6
EDV: End of Discharge Voltage
Capacity, Ah
Qmax
Example: bq27010, bq27210

19. Learning before Fully Discharged
4.5 4
Voltage (V) 7%
EDV2
3.5 3%
EDV1 0%
EDV0 3
Capacity Q (Ah)
Too late to learn when 0% capacity is reached
Set voltage threshold for given percentage of remaining capacity
True voltage at 7%, 3% remaining capacity depends on current, temperature, and impedance
To prevent system crash and to provide more chances for learning, voltage thresholds that correspond for example to 7% soc can be used.
When threshold is reached, FCC is learned. This works well at fixed load,
However in DSC case load is highly variable.

20. CEDV = OCV(T,SOC) - I*R(T,SOC)
Compensated End of Discharge Voltage (CEDV)
4.5 4
Voltage (V) 7%
CEDV2 (I1)
3.5
30%
CEDV2 (I2) 7% 3
Capacity Q (Ah)
CEDV = OCV(T,SOC) - I*R(T,SOC)
Modeling: R(SOC,T), good for new battery
Calculate CEDV2 (7%) and CEDV1 (3%) threshold at any I and T.
Not Accurate for Aged battery

21. Coulomb Counting Based Gauging
Advantages
Not influenced by distortions of voltage measurement
Accuracy is defined by current integration hardware
Gauging error: 3-10% depending on operation conditions and usage
Disadvantages
Learning cycle needed to update Qmax
Battery capacity degradation with aging
Qmax Reduction: 3-5% with 100-cycles
Gauging error increases 1% for every 10-cycles without learning
Self-discharge has to be modeled: Not accurate
Key Parameter related to Aging: Impedance ?

22. Advantages for typical gas gauges
4.2
Open Circuit Voltage
(OCV)
I•RBAT
Quse
3.6
Battery Voltage (V)
EDV
3.0
2.4
Battery Capacity
Qmax
Very accurate gauging from OCV without load (relaxation)
Very accurate gauging with Coulomb Counting with load

23. Current integration gas gauge: CEDV = OCV(T,SOC) - IR(T,SOC, Aging)
Issue Review
4.2
Open Circuit Voltage
(OCV)
I•RBAT
Quse
3.6
Battery Voltage (V)
EDV
3.0
2.4
Battery Capacity
Qmax
Voltage Based gas Gauge: V = OCV(T,SOC) - IR(T,SOC, Aging)
Current integration gas gauge: CEDV = OCV(T,SOC) - IR(T,SOC, Aging)
Problem: Battery Impedance

24. Finish

25. Back Up Slides: Impedance Track Reference

26. 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)

27. Single Cell Impedance Track (IT) Basic Terminology and Relationships
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

28. Single Cell Impedance Track (IT) Fuel Gauge Introduction

29. 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.
4.5 4 Q
15%
Voltage, V
3.5 3%
EDV 3 0% 1 2 3 4 5 6
Capacity, Ah Q
FCC
max
Error check-points
Check point at 0% is not meaningful – EDV
is the voltage where system crashes!

30. 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)

31. Single Cell Impedance Track (IT) Error Definition and Calculation
Remaining Capacity (RM) Error
RM calculated = FCC - Qstart - PassedQ
(RM reported is the RM reported by bq275xx Impedance Track TM algorithm)

32. Example error plots