Single-Cell Gauging 101

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

Outline Battery chemistry fundamentals Classic fuel gauging approaches voltage based coulomb counting Impedance Track and its benefits

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

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

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

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

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)

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

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 ?

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

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

Impedance Differences for New Cells RSER Rhf R1 R2 C2 Manufacturer 1 Manufacturer 2 0 0.062 0.084 0.11 0.13 0.15 R(Z) -  - Im (Z) -  0.05 0.025 1 mHz 1 kHz 1 mHz 1 kHz 0 0.042 0.064 0.086 0.11 0.13 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%

Battery – Transient Response 3.830 3.855 3.880 3.905 0 1000 2000 3000 4000 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 0 500 1000 1500 2000 2500 3000 3500 Time (Second) 3.250 3.275 3.300 3.325 Battery Voltage (V) Load Removal L C1 RSER Rhf R1 R2 C2

Voltage Relaxation and State of Charge Error 3.2 3.45 3.7 3.95 4.3 100 50 -50 SOC % Battery Cell Voltage (V) SOC % Error 3.2 3.45 3.7 3.95 4.3 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%

SOC Error of Voltage-Based Fuel Gauging Error for a New Cell Error Evolution with Aging 15 11.25 7.5 3.75 0 20 40 60 80 100 SOC % SOC Error % 0 20 40 60 80 100 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

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

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

Learning before Fully Discharged 4.5 4 Voltage (V) 7% EDV2 3.5 3% EDV1 0% EDV0 3 0 1 2 3 4 5 6 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.

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 0 1 2 3 4 5 6 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

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 ?

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

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

Finish

Back Up Slides: Impedance Track Reference

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)

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

Single Cell Impedance Track (IT) Fuel Gauge Introduction

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!

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)

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)

Example error plots