Report

Single-Cell Gauging 101 1 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 Run Time 6:23 • Full Charge Capacity 73% 2 Outline • Battery chemistry fundamentals • Classic fuel gauging approaches – voltage based – coulomb counting • Impedance Track and its benefits 3 Single-Cell Gauging 101 Part 2: Classic fuel gauging approaches Goal: Full Use of Available Battery Capacity 100% + Charging Voltage Tolerance 80% 60% Actual Useful Capacity 40% 20% 0% Capacity Shutdown Uncertainty due to inaccurate gauging • Only 80-90% of Available Capacity may actually be used! • High Accuracy Gas Gauge Increases the Battery Run-time 5 Traditional Battery Pack-Side Gas Gauge PACK+ Power Management Host Processor TI OMAP GPIO I2C/HDQ Gas Gauge PACKProtector TS Battery Pack 6 System-Side Impedance Track Fuel Gauge PACK+ Power Management Host P TI OMAP GPIO Gas Gauge TS Protector PACKPortable Devices 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) 8 How to Implement a Fuel Gauge? • Voltage Based: SOC = f (VBAT) • Coulomb Counting: Q i dt • Impedance Track: Real time resistance measurement V = VOCV - I •RBAT 9 Voltage Based Fuel Gauge Battery Voltage (V) 4.2 Open Circuit Voltage (OCV) I•RBAT 3.8 3.4 3.0 EDV Battery Capacity Quse Qmax • Applications: low end cellular phone, DSC,… • Pulsating load causes capacity bar up and down V VOCV - I R BAT • Accurate ONLY at very low current ? 10 Battery Resistance V = VOCV - I •RBAT? 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 11 Impedance Dependent on Temperature and DOD Impedance is strongly dependent on temperature, State of Charge and aging SOC = Full Charged Q Qmax 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 12 Impedance Differences for New Cells Rhf R1 R2 RSER L C1 C2 Manufacturer 1 0.05 - Im (Z) - - Im (Z) - 0.05 1 mHz 0.025 Manufacturer 2 1 kHz 1 mHz 0.025 1 kHz 0 0 0 0.062 0.084 0.11 R(Z) - 0.13 0.15 0 0.042 0.064 0.086 R(Z) - 0.11 0.13 • Low-frequency (1 mHz) impedance variation 15% • At 1C rate discharge, 40-mV difference, causes maximum SOC error of ±26% 13 Battery Voltage (V) Battery – Transient Response 3.905 Load Removal 3.880 • Different voltage at different instants 3.855 3.830 0 1000 *C/3 rate current used for both tests Battery Voltage (V) • Complete relaxation takes about 2000 seconds 2000 3000 Time (Second) 4000 • Voltage difference between 20 and 3000 seconds is over 20 mV 3.325 3.300 3.275 3.250 Load Removal 0 500 1000 1500 2000 2500 3000 3500 Time (Second) Rhf R1 R2 RSER L C1 C2 14 Voltage Relaxation and State of Charge Error 100 SOC % 20 SOC % Error 10 50 0 0 -10 -50 3.2 3.45 3.7 3.95 Battery Cell Voltage (V) 4.3 -20 3.2 3.45 3.7 3.95 Battery Cell Voltage (V) 4.3 • ±20mV difference • Error depends on particular voltage at the moment of estimation • Maximum error reaches 15%, average error 5% 15 SOC Error of Voltage-Based Fuel Gauging Error for a New Cell Error Evolution with Aging 100 100 1515 13.13 Relaxation Error Cell-to-Cell Variation Total Error 11.25 % Error SOCSOC Error – % SOC Error – % SOC Error % 11.25 9.38 7.5 7.5 5.63 3.75 3.75 1.88 0 00 0 300 Cycles 200 Cycles 100 Cycles 0 Cycles 87.5 7575 62.5 50 50 37.5 25 25 12.5 0 20 20 40 60 40 SOC – %60 SOC % 80 80 100 100 00 0 V = VOCV - I • R BAT 20 20 40 60 40 60 SOC – % SOC % 80 80 100 100 ? • 20-mV relaxation measurement error • 15% cell-to-cell resistance tolerance • Battery resistance doubles every 100 cycles 16 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 17 Coulomb Counting Based Gauging • Battery is fully charged • During discharge capacity is integrated • Qmax is updated every time full discharge occurs Q i dt 4.5 Li-Ion Battery Cell Voltage 0.2C Discharge Rate 4.0 Q 3.5 3.0 EDV: End of Discharge Voltage 0 EDV 1 2 3 4 Capacity, Ah 5 6 Qmax Example: bq27010, bq27210 18 Learning before Fully Discharged Voltage (V) 4.5 4 3.5 3 0 EDV2 7% EDV1 EDV0 1 3% 0% 2 3 4 Capacity Q (Ah) 5 6 • 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 19 Compensated End of Discharge Voltage (CEDV) Voltage (V) 4.5 4 3.5 7% CEDV2 (I1) CEDV2 (I2) 30% 7% 3 0 1 2 3 4 Capacity Q (Ah) 5 6 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 20 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 V = VOCV - I •RBAT ? 21 Advantages for typical gas gauges Battery Voltage (V) 4.2 I•RBAT 3.6 3.0 Open Circuit Voltage (OCV) EDV 2.4 Battery Capacity Quse Qmax • Very accurate gauging from OCV without load (relaxation) • Very accurate gauging with Coulomb Counting with load 22 Issue Review Battery Voltage (V) 4.2 I•RBAT 3.6 3.0 Open Circuit Voltage (OCV) EDV 2.4 Battery Capacity Quse Qmax • Voltage Based gas Gauge: V = OCV(T,SOC) - IR(T,SOC, Aging) • Current integration gas gauge: CEDV = OCV(T,SOC) - IR(T,SOC, Aging) Problem: Battery Impedance 23 Finish Back Up Slides: Impedance Track Reference 25 Single Cell Impedance Track (IT) Basic Terminology and Relationships • OCV – Open Circuit Voltage • Qmax – Maximum battery chemical capacity Qmax PassedQ = |SOC1 - SOC2 | (SOC1/SOC2 is correlated from OCV table after OCV1/OCV2 measurement) • SOC – State of Charge PassedQ* SOC = 1 Qmax • (* From Full Charge State) • RM – Remaining Capacity RM = ( SOCstart - SOCfinal ) × Qmax • (SOC start is present SOC, SOC final is SOC at system terminate voltage) 26 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 RSOC RM 100 FCC 27 Single Cell Impedance Track (IT) Fuel Gauge Introduction 28 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 4.5 Voltage, V 4 Q 15% 3.5 3% EDV 3 0 1 2 3 Capacity, Ah 4 5 0% 6 Q FCC max Error checkpoints Check point at 0% is not meaningful – EDV is the voltage where system crashes! 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. 29 Single Cell Impedance Track (IT) Error Definition and Calculation • Relative State of Charge (RSOC) Error RSOC Error = RSOC calculated - RSOC reported RSOC calculated FCC Qstart PassedQ 100 FCC (RSOC reported is the RSOC reported by bq275xx Impedance Track TM algorithm) 30 Single Cell Impedance Track (IT) Error Definition and Calculation • Remaining Capacity (RM) Error RM Error RMcalculated RMreported FCC RM calculated = FCC - Qstart - PassedQ (RM reported is the RM reported by bq275xx Impedance Track TM algorithm) 31 Example error plots True vs reported RSOC RSOC error 12 2 11.5 1 11 RSOC error Voltage 0 10.5 10 1 9.5 2 9 3 8.5 0 20 40 60 80 100 4 0 10 20 30 40 RSOC 60 70 80 90 100 RSOC SMB RSOC true RSOC SMB RSOC Remaining capacity test Relative RemCap error 12 4 11.5 3.13 11 2.25 Relative RemCap error, % Voltage 50 10.5 10 9.5 1.38 0.5 0.38 1.25 9 2.13 8.5 0 1 2 3 4 5 3 0 12.5 Capacity, mAh SMB remaining capacity true remaining capacity 25 37.5 50 62.5 75 87.5 100 SOC SMB RSOC 32