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Battery Monitoring Basics

Battery Monitoring Basics. Section 1 – Basic Concepts. What does a battery monitor do? How to estimate battery capacity? Voltage lookup Current integration Factors affecting capacity estimation Other functions Safety and protection Cell balancing Charging support

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Battery Monitoring Basics

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  1. Battery Monitoring Basics

  2. Section 1 – Basic Concepts • What does a battery monitor do? • How to estimate battery capacity? • Voltage lookup • Current integration • Factors affecting capacity estimation • Other functions • Safety and protection • Cell balancing • Charging support • Communication and display • Logging

  3. CHG DSG Vbatt VPACK ICHG VCHG VDSG Gas Gauge comm Battery Charger Load Tbatt IDSG Ibatt Rs System Cell Monitor What does a battery monitor do? • Capacity estimation • Safety/protection • Charging support • Communication and Display • Logging • Authentication Battery Subsystem

  4. How to estimate battery capacity? • Measure change in capacity • Voltage lookup • Coulomb counting • Develop a cell model • Circuit model • Table Lookup

  5. Voltage lookup • One can tell how much water is in a glass by reading the water level • Accurate water level reading should only be made after the water settles (no ripple, etc) • One can tell how much charge is in a battery by reading well-rested cell voltage • Accurate voltage should only be made after the battery is well rested (stops charging or discharging) mL marks I(t) V(t)

  6. Level rises slower OCV Curve OCV Curve Level rises same rate Capacitor Level rises faster Full charge voltage Full charge voltage Voltage Voltage Level rises same rate End of discharge voltage End of discharge voltage 100% 100% 0% 0% Fullness Fullness Battery OCV curve

  7. OCV(DOD) 4300 4100 3900 3700 Voltage_a Voltage_a(DOD) 3500 Poly_a(DOD) 3300 3100 2900 0 0.2 0.4 0.6 0.8 1 1.2 DOD OCV voltage table: DOD representation Vmax Vmin Flat Zone DOD = Depth of Discharge SOC = State of Charge DOD = 100% - SOC

  8. Current integration • One can also measure how much water goes in and out • In batteries, battery capacity changes can be monitored by tracking the amount of electrical charges going in/out • But how do you know the amount of charge, , already in the battery at the start? • How do you count charges accurately? mL marks I(t) Voltage

  9. Basic Smart Battery System CHG DSG VPACK Vbatt ICHG VCHG VDSG Gas Gauge comm Tbatt Battery Model Load Charger IDSG Ibatt Rs

  10. DC model Transient model Circuit model • VOC a function of SOC • Rint is internal resistance • Rs and Cs model the short term transient response • RL and CL model the long term transient response • Vbatt and Ibatt are the battery voltage and current • All parameters are function of temperature and battery age

  11. Table lookup • Large, multi-dimensional table relating capacity to • Voltage • Current • Temperature • Aging • No cell model • Apply linear interpolation to make lookup “continuous” • Memory intensive

  12. Factors affecting capacity estimation • PCB component accuracy • Instrumentation accuracy • Cell model fidelity • Aging • Temperature

  13. Gas Gauge R+ R- Rs PCB component accuracy • Example • Current sensing resistor • Trace length (resistance)

  14. Voltage ADC count Instrumentation accuracy • ADC Resolution • Sampling rate • Voltage drift / calibration • Noisy immunity

  15. DC model Transient model Battery model fidelity • Steady-state (DC) • Transient (AC) • Capacity degradation • Aging • Overcharge

  16. Model parameter extraction • Extract battery model parameter values using actual collected battery data • Open circuit voltage (OCV) • Transient parameters (RC) • DC parameters (Ri) • Least square minimization • Extraction process can be hard and time consuming

  17. Temperature • Temperature is important for • Capacity estimation • Safety • Charging control • Temperature impacts model parameters • Resistance • Capacitance • OCV • Max capacity

  18. Safety • High operating temperature • Accelerates cell degradation • Thermal runaway and explosion • LiCoO2 – Cathode reacts with electrolyte at 175°C with 4.3 V • Cathode coatings help considerably • LiFePO4 shows huge improvement! Thermal runaway is > 350°C OCV = 4.3 V Heat Flow (W/g) Thermal Runaway 100 125 150 175 200 225 250 Temperature (°C)

  19. Cell Safety Safety Elements • Pressure relief valve • PTC element • Aluminum or steel case • Polyolefin separator • Low melting point (135 to 165°C) • Porosity is lost as melting point is approached • Stops Li-Ion flow and shuts down the cell • Recent incidents traced to metal particles that pollutes the cells and creates microshorts

  20. Trip-Over Trip Alert Trip Trip Margin (level) Trip Level time Trip Margin (time) Trip-Under Trip Alert Trip Trip Level Trip Margin (level) time Trip Margin (time) Safety and protection • Short circuit • Over/under (charge/discharge) current • Over/under voltage • Over temperature • FET failure • Fuse failure • Communication failure • Lock-up • Flash failure • ESD • Cell imbalance

  21. Overcurrent Protection Details

  22. Basic Battery-Pack Electronics Charge MOSFET Discharge MOSFET Q2 Q1 Chemical Fuse Pack+ Gas Gauge IC AFE SMD LDO Second Safety OVP IC bq29412 SMBus Overvoltage Undervoltage OCP Cell Balancing SMC I2C Temp Sensing RT bq20z90 bq29330 Voltage ADC Sense Resistor Rs Current ADC Pack– • Measurement: Current, voltage, and temperature • bq20zxx gas gauge : Remaining capacity, run time, health condition • Analog front end (AFE)

  23. JEITA/BAJ Guidelines for Notebook • Do not charge if T< 0°C or T> 50°C • Minimize temperature variation among cells • How do we collect temperature information? Upper-Limit Charge Current Upper-Limit Voltage: 4.25 V 4.20 V 4.15 V No Charge No Charge Safe Region T2 T5 T6 T3 T4 T1 (100C) (450C)

  24. >10ºC Variation Between Cells Temperature Profile along Section Line Why Are Battery Packs Still Failing? → Heat Imbalance • Space-limited design causes local heat imbalance • Cell degradation accelerated • Leads to cell imbalance • Single/insufficient thermal sensor(s) compromise safety

  25. Cell Balancing Battery cells voltages can get out of balance, which could lead to over charge at a cell even though the overall pack voltage is acceptable. Cell balance can be achieved through current bypass or cross-cell charge pumping 25

  26. Passive Cell Balancing: Simplest Form • Simple, voltage based • Stops charging when any cell hits VOV threshold • Resistive bypassing turns on • Charge resumes when cell A voltage drops to safe threshold bq77PL900, 5 to 10 series-cell Li-Ion battery-pack protector for power tools

  27. PACK + 1 k W R1 R4 Cell 2 Q2 1 k W bq2084/ R2 R4 bq20zxx Cell 1 Q2 1 k W R3 R Fast Passive Cell Balancing • Needed for high-power packs, where cell self-discharge overpowers internal balancing • Fast cell balancing strength is 10x ~ 20x higher Internal CB RDS(on) Fast CB Where R4 << RDS(on)

  28. Charging support • Inform battery charger proper charging voltage and current • Conform to specification (e.g., JEITA) • Reduce charge time • Extend battery life by: • Avoid overcharging • Precharging depleted and deeply discharged cells

  29. Communication and Display • Communication • To the System or Charger • Industry specification • Display • LED, LCD • Capacity indication • Fault indication

  30. Logging • Works like an airplane “blackbox recorder” • Record important lifetime information • Max/min voltage • Max/min current • Max/min temperature • Record important data for failure analysis • Reset count • Cycle count • Excessive flash wear

  31. Section 2 Battery Fuel Gauging: CEDV & Z-track

  32. Basic Vocabulary Review Capacity Design Capacity [mAh] Qmax, Chemical Capacity [mAh] FCC, Usable Capacity [mAh] RM, Remaining Capacity [mAh] RSOC [%] DOD [%] DOD0, DOD1 [%] Voltages OCV [mV] OCV(DOD) [mV] EDV [mV] EDV 2 [mV] EDV 0 [mV] CEDV [mV] • Current • C-rate [mA] • Coulomb Counting

  33. External battery voltage (blue curve) V = V0CV – I • RBAT Higher C-rate EDV is reached earlier (higher I • RBAT) Open circuit voltage (OCV) I • RBAT How Much Capacity is Really Available? Voltage, V 4.5 4.0 3.5 EDV 3.0 0 1 2 3 4 6 Capacity, Ah Usable capacity : FCC Full chemical capacity: Qmax

  34. Which route is the battery taking? What Does A Fuel Gauge Do? Suppose we are here 4.2V • What is the remaining capacity at current load? • What is the State of charge (SOC)? • How long can the battery run? 3V 0%

  35. Current Integration Based Fuel-gauging • Battery is fully charged • During discharge capacity is integrated • State of charge (SOC) at each moment is RM/FCC • FCC is updated every time full discharge occurs 4.2V Q 0% RM = FCC - Q SOC = RM/FCC 3V FCC

  36. Learning Before Fully Discharged– fixed voltage thresholds • It is too late to learn when 0% capacity is reached  Learning FCC before 0% • We can set voltage threshold that correspond to given percentage of remaining capacity • However, true voltage corresponding to 7% depends on current and temperature 4.2V 7% 3% EDV2 EDV1 0% EDV0 FCC

  37. Learning before fully discharged with current and temperature compensation CEDV CEDV Model: Predict V(SOC) under any current and temperature • Modeling last part of discharge allows to calculate function V(SOC, I, T) • Substituting SOC=7% allows to calculate in real time CEDV2 threshold that corresponds to 7% capacity at any current and temperature OCV 4.2V EDV2 (I1) EDV2 (I2)

  38. OCV curve defined by EMF, C0 OCV corrected by I*R (R is defined by R0, R1, T0) I*R Further correction by low temperature (TC) Reserve Cap: C1 shifts fit curve laterally CEDV Model Visualization Voltage Actual battery voltage curve Battery Low 3% 4% 5% 6% 7% 8% 9%

  39. CEDV Formula • CEDV = CV - I*[EDVR0/4096]*[1 + EDVR1*Cact/16384]* • [1 – EDVT0*(10T - 10Tadj)/(256*65536)]*[1+(CC*EDVA0)/(4*65536)] * age • Where: • CV = EMF*[1 – EDVC0*(10T)*log(Cact)/(256*65536)] • Cact = 256/(2.56*RSOC + EDVC1) – 1 for (2.56*RSOC + EDVC1) > 0 • Cact = 255 for (2.56*RSOC + EDVC1) = 0 • EDVC1 = 2.56 * Residual Capacity (%) + “Curve Fit” factor • Tadj = EDVTC*(296-T) for T< 296oK and Tadj < T • Tadj = 0 for T > 296 oK and Tadj max value = T • age = 1 + 8 * CycleCount * A0 / 65536. 39

  40. Impedance Track Fuel Gauging • Combine advantages of voltage correlation and coulomb counting methods • State of charge (SOC) update: • Read fully relaxed voltage to determine initial SOC and capacity decay due to self-discharge • Use current integration when under load • Parameters learning on-the-fly: • Learn impedance during discharge • Learn total capacity Qmax without full charge or discharge • Adapt to spiky loads (delta voltage) • Usable capacity learning: • Calculate remaining run-time at typical load by simulating voltage profile  do not have to pass 7% knee point

  41. Current Direction Thresholds andDelays 8 • CHG relaxation timed • Enter RELAX mode • Start discharging • Enter DSG mode • DSG relaxation timed • Enter RELAX mode • Start charging • Enter CHG mode 1 2 3 7 6 5 4 Example of the Algorithm Operation Mode Changes With Varying SBS.Current( )

  42. What is Impedance Track? 1. Chemistry table in Data Flash: OCV = f (dod) dod = g (OCV) 2. Impedance learning during discharge: R = OCV – V I 3. Update Max Chemical Capacity for each cell Qmax = PassedCharge / (SOC1 – SOC2) 4. Temperature modeling allows for temperature-compensated impedance to be used in calculating remaining capacity and FCC 5. Run periodic simulation to predict Remaining and Full Capacity 10,000 foot View

  43. Close OCV profile for the Same Base-Electrode Chemistry • OCV profiles close for all tested manufacturers • Most voltage deviations from average are below 5mV • Average DOD prediction error based on average voltage/DOD dependence is below 1.5% • Same OCV database can be used with batteries produced by different manufacturers as long as base chemistry is same • Generic database allows significant simplificationof fuel-gauge implementation at user side

  44. Resistance Update Before Update Discharge direction

  45. Ra Table: Interpolation and Scaling Operation R = (OCV – V) / Avg Current. Averaging method is selectable Resistance updates require updating 15 values for each cell A new resistance measurement represents the resistance at an exact grid point. Exact value found by interpolation All 15 grid points are ratiometrically updated from any valid gridpoint measurement. Changes are weighted according to confidence in accuracy Ra_new Ra_old Grid 0 Grid 14 k: Present grid m: Last visited grid Step 1 Interpolation Step 2 Scale “After” Step 3 Scale “Before”

  46. Timing of Qmax Update

  47. FCC Learning

  48. Modeling temperature Cooling Heating • Based on a heating / cooling model ** • Heating is from the internal resistance • Cooling is from heat transfer to the environment, i.e., • How many thermistors? hc := heat transfer coef A := cell surface area m := cell mass cp := specific heat Ta := ambient temp ** “Dynamic Lithium-Ion Battery Model for System Simulation”, L. Gao, S. Liu and R. A. Dougal, IEEE Transaction on Components and Packaging Technologies, vol. 25, no. 3, September 2002. 48

  49. RemCap Simulation (concept) Start of discharge V (loaded) I*R OCV ΔV > 250mV EDV Vterm Time ΔQ/2 I ΔQ/4 RsvCap Qstart . . . . . ΔQ ΔQ ΔQ Time RemCap Constant Load Example

  50. Z-track Accuracy in Battery Cycling Test • Error is shown at 10%, 5% and 3% points of discharge curve • For all 3 cases, error stays below 1% during entire 250 cycles • It can be seen that error somewhat decreases from 10 to 3% due to adaptive nature of IT algorithm

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