Battery Monitoring Basics

Name: Battery Monitoring Basics
Uploaded: 2017-08-23T15:48:31+00:00
Duration: PTM36S23
Channel: Elwin Webb
Description: Battery Monitoring Basics

Battery Monitoring Basics
Macnica I&C

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 Macnica I&C

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

How to estimate battery capacity?
Measure change in capacity Voltage lookup Coulomb counting Develop a cell model Circuit model Table Lookup Macnica I&C

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) We are talking about relative charge or fullness here, because without knowing the volume of the water, we don’t know the exact amount of water; we could only say it’s half-full, or it’s x% full. So similarly, by voltage look up, we can only tell SOC, but not the remaining charge. V(t) Macnica I&C

OCV curve Capacitor Battery Macnica I&C OCV Curve Voltage Fullness
Level rises same rate Voltage Fullness OCV Curve Full charge voltage End of discharge voltage 0% 100% Capacitor Level rises faster slower Voltage Fullness OCV Curve Full charge voltage End of discharge voltage 0% 100% Battery Macnica I&C

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

Current integration I(t)
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 Macnica I&C

Basic Smart Battery System
CHG DSG VPACK Vbatt ICHG VCHG VDSG comm Gas Gauge Tbatt Battery Model Charger Load IDSG Ibatt Rs Macnica I&C

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 DC model Transient model Macnica I&C

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 Macnica I&C

Factors affecting capacity estimation
PCB component accuracy Instrumentation accuracy Cell model fidelity Aging Temperature Macnica I&C

PCB component accuracy
Gas Gauge Rs R+ R- Example Current sensing resistor Trace length (resistance) Macnica I&C

Instrumentation accuracy
ADC Resolution Sampling rate Voltage drift / calibration Noisy immunity ADC count Voltage Macnica I&C

Battery model fidelity
DC model Steady-state (DC) Transient (AC) Capacity degradation Aging Overcharge Transient model Macnica I&C

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 Macnica I&C

Temperature Temperature is important for
Capacity estimation Safety Charging control Temperature impacts model parameters Resistance Capacitance OCV Max capacity Macnica I&C

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 Temperature (°C) I could show you some EXCITING videos here of laptop batteries catching fire or exploding. You’ve probably seen the video of the burning laptop computer in Japan. Or, just go to youTube and type in laptop battery fire. No kidding , this is a serious danger, and lithium has an explosive potential with energy a significant fraction of TNT. But how do you get a cell to +175 deg C? Usually it involves current. Cell makers are getting this under control. Coatings push the critical temperature up to around +230 deg C. LiFePo4 is at least +350 deg C. Macnica I&C

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 Explain the 4 basic cell safety elements Macnica I&C

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 Alert Trip Trip Margin (time) time Margin (level) Level Trip-Over Trip-Under Macnica I&C

Overcurrent Protection Details
Don’t dwell on these details. Just cover briefly. Here’s an example of how to protect from overcurrent. Note most faults are recoverable and one is not. Note some are firmware detected and some are AFE hardware initiated. Macnica I&C

Basic Battery-Pack Electronics
Discharge MOSFET Charge MOSFET Chemical Fuse Q1 Q2 Pack+ Gas Gauge IC SMD AFE LDO Second Safety OVP IC bq29412 SMBus Overvoltage Undervoltage SMC OCP Cell Balancing 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) Macnica I&C 22

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 JEITA at low temperature, charge current should be reduced. Otherwise Lithium plating can occur. At high temperature, charge voltage should be reduced (Note these are limits, but typical levels are 50 mV less) Individual Cell temperatures must be known to within 5 ºC The question is, How representative is a single-point temperature measurement to the overall thermal condition of a pack? 4.15 V No Charge No Charge Safe Region T1 T2 T5 T6 T3 T4 (100C) (450C) Macnica I&C

Why Are Battery Packs Still Failing?
Temperature Profile along Section Line >10ºC Variation Between Cells → Heat Imbalance Space-limited design causes local heat imbalance Cell degradation accelerated Leads to cell imbalance Single/insufficient thermal sensor(s) compromise safety The reality is that notebook hotspots often results in more than 5°C delta temperature across a pack. Figure x is a thermal infrared imaging of a commercial notebook battery pack, showing a peak temperature of 46°C on the outer surface, and a >10°C delta along the section line. Self Discharge Doubles for 10º C rise The uneven temperature across the battery pack significantly compromises the effectiveness of the temperature throttling or JEITA/BAJ compliant thermal management. Also, in system design one should remember to keep the battery away from heat sources, such as a hot CPU. Macnica I&C

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 Macnica I&C

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 Cell balancing algorithms that only use voltage divergence as a balancing criteria have the disadvantage of over-balancing (or under-balancing) because of the effect of impedance imbalance (Figure 9). The problem is that cell impedance also contributes to voltage divergence during charging. The simple voltage-based cell balancing does not differentiate whether it is capacity or impedance imbalance. Therefore, this type of balancing cannot guarantee that all cells are at 100% in capacity at full charge. bq77PL900, 5 to 10 series-cell Li-Ion battery-pack protector for power tools Macnica I&C

Fast Passive Cell Balancing
Needed for high-power packs, where cell self-discharge overpowers internal balancing Fast cell balancing strength is 10x ~ 20x higher Cell 2 PACK + 1 k W R2 R1 bq2084/ bq20zxx Cell 1 R R3 R4 Q2 Internal CB In extreme conditions, the weak balancing may be over-powered by the rate of cell divergence/imbalance. To improve the strength of the passive cell balancing, external bypass can be established to utilize existing hardware. Typical implementation is shown. The internal balancing MOSFET is first turned on based on a balancing decision for that particular cell, which created a low-level bias current through the external filter resistors R2 and R3 that connect from the cell terminals to the IC pins. The gate-to-source voltage is thus established across R3, and the external MOSFET Q2 is turned on. The external MOSFET Rdson is negligible, and the external balancing current, is controlled by cell voltage and R4 -Weak internal balancing may be over-powered by the rate of cell divergence -Internal FETs can be used to turn on external FETs for fast passive cell balancing -Balancing current set by external resistor and is more accurate -Balancing strength is 10x ~ 20x higher RDS(on) Fast CB Where R4 << RDS(on) Macnica I&C

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 Macnica I&C

Communication and Display
To the System or Charger Industry specification Display LED, LCD Capacity indication Fault indication Macnica I&C

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 Macnica I&C

Section 2 Battery Fuel Gauging: CEDV & Z-track
Macnica I&C

Basic Vocabulary Review
Current C-rate [mA] Coulomb Counting 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] Macnica I&C

How Much Capacity is Really Available?
Voltage, V 4.5 Open circuit voltage (OCV) 4.0 I • RBAT 3.5 EDV 3.0 1 2 3 4 6 Capacity, Ah Usable capacity : FCC Battery voltage depends not only on its state of charge, but also on discharge current. This is caused by voltage drop IR due to battery internal resistance. This drop is higher at high currents, low temperatures, and for aged batteries which have increased resistance. How does it influence run-time? Minimal acceptable voltage will be reached earlier, therefore reducing the “useable” capacity that the battery can deliver. Capacity integrated until voltage reaches EDV under load conditions is called “useable capacity”. Because it depends on current, it has to be evaluated specifically for each application. Note that internal resistance “R” depends on state of charge and increases at the end of discharge, therefore simple modeling assuming fixed R will not give accurate estimation of useable capacity. Also battery manufacturers often report battery impedance at 1kHz. This value can not be used as estimate for internal resistance at DC conditions, because low frequency impedance (that corresponds to DC conditions) is much higher than that at 1kHz (typically 2-3 times higher). Best way to estimate useable capacity is to make a test, or to refer manufacturer discharge curves at load comparable with expected application load. Full chemical capacity: Qmax External battery voltage (blue curve) V = V0CV – I • RBAT Higher C-rate EDV is reached earlier (higher I • RBAT) Macnica I&C 33

What Does A Fuel Gauge Do?
Which route is the battery taking? 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% Before getting down to the CEDV business, let’s review the basis of CEDV: current integration fuel gauging Explain… The idea seems a pretty robust one, except that in practice, you really can’t update capacity at 0% 4/19/2017 Macnica I&C

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 Before getting down to the CEDV business, let’s review the basis of CEDV: current integration fuel gauging Explain… The idea seems a pretty robust one, except that in practice, you really can’t update capacity at 0% FCC 4/19/2017 Macnica I&C

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% EDV2 3% EDV1 0% EDV0 FCC 4/19/2017 Macnica I&C

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) 4/19/2017 Macnica I&C

CEDV Model Visualization
OCV curve defined by EMF, C0 Voltage OCV corrected by I*R (R is defined by R0, R1, T0) I*R Further correction by low temperature (TC) Actual battery voltage curve Parameters EMF and C0 define function OCV(SOC,T) Parameters (R0,R1 and T0) define R(SOC,T). R1 defines the slope of R(SOC) dependence, R0 the magnitude of R, and T0 the slope of R(T) dependence. Parameter TC defines additional resistance increase at temperatures below 21oC: R`(SOC,T). Parameter C1 allows to shift whole function to the left. EDV2 is reached earlier and so reserve capacity is provided. 1 Unit of C1 shifts function by 0.39% Reserve Cap: C1 shifts fit curve laterally Battery Low 3% 4% 5% 6% 7% 8% 9% Macnica I&C

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 = * CycleCount * A0 / 39 Macnica I&C

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 Macnica I&C

Current Direction Thresholds and Delays
8 CHG relaxation timed Enter RELAX mode Start discharging Enter DSG mode DSG relaxation timed Start charging Enter CHG mode 1 2 3 7 6 5 4 Example of the Algorithm Operation Mode Changes With Varying SBS.Current( ) Macnica I&C

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 Macnica I&C

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 simplification of fuel-gauge implementation at user side Macnica I&C

Resistance Update Before Update Discharge direction Macnica I&C

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_old Ra_new Grid 0 Grid 14 k: Present grid m: Last visited grid Step 1 Interpolation Step 2 Scale “After” Step 3 Scale “Before” Macnica I&C

Timing of Qmax Update Macnica I&C

FCC Learning Macnica I&C

Modeling temperature Heating Cooling
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? m := cell mass cp := specific heat hc := heat transfer coef A := cell surface area Ta := ambient temp Heating Cooling ** “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 Macnica I&C

RemCap Simulation (concept)
Start of discharge V (loaded) I*R OCV Δ V > 250mV EDV Vterm Time ΔQ/2 Qstart = PassedChargeDod0 Each delta Q slice is simulated Q increment New dod point is computed using dod = dod0 + PassedCharge/Qmax OCV is looked up from dod Loaded voltage V_load = OCV – I*R If V_load drops below EDV, termination occur. Total simulated Q defines RemCap I ΔQ/4 Qstart ΔQ ΔQ ΔQ RsvCap Time RemCap Constant Load Example Macnica I&C

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 Macnica I&C

CEDV, Impedance Track Comparison
Property CEDV Impedance Track Worst error new, learned +/-2% +/-1% Worst error aged, learned +30% (+/- 15% with age data) Data collection 3 temperatures, 2 rates, Fitting to obtain parameters. 2 weeks Chemistry selection test, Optimization cycle 1 week Instruction flash small large Voltage accuracy requirement 20mV/pack 3mV/pack State of charge initialization (host side requirement) No Yes FCC temperature compensation No (with rare exceptions) FCC rate compensation Learning cycle in production required Not required Macnica I&C

Thank You Macnica I&C

Battery Monitoring Basics

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