Battery Monitoring Basics

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Battery Monitoring Basics
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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

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What does a battery monitor do?
Battery Subsystem

• Capacity estimation
• Safety/protection
• Charging support
• Communication and Display
• Logging
• Authentication

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How to estimate battery capacity?

• Measure change in capacity
• Voltage lookup
• Coulomb counting
• Develop a cell model
• Circuit model
• Table Lookup

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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)
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OCV curve
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OCV voltage table DOD representation
Vmax
Vmin
Flat Zone
DOD Depth of Discharge SOC State of
Charge DOD 100 - SOC
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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
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Basic Smart Battery System
CHG
DSG
Vbatt
VPACK
ICHG
VCHG
VDSG
Gas Gauge
comm
Battery Model
Tbatt
Load
Charger
IDSG
Ibatt
Rs
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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

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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

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Factors affecting capacity estimation

• PCB component accuracy
• Instrumentation accuracy
• Cell model fidelity
• Aging
• Temperature

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PCB component accuracy

• Example
• Current sensing resistor
• Trace length (resistance)

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Instrumentation accuracy

• ADC Resolution
• Sampling rate
• Voltage drift / calibration
• Noisy immunity

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Battery model fidelity

• Steady-state (DC)
• Transient (AC)
• Capacity degradation
• Aging
• Overcharge

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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

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Temperature

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

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Safety

• High operating temperature
• Accelerates cell degradation
• Thermal runaway and explosion
• LiCoO2 Cathode reacts with electrolyte at 175C
  with 4.3 V
• Cathode coatings help considerably
• LiFePO4 shows huge improvement! Thermal runaway
  is gt 350C

OCV 4.3 V
Heat Flow (W/g)
Thermal Runaway
100 125 150 175 200 225
250 Temperature (C)
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Cell Safety

• Safety Elements
• Pressure relief valve
• PTC element
• Aluminum or steel case
• Polyolefin separator
• Low melting point (135 to 165C)
• 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

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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

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Overcurrent Protection Details
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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)

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JEITA/BAJ Guidelines for Notebook

• Do not charge if Tlt 0C or Tgt 50C
• 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
T1
T5
T6
T3
T4
(100C)
(450C)
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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

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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
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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
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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 ltlt RDS(on)
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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

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Communication and Display

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

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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

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Section 2 Battery Fuel Gauging CEDV Z-track
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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

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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

• External battery voltage (blue curve) V V0CV
  I RBAT
• Higher C-rate ?EDV is reached earlier (higher I
  RBAT)

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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
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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
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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
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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 SOC7 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)
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CEDV Model Visualization
Voltage
Actual battery voltage curve
Battery Low
3
4
5
6
7
8
9
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CEDV Formula
CEDV CV - IEDVR0/40961
EDVR1Cact/16384 1 EDVT0(10T -
10Tadj)/(25665536)1(CCEDVA0)/(465536)
age Where CV EMF1 EDVC0(10T)log(Cact)/(
25665536) Cact 256/(2.56RSOC EDVC1) 1
for (2.56RSOC EDVC1) gt 0 Cact 255 for
(2.56RSOC EDVC1) 0 EDVC1 2.56 Residual
Capacity () Curve Fit factor Tadj
EDVTC(296-T) for Tlt 296oK and Tadj lt T Tadj 0
for T gt 296 oK and Tadj max value T age 1 8
CycleCount A0 / 65536.
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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

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Current Direction Thresholds and Delays
8

1. CHG relaxation timed
2. Enter RELAX mode
3. Start discharging
4. Enter DSG mode
5. DSG relaxation timed
6. Enter RELAX mode
7. Start charging
8. Enter CHG mode

1
2
3
7
6
5
4
Example of the Algorithm Operation Mode Changes
With Varying SBS.Current( )
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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
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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

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Resistance Update
Before Update
Discharge direction
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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
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Timing of Qmax Update
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FCC Learning
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Modeling temperature

• 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.
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RemCap Simulation (concept)
Start of discharge
V (loaded)
IR
OCV
? V gt 250mV
EDV
Vterm
Time
?Q/2
I
?Q/4
RsvCap
Qstart
. . . . .
?Q
?Q
?Q
Time
RemCap
Constant Load Example
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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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CEDV, Impedance Track Comparison
Property CEDV Impedance Track
Worst error new, learned /-2 /-1
Worst error aged, learned 30 (/- 15 with age data) /-2
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) Yes
FCC rate compensation No (with rare exceptions) Yes
Learning cycle in production required Not required
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Thank You