Battery Monitoring

Open Cell Voltage - DC

• Voltage is function of SOC

• The voltage across positive and negative terminal of battery while not discharging current to a load

• The internal resistance of the battery causes the OCV to decrease when current is discharged from it.

Open

v(t) = OCV(z(t)) − i(t)R0.

Open Cell Voltage - AC

• Voltage is function of SOC

Open

• In this model, R0 is internal resistance, R1 & C1 model the short term transient response, and R2 & C2 model the long term transient response.

Table Lookup

• The table relates capacity to voltage, current, temperature, and age.

• Not practical takes time to collect and test data

• Does not require a cell model

Voltage Lookup

• Accurate voltage measurements can be made while a cell is resting (ensure that cell has been resting for a considerable amount of time)

• The relation between capacity and voltage is non-linear. A typical model is shown below

Factors that Affect Capacity Estimation

• PCB component accuracy

  • Ideally components with low drift/offset

  • Trace length may affect resistance

• Instrumentation accuracy

  • ADC resolution

  • Sampling rate - Is the sampling fast enough to capture waveforms and integrate

  • Voltage drift (RESEARCH THIS MORE)

  • Noise immunity

• Cell model accuracy

  • Capacity is susceptible to degradation upon repeat cycles, how does the model accurately capture this.

  • Hard and time consuming to extract adequate parameters using collected data, for resistances and OCV

• Temperature

  • Impact parameters, especially resistance, and including max capacity, OCV, capacitance

• Aging

Protection

• Protect against short circuits, over/under voltage, FET failure, comm failure

• AFE lowers the voltage before inputting them into ADC, and can be programmed to turn off the MOSFET’s

Cell-Balancing (Example)

• Each cell is controlled by FET on the voltage input pin, which is pulled to ground when a cell needs to be bealnced.

• Balancing occurs by diverting some of the current to the 40 ohm resistor, which is controlled by the External FET as chargin occurs

Other Features of BMS

• Communication with host using I2X, SMBus, etc

• Displays with LEDs to indicate SOC or errors

• Logging to record max/min measurements and data for failure analysis (similiart to blackbox

Current Integration Based Fuel-Gauging

Coulomb Counting

• Involves counting the charges going into the battery by integrating the current

• Requires knowledge of the initial amount of charges

• Defining Q as the total capacity of the battery in Ah/mAh leaves us with the following equation sets of equations

• Thus we have RM = FCC - Q

• SOC is calculated as RM/FCC

• FCC is updated at every full discharge

• Learning when we reach 0% SOC is not ideal, since it’s too late.

• Therefore certain voltage thresholds (EDV 1, EDV 2,…) can be set

• Useful since these EDVs are fixed values, and can correct the SOC based on the OCV-SOC relation when coulomb counting becomes inaccurate.

Compensated EDV (CEDV)

• The model used to calculate the EDV values based on current SOC, current, and temperature.

• We want to predict the actual battery voltage curve

• Start with OCV, then correct it using the internal resistance and current

• Then correct it using the value of the low temperature, leaving us with the green curve which accurately resembles the actual battery voltage curve

CEDV Formula

Impedance Track Fuel-Gauging

• Includes advantages of both voltage lookup and coulomb counting

• Allows for SOC updates, where initial SOC can be measured when voltage is relaxed, in addition to self-discharge

• Impedance can be measured during discharge

• Total capacity can be measure without full discharge or charge