Week 7 State of charge estimation
Challenge #7:
- Simulate the 3 test cases from harness dashboard and write a detailed report on the results
- What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation?
Answer: 1
Test Case #1:
Observations:
- BMS_Driving: No Fault is observed
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- Its hovering between 3.9 & 3.2V - Charging & discharging of the battery is happening during the drive cycle based of energy supplied or drawn from the pack |
- Increases from 295 until 302 F - Based on the location of the cells in the battery pack, the temperature variations are different |
- Dropped from 0.8 to approx. 0.45 – 0.54 - The SOC drop varies with different cell due to different cell temperature |
- Based the connected load, the pack current is hovering between +18A (charging) and -70A (discharging) |
- BMS_Standby: No Fault is observed
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- The load is disconnected, and voltage seems to be stabilized at 3.7 V |
- The cells are getting cooled during the stand by mode - The rate of cooling is different for different cells based on its location |
- No variations are observed. - SOC remains constant during standby period |
- Since there is no charge/discharge, the pack current in Zero |
- BMS_Charging: No Fault is observed
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- The cells are getting charged; hence the cell voltage increases from 3.7V back to 4.2 V - The cells are getting charged by CVCC modes. |
- The cells temperatures are increasing until the cell voltage reaches the nominal value of 4.2V (CC charging) and there after starts to drops |
- Once the charging is over, SOC increases back to 100% |
- Initially current is constant when is the cells are getting charged under CC. - After the pack current decreases gradually when the cell voltage remains constant |
- BMS_Standby: No Fault is observed
All the above parameters seem to be steady
Test Case #2:
Observations on BMS States
- BMS_Driving
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- Its hovering between 3.9 & 3.2V - Charging & discharging of the battery is happening during the drive cycle based of energy supplied or drawn from the pack |
- Increases from 295 until 302 F - Based on the location of the cells in the battery pack, the temperature variations are different |
- Dropped from 0.8 to approx. 0.45 – 0.54 - The SOC drop varies with different cell due to different cell temperature |
- Based the connected load, the pack current is hovering between +18A (charging) and -70A (discharging) |
- BMS_Charging
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- Cell voltage increases from around 3.9V to 4.1V after which the cell disconnected due to fault situation |
- During the charging period, the cell temperature steadily increases to 323 F. - Once the cell temperatures raises above this limit value, the battery gets disconnected and temperature starts to drop |
- SOC increases from 0.35 to 0.75 during the charge cycle |
- The charging current is observed to be 30A |
- BMS_Fault
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- During the fault situation, the charging current gets disconnected and terminal voltage is observed to be around 3.85-3.9V |
- The cell temperatures drops gradually once the BMS initiates fault state |
- During the fault condition, the battery stops charging and SOC remains at 0.75 |
- Once the BMS state turns to fault, the pack current drops to zero |
Test Case #3:
Observations on BMS States
- BMS_Charging
|
Cell Voltage |
Cell Temperature |
SOC |
Pack Current |
|
- The cell voltages reach around 4.15 to 4.2 during the charging period. - After SOC reached 100%, there is a small drop in terminal voltages of the cells indicating self-discharge. |
- Increases to 292.5 during the time when the cells reach 100% SOC. - After that, the cell temperatures drop gradually indicating that the cells are getting cool down. |
- During this charging cycle, the SOC increases gradually from 0.80 to 1 for certain cells but for other cells, its found to reach around 0.94. |
- Once the cell voltages reaches the nominal voltage, the pack current starts to drop. - And when the SOC reaches 1%, the current drops to zero. |
Answer: 2 - Coulomb Counting method:
Coulomb counting is a method of estimating the SOC of a battery or a battery pack.
Basic Concept:
Current is defined as rate of flow of charges.
- I = dq/dt or dq = I x dt
Coulomb counting works by integrating the active flowing current (measured in amps) over time to derive the total sum of energy entering or leaving the battery pack. This gives the capacity in Ah (Ampere hour).
Discharging:
- This capacity divided by the nominal capacity of the battery is basically Depth of Discharge (DoD)
- Calculated capacity is the capacity which is drawn from the battery during the discharge cycle
Charging:
- Calculated capacity is the capacity which is supplied to the battery during the charge cycle
Mathematically SOC is calculated via coulomb counting method by the below equation:
- µn: Nominal Capacity of the battery
- i(t): battery current at time instant ‘t’ this will be -ve during discharge & +ve during charging
- Δt: step time
- SOC(t): State of charge at time ‘t’
- SOC(t-1): State of charge at time ‘t-1’
Below is the model being used in the battery model for Coulomb counting in SOC estimation:
- The battery current is divided by 3600sec to get the output as Ah
- Based on the input temperature, from the 1-D look up table, the respective nominal battery capacity is found and input to the next divide block
- The instantaneous Ah (battery capacity) is divided by the nominal battery capacity using the divide block
- Then the output is fed to an Discrete-Time Integrator which adds up all the instantaneous SOC to calculate the SOC of the battery pack and cell
Below is another Simulink model which is designed based on Coulomb counting principle: