Week 7 State of charge estimation
CHALLENGE 7(STATE OF CHARGE ESTIMATION)
AIM:
TO STUDY ABOUT STATE OF CHARGE ESTIMATION.
Objective:
STUDY#1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results
STUDY#2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
Downloading and open closedloop BMS harness dashboard model:
Download the Zip file and extract to a folder.The zip file link is given below
Open Matlab software and open the simulink model file of 'Closed loop BMS harness dashboard' in Tests-->System-->BMS_Closedloop_Harness_Dashboard model file.
Then give open project and model in pop-up dialog box to install all the model related library files into the current folders and workspace.
The model file open and the model are above as,
Blocks in model:
1)Dash board
- Test Sequence Varient(Selector).
- Manual Varient(Selector).
- BMS_Testsequence state request(Subsystem) to provide state request commands.
- Lamps to indicate Fault state,High temperature,Over voltage,Over current,charging ,voltage sensor operation,inverter operation,low temperature,Under voltage.If any fualt in these red indication will intimate us and all in good conditions indicated by green indication.
- Rotating switch for manual Varient changes while running simulation.
- In scope results the SOC of module,Module current,Individual cell voltages,Individual cell temperature,Battery state,Cell balancing states are seen as outputs.
2) BMS_Closed loop
- There are two subsystems in this main block: the BMS ECU and the PLANT units.
- As part of the BMS input feedback loop, the plant output from the plant unit is sent into the BMS ECU.
- The BMS ECU unit receives the state request.
- The 'From BMS' command is used to input BMS output commands to the plant unit.
- The status request is then sent to the Plant unit.
- BMS input,BMS Output, and BMs info are the closed loop model's final outputs.
In BMS ECU unit,the current power limit calculations are done used in State machine block,Soc Estimations,Cell balancing logics as shown below,
The battery pack unit, pre charge unit, and charger/load units are all formed in the Plant unit.
There are two sorts of battery pack configurations: 1 module (6 cells) and 16 modules (96 cells).
In this case, we've chosen 1 module to reduce simulation running time.
STUDY#1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results
We have to test three cases for this model by selecting test sequence varient and select the cases one by one as follows,
Test case 1:
Select 1:From test sequence varient dialog box and click apply as shown below.
This selection consist of selection changes in masked BMS_State request subsystem as,
This test case consist of the fallowing sequence operations as,
Here from the above test sequence editor figure it was set as,
- For the first 3000 seconds, drive.
- After 3000 seconds, balancing procedures begin for the next 1000 seconds.
- Then, for the following 5000 seconds, charging and balancing take place.
- Following that, a post-charge balancing process occurs until the simulation time ends.
Results:
Run the simulation and results are shown as,
The lamp indications are showing green for overall simulation period showing that there are no faults occurs during the sequences.
From the results we can understand that,
Cell Voltages and Pack Current-
For the first 3000 seconds, all 6 cells are in drive mode, when discharging happens primarily, and for a shorter period of time, between 1000 and 2000 seconds.
In this range, the voltages vary from 3.1V(min) to 3.9V(max).
Cell balancing happens between 3000 and 4000 seconds, and the module is in standby mode with a steady voltage of roughly 3.7V.
After 4000 seconds of charging, the voltage rises from 3.7V to 4V. At 4V to 4.2V, the constant current is approximately 30 A, and after 4.2 V, the constant voltage drops the current from 30A to 2.6A.
At the completion of the charging process, cell balancing can take up to 9000 seconds.
Post-charge cell balancing begins after 9000 seconds. All cell voltages are balancing to a value of 4.1 V with regard to cell 1, as indicated below; at this point, the pack is in standby mode.
Cell temperatures:
For the first 3000 seconds, the temperature range of cells steadily increases from 288 K to 302 K.
For all stages, cell 1 has a lower temperature while cell 6 has a higher temperature.
The cell temperatures are lowered somewhat after 3000 seconds to 4000 seconds because no current runs through the cell in standby mode.
After 4000secs, the temperature begins to rise exponentially up to the constant current zone and then begins to fall as current flow decreases in the constant voltage and cell balancing regions up to 9000secs.
Cell 1's maximum temperature was raised to 302K, while cell 6's temperature was boosted to 317K.
After 9000 seconds, the temperatures continue to fall and remain constant until they approach the ambient temperature.
BMS State:
The battery module was in drive mode for 3000 seconds (discharging).
The battery module was in balanced standby mode from 3000 to 4000 seconds, as seen in the result figure.
The battery module is in charging and balancing mode after 4000 seconds and up to 9000 seconds.
After 9000 seconds, only the post-charge balancing mode is active till the simulation ends.
SOC:
The initial state of charge of the battery module is 80%.
The yellow curve here indicates SOC estimate using the coulomb counting approach.The blue plot depicts the estimate of SOC using the Unscented Kalman filter approach.
The extended Kalman filter approach is used to estimate SOC in the red plot.Among the three procedures, UKF and EKF estimate more precisely than the coulomb counting methodology.
During driving mode, SOC drops to about 45 percent after 3000 seconds.
As cell balancing happens in standby mode, the SOC remains constant at 45 percent after 3000 seconds to 4000 seconds.
Module in charging mode from 4000 secs to 9000 secs.(4000–7000=CC mode, 7000–9000=CV mode).
As observed in the charts above, after 9000 seconds, the SOC becomes steady at around 94 percent.
Balanceing cells:
Initially upto 3000 secs balancing was not performing.
After 3000 seconds, cell balancing begins on cell 6 and cell 2 as they increase to 1 as seen above since their voltage is greater than all other cell voltages.
Following this step of charging and balancing, post-charge balancing happens as Cell 6 and Cell 2 remain in one state until the completion of the simulation.
Because cell 1 (yellow) has a lower voltage than the other cells, it remains in the 0 state throughout the simulation.
At the end of the simulation, all cell voltages attempt to balance evenly, as shown in the single cell voltages plot previously.
Test case 2:
Now select test case 2 and click apply in test sequence varient dialog box as shown below,
The case is altered to second as shown below,
This test case consist of drive mode for 10000secs and after that charging occurs till end of simulation.
Now run the model and results can be viewed.
Results:
The dashboard results are shown as without any faults occured.
The scope result plotted as,
Here in these plots only driving mode of battery module is plotted.
Cell Voltages and Pack Current-
The cell voltage ranges from 3.9V to 3.05V as the cutoff voltage up to 9000 seconds. At 9000 seconds, the whole capacity was depleted.
During the overall drive, the pack current discharges to a maximum of 70A and slightly charges.
Some standby mode circumstances exist in the transition between drive modes.
Cell temperature:
Cell 1 has less temperature variance, ranging from 288K to around 302.5K.
Cell 6 has a greater temperature range, ranging from 288K to roughly 322.5K.
As these periods are in stand by modes, the cell temperatures are somewhat decreasing in between driving modes.
Battery module state:
Entire sequence is performing in drive mode as shown from the above plot.
Small duration of standby modes are not plotted in this plot as simulation upto 10000secs command is given as drive mode.
But in actual practical situation standby modes are not present present as battery current drains by other small auxilaries of EV's.
SOC of module:
As demonstrated in the above plot, the SOC decreases from 80 percent to 0 percent (as current is drained for a longer period of time) in the full driving mode.
The SOC remains constant at standby modes in between them, as demonstrated.
Balancing:
Here balancing of cells are not performed as shown as balance states are in 0 for entire drive mode.
Test case 3:
Now select test case 3 and click apply in test sequence varient dialog box as shown below,
The test case is altered to third as shown below,
This test case consist of charging for entire duration of time.
Run the model and results can be viewed.
Result:
The dashboard lamps indications are shown as,
All indications are in green indicates no fault conditions are occured.
The plots are shown as,
Cell voltages and pack current:
Initially, the voltage in cell 2 climbs from 3.9V (approx) to 4.2V and then stabilises at 4.15V.
Cell 1 travels from 3.85V to 4.1V and then stabilises at 4.08V.
The battery pack current increases from 0A to 30A and then gradually decreases to 2.5 A at the conclusion of complete charge for up to 2000 seconds.
The pack is completely charged to 100% after 2000 seconds.
The current flow will be turned off after full charging, as indicated in the result plot.
Cell temperatures:
The cell temperatures continue to increase heat upto full charge and after that the temperatures get started to reduces to ambient temperature.
As shown from result plot the temperatures start decreases after full charge(at 2000 secs).
BMS state:
Entire battery pack sequence is operating in charging mode as shown above.
SOC:
The pack SOC is fully charged at 2000secs and remains fully charged for the remainder of the sequence, as illustrated above.
The true condition of charge is roughly 93 percent as revealed by UKF and EKF procedures as full charge.
Cell Balancing:
Here from 0sec the cell 6 and 2 starts to balance with respect to cell 1 and all cells are try to balancing at end of simulation.
Conclusion:
Hence from above three test cases the performance of BMS on various factors are discussed and analysed.
B)To explain coulomb counting for finding the SOC estimation in BMS.
Coulomb counting:
SOC(State Of Charge) is defined as the rate of available capacity to its maximum capacity when a battery is fully charged and describes the remaining percentage of the battery
capacity.
To find this SOC % we are using coulomb counting method.
The coulomb counting method measures the discharging current of the battery and integrates the discharging current over time in order to estimate SOC.
Which can be estimated from discharrging current(I(t)),previously estimated SOC values(SOC(t-1)).The relation can be given as,
Modified Coulomd counting:
In this technidue discharge current is corrected with a function and corrected current is found out for accurate SOC estimation.
The function can be given as,
Where K0,K1,K2 are constant values obtained from the practice experimental data.
The SOC relation can be given as,
SOC estimation in BMS model:
To see the model of coulomb counting technique in BMS model click the subsystem of SOC_Estimation and click Coulomb counting.
It is developed as shown below,
In this case, the discharge current from the battery is delivered to gain with a value of 1/3600 in order to convert current to Ahr.
Due to temperature changes, the value is divided by the capacity. Then, using a discrete integrator, we integrated with a gain of 1 and a sample time of -1.
Saturation limits range from 0 to 1.
We'll take the SOC values ranging from 0 to 1 and multiply them by 100 to get the percentage of SOC.
