Week 7 State of charge estimation
Challenege URL:- https://drive.google.com/drive/folders/1dRmy6qu5csbhqZ38OcRzC0gL4oV1ziwL?usp=sharing
To open the Lithium Ion Battery Management algorithm firstly we have to download the zip file from the link given in the task and the link has also been attached below for refrence.
After downloading the zip file we first have to extract the zip file to a folder and then I choose again Battery_System_R2020a to extract to a a folder.
Then to get access to the Close Loop BMS harness dashboard we have to check for the following model Test>System>BMS_ClosedLoop_Harness_Dashboard model file.
After opening this file Matlab will ask the following question:-
I have selected to Open Project and Model. ANd after that the whole project will be loaded in the Matlab in current folder and workkspace.
After this the BMS_ClosedLoop_Harness_Dashboard model file is open as:-
Block of Model:-
- Test sequence Variant (it has 3 different test condition)
- Manual Variant (selection)
- BMS_StateRequest subsytem to provide the state request commands.
- Indictaing lamps are there to show any fault state, High Temp, Overvoltage, Charger, Voltage Sensor, Overcurrent, Low temp, Under Voltage, Inverter state of a battery. Green light tell us that all states are Ok and battery is good to charge and discharge and red indication will indicate the fault state.
- Rotating switch for manual variant to change the position during simulation run time
- BMS_Closed Loop:- this subsystem contain BMS ECU and Plant as two main subsytem and lets see this block in details:-
BMS ECU:- The BMS ECU get have two input which are BMS_Input which is feedaback from the Plant model and The state Request is coming from the state machine . This is generating output as BMS Output and Info which have current power limit calculation in State Machine, SOC estimation and Cell balancing logic as shown below:-
Plant:- This subsytem contain Battery pack, precharge circuit and charger / loads.
In the battery pack there are two configuration which are 1 module (6 cells) and 16 module (96 cells).
Here we have selected 1 module for simple calaulation and less simulation time.
- In the scope SOC of module, Cell voltages, Pack Current, Cell Temperature, BMS state and cell balancing command are seen as output.
Task:- 1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results
For simulating the three test cases we have to go to the Test sequence Vaeiant and select there Test Case one by one.
a) Test Case 1:-
The test case 1 is selected as shown below in the dialog box and click OK to implement it.
We can see the Test sequence in BMS_StateRequest>BMS_TestSequenece and there we can see that the test case we have selected is as shown below:-
Now to see what are the test condition we have to double click on the Test Sequence and the test sequence will be shown as follows:-
The editor figure tell us about the test sequence and it is as following:-
- Driving is done for first 3000 sec.
- Then after driving Balancing of cell ocuurs for the next 1000 sec duration.
- After the balancing Charging and balancing occur for next 5000sec
- Post Charge Balancing is occuring after 9000sec till 20000 sec (i.e. simulation end time.)
Results of Test Sequence 1:-
Simulation is ran for 20000 sec and the result are as below:-
The Lamp indication were green for all states which means the model is fault free.
i) Driving (Discharging) mode (0-3000 sec):- All the cells for the first 3000 sec are in driving mode so the dicharge of all the cell occurs and its varies in the range of 3.1 to 3.9 V (min and max.).
Cell temprature increases from 288K to 302 K.
SOC also reduces from 80 % to 45% for EKF technique.
ii) Balancing mode (3000-4000 sec) :- After 3000sec for 1000 sec, cell balancing occur and the Modules are in standby mode and they are having constant voltage near about 3.7V.
Cell temperature reduces slowly as no current flow through the cells during balancing.
SOC is also constant during this period i.e. 45% for EKF technique.
iii) Charging and Balancing (4000- 9000 sec):- After 4000 sec the charging occurs and the cell voltages increased from 3.7 to nearly about 4.2 V in Constant current mode with current of nearly 30A and then the constant voltage charging starts and current reduces from 30A to 2.6 A Charging and balancing of cells occur till 9000 sec.
Cell temperature also start to increase exponentially upto constant current region and reduces as current flow reduces in constant voltage mode.
SOC also increases from 45% to 94% in this mode for EKF technique.
iv) Post charge Balancing (9000- 20000sec):- After 9000 sec the post cell balancing starts under standby mode.
Cell temperature reduces in this mode and tries to reach the ambient temperature.
SOC remains constant to 94%.
b) Test case -2:-
The test case 2 is selected as shown below in the dialog box and click OK to implement it.
We can see the Test sequence in BMS_StateRequest>BMS_TestSequenece and there we can see that the test case we have selected is as shown below:-
The editor figure tell us about the test sequence and it is as following:-
- Driving is done for first 10000 sec.
- Then Charging is done for next 10000 sec.
Results of Test Sequence 2:-
Simulation is ran for 20000 sec and the result are as below:-
The Lamp indication were green for all states which means the model is fault free.
i) Driving Mode (0-10000 sec):- The cell voltage varies from 3.9 V to 3.05 volt ehich is cut off voltage of cell and the entire cell is drained upto 8692 sec.
Cell temperatures varies from 288K to 322.5K.
SOC reduces from 80% to 0% (cell got fully drained) in the drive mode under 8692 sec.
Cell balancing is not performed here.
ii) Charging Mode (10000- 20000 sec): The cell got drained earlier before reacching this mode.
c) Test case- 3
The test case 3 is selected as shown below in the dialog box and click OK to implement it.
We can see the Test sequence in BMS_StateRequest>BMS_TestSequenece and there we can see that the test case we have selected is as shown below:-
The editor figure tell us about the test sequence and it is as following:-
- Charging is done for entire simulation period i.e. 20000 sec.
Results of Test Sequence 3:-
The Lamp indication were green for all states which means the model is fault free.
i) Charging (0-20000 sec):- Cell Voltages increases from there intial values and after 2000 sec they got constant .
Pack current initally start from 0 to 30A and then start decreasing when the constant voltage mode is there to 2.5 A and got full charged upto 2000 sec.
SOC was initally 80% and in 2000 sec it got fully charged that is 100%.
Cell temperature starts to incraeses initally and when battery got fully charged then decrease down to the ambient temperature.
Task- 2 What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
Coulomb Counting Method:-
The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC. Coulomb counting method is done to estimate the SOC(t) , which is estimated from the discharging current,I(t) , and previously estimated SOC values,SOC(t-1) .
SOC is calculated by the following equation:
But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life
Modified Coulomb Counting Method
To improve the Coulomb counting method, a new technique called modified Coulomb counting method is proposed. The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation. The corrected current is the function of discharging current. There is a quadratic relationship between the corrected current and discharging current of battery. By practice of experimental data, corrected current is calculated by the following form:
where k2 , k1 and k0 are constant values obtained from the practice experimental data.
In modified Coulomb counting method, SOC is calculated by the following equation:
SOC estimation in Battery model:-
To observe the coulomb counting method we need to open the BMS_ClosedLoop>BMS ECU and then on SOC_ Estimation block
Here we see that there are three method for calculation of SOC estimation and these are:-
i) Coulomb counting
ii) Unscented Kalman filter
iii) Extended laman filter.
By opening the coulomb counting method we get to see the gain, 1-D look Up table, Product and Discrete Time Integrator block
The discharge current from the battery given to a gain having a value of 1/3600 to convert A to Ah.
That current is divided by the capacity due to temperature change and then intregrated with the gain value of -1 wo=ith sample time of -1 as discrete type integrator is used here.
The saturation limits are given which are 1 to 0. So that we get the SOC value from 0 to 1 multiplied with 100 to get percentage of SOC.
This Coulomb Counting methos is simple and require less computational compared to other two methods.