Week 7 State of charge estimation
1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results.
Ans :
Here, we have to simulate 3 test cases from harness dashboard. We need to, select the harness dashboard from project file which we downloaded with the reference link https://in.mathworks.com/matlabcentral/fileexchange/72865-design-and-test-lithium-ion-battery-management-algorithms . The battery model harness dash board is represented below.
when Test Sequence Variant is 1.
After simulating the model for 20000 seconds as given default.
As we look at the test variant as shown below, it states that at the start of the simulation the model will be in the driving condition, thereafter, after 3000 seconds it will be in balancing mode, after 1000 seconds it will be in charging and balancing mode and after 5000 seconds it will be in post charge balancing mode.
It can be observed from the plot that the vehicle is initially in the driving state and charging was done till 90%. After the charging mode it reaches to the standby mode which can be seen from the BMS state plot. From the Cell temperatures plot it is observed that the battery temperature rises in driving and charging. The battery voltage was increased to 4.2 V while charging. The current plot shows the current value supplied to the vehicle while driving. The SOC was observed to be decreased around 50% in driving condition, which increased while charging.
when Test Sequence Variant is 2.
After simulating the model for 20000 seconds as given default.
It was observed from the plot that the battery discharges completely in 8692 seconds and the vehicle does not go forward and the charging state was not achieved at 10000 seconds. Hence the vehicle comes to standby condition and simulation stops midway.
when Test Sequence Variant is 3.
After simulating the model for 20000 seconds as given default..
from the condition below for third variant of the simulation. It is just the continuous charging condition from the start till the end of 20000 seconds of simulation.
The plot it can be seen that the battery starts charging and the SOC starts increasing from beginning to around 2000 seconds and stays stable at 100% continuously from 2000 seconds until 20000 seconds. The peak current plot drops from 0 to 2000 seconds and stays constant as the charge is not used anywhere as the driving is not present.
From the above simulated 3 variants of the simulation carried out it was observed that,
- In the first simulation all three conditions were simulated such as driving charging and the stable state hence the charging and discharging were observed and in the end the constant 100% SOC was observed because of the standby state of the vehicle.
- In the second simulation it was just the discharging as the SOC dropped to 0% in the driving state within 8600 seconds and the charging state was not achieved at 10000 seconds.
- In the third simulation, only charging was observed for the first 2000 seconds and the SOC remained at 100% constant for the next 18000 seconds.
Study 2:-What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
coulomb counting:-
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 , which is estimated from the discharging current, , and previously estimated SOC values, . 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,k1andk0k2,k1andk0are constant values obtained from the practice experimental data.
In modified Coulomb counting method, SOC is calculated by the following equation:
The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.
But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life .
For example we can look into the project file we downloaded. A BMS system in the file is represented below.
BMS algorithm models , protects , reports the measurement from battery pack.
By Clicking on the BMS ECU we get the Subsystem
By clicking on the SOC estimation block we get
From above figure we observe the Coulomb Counting block.
A subsystem opens when we click on the Coulomb Counting
- This system does the estimation by coulomb counting equation mentioned above.
- Advantage of coulomb counting method is its simplicity and low computational cost.
- Temperature and current from the battery is given as inputs.
- Current signal is given to a gain block and temperature is given to capacity since it varies according to temperature.
- These inputs are given for integration where current will be integrated with respect to time.
- This outout is given to another block where it is divided by capacity.
- Hence the SOC is estimated.