Week 7 State of charge estimation

1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results

2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?

Solution-

Study 1:-Simulate the 3 test cases from harness dashboard and write a detailed report on the results

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.

1)Click on the Test Sequence Varient Block and select test case as 1-

Now by clicking on the BMS Sequence block we get the chart

When simulation will start, the model will be in driving condition. After 3000 seconds it will be in standby mode and after 1000 seconds it will be in charging mode.

Now by Running the simulation.

Results:-

Here all the indicators are green. So we can see no errors & we check the plot for this model and is shown below.

From the plot we can see the vehicle is initially in driving state and charging was done till 90%. After charging it reaches to standby mode. It can be seen in BMS state plot.Battery temperature increases in driving and charging. Battery voltage was increased to 4.2V while charging. Current plot shows the value of current supplied to vehicle while driving. stae of charge decreased up to 50% in driving condition and it was increased while charging.

2) Now change the test sequence varient in case two.

Now by clicking on the BMS state request block-

Result:-

From the above graph , a fault condition occured in second case. So even after charging was started, the stae of charge didn't increase. It continue to decrease.

3) Now by changing the test Sequence Value to 3

This condition has only one phase & that is charging.Thus 0 to 20000 seconds simulation runs.

Result:-

• Here we can see that the stae of charge reached 100% in charging and remained in stand by mode. Cell temperature also decrased when stsndby mode was acheived.

From the 3 cases we observer that only charging takes place thus SOC was 80% at first , then SOC(stae of charge) increases to 100% in 2000 seconds then SOC(stae of charge) remains constant at 100%.

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(stae of charge) . Coulomb counting method is done to estimate the , which is estimated from the discharging current, , and previously estimated SOC(stae of charge) 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,k1 & k0 are constant values obtained from the practice experimental data.

In modified Coulomb counting method, SOC(stae of charge) 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& 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 below block

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 eqn mentioned above.
• Advantage of coulomb counting method is its simplicity & 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 c\n will be integrated with respect to time.
• This outout is given to another block where it is divided by capacity.
• Hence the SOC(stae of charge) is estimated.

Conclusion:

The 3 test cases from harness dashboard were simulated and discussed successfully along with the coulomb counting done by BMS for SOC estimation.