Week 7 State of charge estimation

AIM: To Study the State of Charge Estimation and brief about coulomb counting and how BMS implements coulomb counting for SOC estimation

OBJECTIVES:

• Simulate the 3 test cases from the 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.

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

Now,at first 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 Variant Block and select test case as 1

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

Results:

Here all the indicators are green. So, we can see no errors and 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. SOC decreased up to 50% in driving condition and it was increased while charging.

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

Now by clicking on the BMS state request block

Result:

From the above graph, a fault condition occurred in second case. So even after charging was started, the soc didn’t increase. It continues to decrease.

3) Now by changing the test Sequence Value to 3

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

Result:

Here we can see that the soc reached 100% in charging and remained in standby mode. Cell temperature also decreased when standby mode was achieved.

From the 3 cases we observer that only charging takes place thus SOC was 80% at first, then SOC increases to 100% in 2000 seconds then SOC 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.

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

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:

The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method [1].

From the given model by MathWorks as shown below of BMS Closed Loop model, when seen inside the BMS ECU,

BMS algorithm models, protects, reports the measurement from battery pack.

By Clicking on the BMS ECU we get the Subsystem

And then inside the SOC estimation block

By clicking on the SOC estimation block we get

The Coulomb counting block can be observed on the top of the model, where the Coulomb counting was carried out. The model for the same can be seen below.

In this subsystem, we can see various methods to estimate the SOC. In this, the first block represents the coulomb counting method. Using the method, we will estimate the SOC. If we open that subsystem, we can observe what is happening or what blocks we have taken to estimate SOC and all. The following figure shows the system under Coulomb counting method.

So here we are taking the current and temperature parameters of the cells and giving them to the gain block and 1-D lookup table respectively. Where the lookup table gives the capacity over time. The capacity will vary with respect to the temperature so that is why we have taken the temperature as input to the look-up table. Now, these outputs from the gain block and look-up table are given to the product block, where the gain is multiplied means the current with respect to gain is multiplied. And the capacity has been integrated. That output is given to the discrete-time integrator block, it performs the integration operation and gives the value of SOC. As we discussed before it applies the formula here over the current and capacity.

The Coulomb counting method is associated with monitoring the input and the output current continuously. Since capacity is the integral of current with respect to time, by measuring the input and the output current, change in capacity or capacity degradation of a battery can be measured easily. In this method, we calculated by dividing the measured capacity (after discharging the battery to 0% SOC value) by its rated capacity. It is an extensively used method by researchers for its simplicity. But, the accuracy of this method is not very high. The main advantage of Coulomb counting is its simplicity, easy implementation, and low computational cost.

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