Week 7 State of charge estimation

Aim:

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:

First we have to simulate the first three cases from harness dashboard. We need to select the harness dashboard from project file which we downloaded with reference link : -https://in.mathworks.com/matlabcentral/fileexchange/72865-design-and-test-lithium-ion-battery-management-algorithms

The harness dashboard is shown below: -

1. Click on the test sequence variant block and choose test case as 1.

2.Now by clicking on the BMS sequence block we get this below chart.

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

The simulation Result:

Here all the indicators are green so we can say that there is no error in this model.

• 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 we select the second test variant:

By clicking the BMS Sequence test block we will get this:

Initially it is a driving condition after 10000 sec it is a charging condition.

Result:

From the above plot a fault condition occurs in the second case while the charging is started but the soc didn’t increase. The battery completely discharge in 8692 seconds and the charging state was not achieved at 10000 seconds battery.

3.Now we select the third test variant:

Looking at 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.

Result:

• Looking at 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.

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.

Double Click on the BMS ECU you will get the another subsystem which is given below:

Now click on the soc estimation you will get this type of subsystem:

When you open the coulomb counting the below system is open.

This system carries out the estimation by coulomb counting equation mentioned earlier. The main advantage of coulomb counting is that of its simplicity and low computational cost. Temperature and current are taken as input from the battery. Current signal is given to the gain block and the temperature is given to capacity as it varies according to the temperature. These inputs are given to integration where current will be integrated w.r.t time. This output is given to another block where it is divided by capacity and henceforth the SOC is estimated.