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:

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

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

The BMS Model consist of BMS ECU and a Plant. The control unit with its various monitoring and control algorithms is connected to a block with the representation of the battery pack and associated circuitly and peripherals. This model contains two various of the battery pack, a small one with just 6 cell series, and a larger 16-module pack, each module containing a 6-cell series string. The architecture is 6s-Ip and 96s-Ip packs, respectively.

• 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.

• Click on the Test Sequence Varient Block and select test case as 1.
• When simulation will start, the model will be in driving condition. After 3000 seconds it will be in standby mode and after 2000 seconds it will be in charging mode. Now by Running the simulation.

• All the lights have turned green as there is no fault or errors in the model.
• 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.

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

• From the above graph , a fault condition occured in second case.

So even after charging was started, the soc didnt increase. It continue 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.
• Here we can see that the soc 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 increases to 100% in 2000 seconds then SOC remains constant at 100%.

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, .

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, k1and k0 k2, k1and 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.
• But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life .
• From the given model by Mathworks as shown below of BMS Closed Loop model, when seen inside the BMS ECU,

Inside the SOC estimation block

• 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.
• 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 wrt time.
• This output is given to another block where it is divided by capacity and henceforth the SOC is estimated.

• A subsystem opens when we click on the Coulomb Counting
• 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.