Week 7 State of charge estimation

Unable to submit the challenge completely due to some technical error, please refer to the below attached PDF for challenge 7, thank you.

Aim:

1. Simulate the three test cases from the harness dashboard and provide a detailed report on the results.

2. Explain coulomb counting and describe how the BMS uses coulomb counting for SOC estimation.

Study 1: Simulate the Three Test Cases from the Harness Dashboard and Provide a Detailed Report on the Results

To begin, we need to simulate the three test cases from the harness dashboard. To do this, select the harness dashboard from the project file downloaded from MathWorks. The battery model harness dashboard is illustrated below.

Test Case 1:

Click on the Test Sequence Variant Block and select test case 1.

Click on the BMS Sequence block to view the chart.

When the simulation begins, the model will start in the driving condition. After 3000 seconds, it will switch to standby mode, and after an additional 1000 seconds, it will enter the charging mode. Now, run the simulation.

Results:

All indicators are green, indicating no errors. The plot for this model is shown below.

From the plot, we can see the vehicle initially in the driving state. Charging occurs until the SOC reaches 90%. After charging, it transitions to standby mode, as seen in the BMS state plot. Battery temperature increases during driving and charging. Battery voltage rises to 4.2V during charging. The current plot shows the current supplied to the vehicle while driving. SOC decreases to 50% during driving and increases during charging.

Test Case 2:

Change the test sequence variant to case two.

Click on the BMS state request block.

Results:

· A fault condition occurred in the second case. Even after charging started, the SOC did not increase; it continued to decrease.

Test Case 3:

Change the test sequence value to 3.

This condition involves only one phase: charging. The simulation runs from 0 to 20000 seconds.

Results:

· The SOC reaches 100% during charging and remains in standby mode. Cell temperature decreases when standby mode is achieved. From these three cases, we observe that only charging takes place, resulting in SOC starting at 80%, increasing to 100% in 2000 seconds, and then remaining constant at 100%.

Study 2: What is Coulomb Counting? Explain How the BMS Implements Coulomb Counting for SOC Estimation.

Coulomb Counting:

Coulomb counting is a method used to estimate the State of Charge (SOC) of a battery by measuring the discharging current and integrating it over time. The principle behind coulomb counting is based on the fact that the SOC of a battery is proportional to the amount of charge removed from or added to the battery. The basic equation for coulomb counting is:

Where:

· SOC(t)\text{SOC}(t)SOC(t) is the state of charge at time ttt.

· SOC(t0)\text{SOC}(t_0)SOC(t0) is the initial state of charge.

· CratedC_{\text{rated}}Crated​ is the rated capacity of the battery.

· Idischarge(t)I_{\text{discharge}}(t)Idischarge​(t) is the discharge current at time ttt.

The accuracy of the coulomb counting method can be affected by several factors, including temperature, battery history, discharge current, and cycle life.

Modified Coulomb Counting Method:

To enhance the accuracy of the traditional coulomb counting method, a modified coulomb counting technique is proposed. This method uses a corrected current, which is a function of the discharging current. The corrected current is derived from a quadratic relationship with the discharging current. The corrected current is calculated using the following formula:

Where:

· k2,k1,k_2, k_1,k2​,k1​, and k0k_0k0​ are constants obtained from experimental data.

The SOC is then calculated using the corrected current as follows:

Experimental results show that the modified coulomb counting method provides better accuracy than the conventional method.

Example from the Project File:

In the project file, the BMS system is responsible for managing, protecting, and reporting measurements from the battery pack.

The BMS ECU subsystem includes the SOC estimation block, which contains the Coulomb Counting block.

This subsystem estimates SOC using the Coulomb counting equation mentioned above.

A subsystem opens when we click on the Coulomb Counting

Advantages of Coulomb Counting:

· Simplicity and low computational cost.

Implementation Details:

· Temperature and current from the battery are inputs.

· The current signal is fed to a gain block, and temperature affects capacity since it varies with temperature.

· These inputs are integrated with respect to time, and the output is divided by capacity to estimate SOC.

This method accurately estimates SOC by considering the integrated current and temperature variations over time.

Conclusion:

Simulating Test Cases:

o Test Case 1: The SOC was successfully managed with the vehicle initially in a driving state, transitioning to standby and charging states, showing appropriate SOC, voltage, and temperature changes.

o Test Case 2: A fault condition prevented SOC from increasing despite charging, highlighting the system's response to faults.

o Test Case 3: The SOC reached 100% during continuous charging and remained constant, demonstrating the system's behavior under prolonged charging conditions.

Coulomb Counting:

o Coulomb counting is a straightforward method for estimating SOC by integrating discharge current over time.

o The modified coulomb counting method improves accuracy by using corrected current values derived from a quadratic relationship with discharge current.

o The BMS implements coulomb counting through a series of steps involving temperature and current inputs, gain blocks, and integration, resulting in an accurate estimation of SOC.