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 ?

Hint :(Download the file from the link and go to SOC estimation model-ECU block)

Solution:

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

The Closed Loop Harness Dashboard model by Mathworks is as shown below

To select one of the 3 test conditions, it can be selected via the Test Sequence Variant block which allows you to select which test needs to be simulated. The first variant was selected initially for the simulation which can be seen below.

As we look at the test variant as shown below, it states that at the start of the simulation the model will be in the driving condition, thereafter, after 3000 seconds it will be in balancing mode, after 1000 seconds it will be in charging and balancing mode and after 5000 seconds it will be in post charge balancing mode.

After simulating the model for 20000 seconds as given default.

Here we can see all the lights have turned green as there is no fault or errors in the model.

It can be 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.

Coming to the second variant condition to simulate.

The second variant simulates the below shown conditions.

Initially it is the driving condition and after 10000 seconds there is charging condition.

It was observed from the plot that the battery discharges completely in 8692 seconds and the vehicle does not go forward and the charging state was not achieved at 10000 seconds. Hence the vehicle comes to standby condition and simulation stops midway.

Coming to the third condition.

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.

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%.

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.

Part 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.

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,

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

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.

A subsystem opens when we click on the Coulomb Counting

1. This system does the estimation by coulomb counting equation mentioned above.

2. Advantage of coulomb counting method is its simplicity and low computational cost.

3. Temperature and current from the battery is given as inputs.

4. Current signal is given to a gain block and temperature is given to capacity since it varies according to temperature.

5. These inputs are given for integration where current will be integrated with respect to time.

6. This outout is given to another block where it is divided by capacity.

7. Hence the SOC is estimated.

Please find below attached files for your reference.