Week 7 State of charge estimation

1). The harness dashboard of battery model is shown below.

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

First Test Case:

Now by clicking on the BMS Sequence block we get the chart (shown below),

As we can se here when simulation will start, the model will be in driving condition. After 3000 seconds it will be in standby mode and after 1000 seconds it will be in charging mode. Now by running the simulation.

Here we can see that all indicators are green. So we can see no errors and we check the plot for this model, which is shown below.

As we can see from the graph, the vehicle is initially in 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 graph. Battery temperature increases in driving and charging. Battery voltage was increased to 4.2V while charging. Graph of current shows the value of current supplied to the vehicle while driving. SOC decreased up to 50% in driving condition and it was increased while charging.

Second Test Case:

The second variant simulates the conditions which is shown below,

Initially it is in driving condition and after 10,000 seconds there is charging condition.

From the above graph, a fault condition occured in second case. So even after charging has been started, the SOC did not increase. It continues to decrease.

Third Test Case:

The third variant simulates the below shown conditions.

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 standby mode has been acheived.

From the all 3 test cases we observe 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%.

2). 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, . SOC is calculated by the following equation:

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.
As we can look in given model by Mathwork of BMS closed loop model,

By Clicking on the BMS ECU we get the subsystem as shown below,

And then by clicking on the SOC estimation, we get into a another subsystem which is shown below,

From above figure we observe the Coulomb Counting block.

A subsystem opens when we click on the Coulomb Counting,

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

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

Temperature and current from the battery is given as inputs.

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.

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

Hence the SOC is estimated.