Week 7 State of charge estimation
Aim : To simulate the model of design and test lithium ion battery management algorithm for differnt three cases .
Objective / Questions :
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 ?
Answer :
1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results
we used below link download the model which we want to simulate as link as below
after download we can extract the given zip file and than go to palnt folder in that we get the model mat file so we goto matlab window in that select simulink open it than select the open file option than choose the model file and opent it than we select and here we simulate the harness dashboard so we go matlab window in that system folder which is open that time we select them model so here select harness dashboard that window or model shown in below
we see this dashboard so we can use this to simulate
in that we choose the case by test sequence variant block double click on it and than selecte required block shown in below
choose test case in that we select 1 case than run it for 20000 sec than show the below harness dashboard model
Here we can see all the lights have turned green as there is no fault or errors in the model.
we get the results as below
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.
here we click on BMS test request and we show that as below
here done the 1 case so we click on it than show the below chart
in that we see that driving after 3000 sec and balancing 1000 sec and charging start after 5000 sec than next step is post charge balancing
after that we select second case as shown in below
tan run it than we see harness model
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.
to same think done click on BMS test request and show below figure
and to click on it we show below chart
now we select the third test
than we run it
in that we goto BMS State request block we show the below figure
we can see that first 2 case done and now open the 3 cases and we can open it we sohw the below chart
looking at the condition above chart for case 3 of the simulation. It is just the continuous charging condition from the start till the end of 20000 seconds of simulation.
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 formula
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
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.
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 with respected to time. This output is given to another block where it is divided by capacity and henceforth the SOC is estimated.