Week 7 State of charge estimation
Study 1 - Simulate the 3 test cases from harness dashboard and write a detailed report on the results.
Click on the Test Sequence Variant and select Test1
Click on the BMS sequence Block
The Chart shows when the simulation starts the model will be in Driving condition. After 3000Sec it will be in standby mode and after 1000secs it will be in Charging mode. Now Run the simulation.
Results : All indicators are shown in green colour which means no fault has occured.
The Plot shows that the vehicle is initially in driving state and charging was done till 90%. After Charging it reaches to standby mode. It can be seen in BMS state plot.Battery Temperature increases in driving and charging mode. Voltage of battery increased to 4.2V while charging. Current Plot shows the value of supplied current to vehicle while driving. SOC decreased upto50% in driving and it was increased while charging.
2) Change the Test sequence variant to 2
In the test 2 we can see that the driving stage is upto 1000seconds and then the charging starts.
Results:
The SOC is decreasing in the above graph in charging mode which shows a fault in the 2nd Test.
Now change the Test sequence Variant to 3
This Test has only charging mode that runs for 0 to 20000secs.
Results:
The Cell Temperature is decreased when the standbymode was achieved. The SOC rached to 100% in charging and remained in standby mode. From Case 3 we can observe that only charging takes place thus SOC was 80% initially then it increases to 100% in 20000secs and remains constant.
2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
Coulomb counting is a technique used to track the State of Charge of a battery pack. It works by integrating the active flowing current (measured in amps) over time to derive the total sum of energy entering or leaving the battery pack. This produces a capacity that is typically measured in Amp-hours.The coulomb counting method measures the discharging current of a battery and integrates the dicharging current over time in order to estimate SOC.
The SOC is calulated by following equation:
But there are several factors that affect the accuracy of coulomb counting method including temperature battery history, discharge current and cycle life.
To Improve the coulomb counting method a new technique called modified coulomb method is proposed. The modified coulomb counting method uses the corrected current to improve the accuracy of the 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 following form
Where K2 K1 and k0 are constant values obtained from the practice experimental data. In modified method SOC is calculated by Following equation:
The experimental results shows that the accuracy of modified coulomb counting method is suprior to the conventional coulomb counting method. But there are several factors involve that affect the accuracy of coulomb counting method including temperature, Battery histroy, Discharge current and Cycle life.
BMS System:
Click on the SOC Estimation Block where we can see coulomb counting block.
Coulomb Counting block and its subsystem shown below.
- 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 battery is given as inputs.
-Current signals is given to a gain block and temperature is given to capacity since it varies according to temperature.
-These inputs are given for intehgration where current will be integrated with respect to time.
- This output is given to another block where it is divided by capacity.
- This is how the SOC is estimated.