Week 7 State of charge estimation
Aim:
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 ?
Theory:
Matlab model to simulate the 3 test conditions from harness dashboard and to explain how BMS implements coloumb counting is as given below:
Part1:
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 teh 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.
1st variant:
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 sec it will be in balancing mode, after 1000 sec it will be in charging and balancing mode and after 5000 sec it will be in post charge balancing mode.
After simulating the model for 20000 sec as given as 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 that charging mode it will 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 4.2v 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.
2nd variant:
The second variant simulates the below shown conditions.
Initially ,it is the driving condition and after 10000 sec there is charging condition.
It was observed from the plot that the battery discharges completely in 8692sec and the vehicle does not go forword and the charging state was not acheived at 10000sec.Hence the vehicle comes to standby condition and simulation stops midway.
3rd variant condition:
Looking at the condition below for third variant of the simulation.It is just the condition charging condition from the start till the end of 20000 sec 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 sec and stays stable at 100% continuously from 2000 sec untill 20000 sec.The peak current plot drops from 0 to 2000 sec and stays constant as the charge is not used anywhere as the driving is not present.
From the above 3 simulated 3 variants of the simulation carried out it was observed that,
In the first simulation all the 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 stanby state of the vehicle.
In the second simulation it was just the dischrge as the SOC dropped to 0% in the driving state within 8600 sec and the charging state was not achieved at 10000sec.
In the third simulation, only charging was observed for the first 2000sec and the SOC remained at 100% constant for the next 18000 sec..
Part 2:
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 previous 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 Coloumb Counting Method:
To improve the coulomb counting method,a new technique called modified Coulomb cunting method is proposed.The modified Coulomb counting method uses the corrected current to improve the accuraacy 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 the constants 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 Coulumb counting method is superior to the convinental Coulumb counting method[1]
From the given model by Mathworks as shown below of BMS Closed loop model,when seen inside the BMS ECU,
This is 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 respect to time.This output is given to another block where it is divided by capacity and henceforth the SOC is estimated.
Conclusion:
Since we tested for three variant conditions from the harness dashboard were simulated ,coulomb counting done by Battery management system for SOC estimation.