Week 7 State of charge estimation

Aims:

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:

here given in question 

Matlab model to simulate the 3 test conditions from harness dashboard and to explain how BMS implements coulomb counting is as given below:

https://in.mathworks.com/matlabcentral/fileexchange/72865-design-and-test-lithium-ion-battery-management-algorithms

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.

the test variant as shown below, it states start of the simulation model  in the driving condition, after  3000 seconds it will be in balancing mode, after 1000 seconds  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.

  plot the vehicle is initially driving state charging 90%. to the standby mode  from the BMS state plot. From the Cell temperatures plot  is observed in battery temperature rises in driving  charging. The battery voltage was increased to 4.2 V while charging. The current value supplied to vehicle while driving. SOC  decreased  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.

the battery discharges completely in 8692 seconds  vehicle does not go forward the charging state was not achieved  10000 seconds. the vehicle comes to standby condition and simulation stops midway.

Coming to the third condition.

at the condition third variant simulation. It continuous charging condition start till end of 20000 seconds of simulation.

 the battery starts charging SOC starts increasing beginning to around 2000 seconds stable at 100% continuously from 2000 seconds until 20000 seconds. The peak current drops  0 to 2000 seconds and stays constant as the charge is not used anywhere as the driving is not present.

From above simulated 3 variants of the simulation carried out it was observed that,

•  first simulation all three conditions were simulated driving charging and stable state hence charging and discharging in end constant 100% SOC standby state of the vehicle.
•  second simulation  discharging SOC dropped  0% in driving state 8600 seconds charging state  not achieved at 10000 seconds.
•  third simulation, only charging observed for first 2000 seconds SOC remained 100% constant ,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 discharging current battery and integrates discharging current over time in order SOC. Coulomb counting  SOC(t), which is estimated  discharging current, I(t), estimated SOC values, SOC(t-1). SOC is calculated by the following equation:

several factors  affect accuracy Coulomb counting method including temperature, battery history, discharge current, and cycle life.

Modified Coulomb Counting Method

Coulomb counting method a new technique called modified Coulomb counting method is proposed.  current to improve the accuracy of estimation.

corrected current is discharging current. The quadratic relationship between corrected current and discharging current of battery. 

where k₂, k₁ and k₀ are constant values 

In modified Coulomb counting method, SOC is calculated 

The results show accuracy modified Coulomb counting method is superior to the conventional Coulomb counting method 

 BMS Closed Loop model, seen inside the BMS ECU,

And then inside the SOC estimation block

where the Coulomb counting was carried out. The model for the same can be seen below.

This system carries coulomb counting equation main advantage of coulomb counting of its simplicity and low computational cost. Temperature and current as input from battery. Current signal is given to gain block  temperature is given capacity it varies according  temperature. inputs  given integration current integrated wrt time. output is to another block is divided capacity SOC is estimated.

Conclusion:

• The 3 test cases from harness dashboard were simulated and discussed successfully along with the coulomb counting done by BMS for SOC estimation.