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
coulomb counting is as given below:-
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.
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 seconds it will be in balancing mode, after 1000 seconds it will be 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.
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.
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.
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.
Coming to the third condition.
Looking at the condition below for third variant 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 ol
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.
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 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:
1) SOC (t) = SOC (t - 1) +I(t)/Qn Del(t)
Or
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 form:
where k2, k and k 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 wrt time. This
output is given to another block where it is divided by capacity and henceforth the 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.
REFERENCE:-