Week 7 State of charge estimation
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 mentioned as below.
It consists of the test sequence variant subsystem which holda three different test cases. This dashboard also helps in determing the fault states and other parameters as shown in the diagram below. Green color indicates that the system is working without any errors while red signal will indicate that there is a fault in the system.The next step is to "Run Simulation". This is done to avoid the errors while running the model.
After the compilation is successful the scope opens up. The scope consists of the measurements of cell voltages, cell temperatures, SOC, pack current, BMS State.
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.
Test case 1
scope 1
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 inbalancing mode, after 1000 seconds it will be in charging and balancing mode and after5000 seconds it will be in post charge balancing mode.simulating the model for 20000 seconds as given default.
It can be observed from the plot that the vehicle is initially in the driving state andcharging was done till 90%. After the charging mode it reaches to the standby modewhich can be seen from the BMS state plot. From the Cell temperatures plot it isobserved that the battery temperature rises in driving and charging. The battery voltagewas increased to 4.2 V while charging. The current plot shows the current value suppliedto the vehicle while driving. The SOC was observed to be decreased around 50% indriving condition, which increased while charging.
Test case-2
second variant simulates as shown the below conditions
Scope-2
The scope shows the various parameters for the BMS model under second test case. After 10,000 sec when the drivingcycle is over, from the plot that the battery discharges completely in 8692 secondsand the vehicle does not go forward and the charging state was not achieved at 10000seconds.
Test case-3
Scope-3
There is only a single phase here for the third test case, i.e. the charging phase.
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.Here the SOC increases from 80 to 100% and then remains in standby mode.
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 andthe stable state hence the charging and discharging were observed and in the end theconstant 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 thedriving state within 8600 seconds and the charging state was not achieved at 10000seconds.
In the third simulation, only charging was observed for the first 2000 seconds and theSOC 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 ?
Coulomb Counting Method.
The Coulomb counting method is associated with monitoring the input and the output current continuously.Since capacity is the integral of current with respect to time, by measuring the input and the output current, change in capacity or capacity degradation of a battery can be measured easily.In this method,SoH is calculated by dividing measured capacity (after discharging the battery to 0% SoC
SOC is calculated by the following equation
But there are several factors that affect the accuracy of Coulomb counting methodincluding temperature, battery history, discharge current, and cycle life.
Modified Coulomb Counting Method
To improve the Coulomb counting method, a new technique called modified Coulombcounting method is proposed. The modified Coulomb counting method uses thecorrected current to improve the accuracy of estimation.
The corrected current is the function of discharging current. There is a quadraticrelationship between the corrected current and discharging current of battery. Bypractice of experimental data, corrected current is calculated by the following form:
where k2, k1and k0 are constant values obtained from the practice experimental data.
In modified Coulomb counting method, SOC is calculated by the following equation:
From the given model by Mathworks as shown below of BMS Closed Loop model, when seen inside from the BMS ECU
SOC estimation block
Coulomb counting block can be observed on the top of the model, where theCoulomb counting was carried out. The model for the same can be seen below.
inside the coloumb counting block
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.
References:
2.https://www.hindawi.com/journals/isrn/2013/953792/