Week 7 State of charge estimation

1. Simulate the 3 test cases from harness dashboard and write a detailed report on the results.

To open the lithium ion battery management algorithm firstly we have to download the zip file from the link given in the task and the link has also been attached below for refrence.

After downloading the zip file we first have to extract the zip file to a folder and then l choose again battery system to extract to a folder.

Then to get access to the close loop BMS harness dashboard we have to check for the following model Test>System>BMS_Closedloop_Harness_doashboard model file .

I have selected to open project and model. And after that the whole project will be loaded in the Matlab in current folder and workspace.

After this the BMS_Closedloop_harness_dashboard model file is open as:-

Block of model:-

Test sequence variant

Manual variant

BMS_staterequest subsystem to provide the state request commands.

Indicating lamps are there to shown any fault state, High Temp, Overvoltage, Charger, voltage sensor, overcurrent, low temp, under voltage, inverter state of a battery.

Green light tell us that all states are ok and battery is good to charge and discharge and red indication will indicate the fault state.

Rotating switch for manual variant to charge the position during simulation run time.

BMS_CLOSED LOOP:- This subsystem contain BMS ECU and plant as two main subsystem and lets see this block in details:-

BMS ECU:- The BMS ECU get have two input which are BMS_Input which is feedback from the plant model and the state request is coming from the state machine. This is generating output as BMS output and info which have current power limit calculation in state machine, SOC estimation and cell balancing logic as shown below:-

Plant:-

This subsystem contain battery pack, pre-charge circuit and charger/ loads.

In the battery pack there are two configuration which are 1 module (6 cells) and 16 module (96 cells).

Here we have selected 1 module for simple calculation and less simulation time.

In the scope SOC of module, cell voltage, pack current, cell temperature, BMS state and cell balancing command are seen as output.

Task :- 1 simulate the 4 test cases from harness dashboard and write a detailed report on the results.

Test case 1

The test case 1 is selected as shown below in the dialog box and click ok to implement it.

We can see the test sequence in BMS_ststerequest> BMS_testsequenece and there we can see that the test case we have selected is as shown below:

Now to see what are the test condition we have to double click on the test sequence and test sequence will be shown as follow:

The editor figure tell us about the test sequence and it is as following:

Driving is done for first 3000 sec.

Then after driving balancing of cell occurs for the nest 1000 sec duration.

After the balancing charging and balancing occur for next 5000 sec.

Post charge balancing is occurring after 9000 sec till 20000 sec.

Results of test sequence 1 :-

Simulation is ran for 20000 sec and the result are as below :-

The lamp indication were green for all states which means the model is fault free.

i)Driving ( discharging ) mode (0-3000 sec)

All the cells for the first 3000 sec are in driving model so the discharge of all the cell occurs and its varies in the range of 3.1 to 3.9V ( min and max)

Cell temperature increases from 288k to 302k.

SOC also reduction from 80% to 45% for EKF technique

ii) Balancing mode (3000 – 4000 sec)

After 3000 sec for 1000 sec, cell balancing occur and the modules are in standby mode and they are having constant voltage near about 3.7V.

Cell temperature reduce slowly as no current flow through the cells dring balancing.

SOC is also constant during this period i.e 45% for EKF technique.

iii) Charging and balancing (4000-9000 sec)

After 4000 sec the charging occurs and the cell voltage increased from 3.7 to nearly about 4.2 V in constant current mode with current of nearly 30A and then the contant voltage charging starts and current reduces from 30 A to 2.6 A charging and balancing of cells occure till 9000 sec.

Cell temperature also start to increase exponentially upto constant current region and reduces as current flow reduces in constant voltage mode.

SOC also increase frome 45% to 94% in this model for EKF technique.

IV) post charge balancing (9000 – 20000 sec)

After 9000 sec the post cell balancing starts under standby mode:

Cell temperature reduce in this mode and tries to reach the ambient temperature SOC remains constant to 94%.

Test case 2 :

The test case 2 is selected as shown below in the dialog box and click ok to implement it.

We can see the test sequence in BMS STATEREQUEST > BMS_Testsequenece and there we can see that the test case we have selected is as shown below:-

The editor figure tell us about the test sequence and it is as following:-

Driving is ddone for first 10000 sec.

Then charging is done for next 10000 sec.

Results of test sequence 2:-

Simulation is ran for 20000 sec and the result are as below:-

The lamp indication were green for all states which means the model is fault free.

Driving mode (0-10000 sec):-

The cell voltage varies from 3.9V to 3.05 volt which is cut off voltage of cell and the entire cell is drained upto 8692 sec.

Cell temperature varies from 288K to 322.5k.

SOC reduces from 80% to 0% ( cell got fully drained ) in the drive mode under 8692 sec.

Cell balancing is not performed here.

Charging mode (10000 – 20000 sec):

The cell got drained earlier before reaching this mode.

Test Case -3

The test case 3 is selected as shown below in the dialog box and click ok to implement it.

We can see the test sequence in BMS_Staterequest> BMS_Testsequencee and there we can see that the test case we have selected is as shown below:-

The editor figure tell us about the teast sequence and it is as following :=

Charging is done for entire simulation period i.e. 20000 sec.

The lamp indication were green for all states which means the model is fault free.

Charging (0-20000 sec) :-

Cell voltages increases from there initial values and after 2000 sec they got constant.

Pack current initially start from 0 to 30A and then start decreasing when the constant voltage mode is there to 2.5 A and got full charged upto 2000 sec.

SOC was initially 80% and in 2000sec it got fully charged that is 100%.

Cell temperature starts to increases initially and when battery got fully charged then decrease down to the ambient temperature.

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 intergrates the discharging current over time in order to estimates 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:

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, k1 and k0 are current 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

From the given model by mathworks as shown below of BMS closed loop model, when seen inside the BMS ECU,

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 emotional 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 its varies according to the temperature. These inputs are given t=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.