Week 7 State of charge estimation

Q1)

I have selected to Open Project and Model. ANd after that the whole project will be loaded in the Matlab in current folder and workkspace.

After this the BMS_ClosedLoop_Harness_Dashboard model file is open as:-

Block of Model:-

a) Test sequence Variant (it has 3 different test condition).

b) Manual Variant (selection). BMS_StateRequest subsytem to provide the state request commands.

c) Indictaing lamps are there to show 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.

d) Rotating switch for manual variant to change the position during simulation run time.

e) BMS_Closed Loop:- this subsystem contain BMS ECU and Plant as two main subsytem and lets see this block in details:-

BMS ECU:- The BMS ECU get have two input which are BMS_Input which is feedaback 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.

Plant:- This subsytem contain Battery pack, precharge circuit and charger / loads.

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

Test Cases :-

The test case 1 is selected as shown below in the dialog box and click OK to implement it. likewise we can select the all three tests accordingly and run the tests.

Result:-

a) Test case 1:-

i) Driving (Discharging) mode (0-3000 sec):- All the cells for the first 3000 sec are in driving mode so the dicharge of all the cell occurs and its varies in the range of 3.1 to 3.9 V (min and max.). Cell temprature increases from 288K to 302 K. SOC also reduces from 80 % to 45% for EKF technique.

ii) Balancing mode (3000-4000 sec) :- After 3000sec 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 reduces slowly as no current flow through the cells during 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 voltages increased from 3.7 to nearly about 4.2 V in Constant current mode with current of nearly 30A and then the constant voltage charging starts and current reduces from 30A to 2.6 A Charging and balancing of cells occur 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 increases from 45% to 94% in this mode for EKF technique.

iv) Post charge Balancing (9000- 20000sec):- After 9000 sec the post cell balancing starts under standby mode. Cell temperature reduces in this mode and tries to reach the ambient temperature. SOC remains constant to 94%.

b) Test Case 2:-

i) Driving Mode (0-10000 sec):- The cell voltage varies from 3.9 V to 3.05 volt ehich is cut off voltage of cell and the entire cell is drained upto 8692 sec. Cell temperatures 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.

ii) Charging Mode (10000- 20000 sec):- The cell got drained earlier before reacching this mode.

c) Test Case 3:-

Charging (0-20000 sec):- Cell Voltages increases from there intial values and after 2000 sec they got constant. Pack current initally 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 initally 80% and in 2000 sec it got fully charged that is 100%. Cell temperature starts to incraeses initally and when battery got fully charged then decrease down to the ambient temperature.

Q2)

Coulomb Counting Method:- 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:-

But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life.