Week 7 State of charge estimation

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

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

downloading model from this URL

Model file open:

in the above file shows the dashboard following blocks shows:

• test sequences variant
• manual variant
• BMS_stateRequest
• BMS_closedloop
• Diffrent lamp indicating fault state, high temp, over voltage, charger, voltage sensor, over current, low temperature, under voltage and inverter

BMS_closedLoop subsystem:

• In this BMS_closedLoop block consist of two subsystems,BMS ECU and PLANT units.
• The plant output from plant unit given as input to BMS ECU as BMS_input feedback loop.The state request is given to BMS ECU unit.
• The BMS Output commands given as input to plant unit as 'From_BMS'.Alsp the state request is given to Plant unit.
• The final outputs from closed loop model are BMS_input,BMS_Output,BMs_info.

BMS ECU:

In BMS ECU unit consists the current power limit calculations are done used in State machine block,Soc Estimations,Cell balancing logics in the system.

PLANT sunsystem:

In Plant unit subsystem consists,the battery pack unit,Pre charge unit,charger/load units are created. in battery packs there are two types of configurations as 1 module(6 cells) and 16 modules(96 cells).

1)To run three test cases from harness closed loop BMS control.

We have to test three cases for this model by selecting test sequence varient and select the cases one by one as fallows,

Test case 1: Select 1 from test sequence varient box and click apply as shown below.

after selecting the test sequence 1 the BMS_stateRequest subsystem changing to 1

test case sequence:

OUTPUT:

The lamp indications are green for overall simulation period showing that there are no faults occurs during the sequences.

Cell Voltages and Pack Current-

For first 3000secs all 6 cells are in drive mode where discharging occurs majorly and in standby mode for less duration at approximately 1000 to 2000 secs. The voltages varies from 3.1V to 3.8V in this range. From 3000 secs to 4000secs cell balancing occurs and in this stage the module in standby mode and voltage is constant at nearly 3.7V. After 4000secs charging occurs,voltage raised from 3.7V to 4V. At 4V to 4.2V at constant current as nearly 30 A and after 4.2 V as constant voltage,current reduces from 30A to 2.6A at end of the charging and cell balancing occurs upto 9000Secs. After 9000 secs post charge cell balancing starts.

Cell temperatures:

Initially for 3000Secs the temperature range of cells increases gradually from ambient temperature 290K to 305K. After 3000secs upto 4000secs the cell temperatures are reduced slightly as no current flows through cell in standby mode. after 4000Secs temperature starts increases gradually upto constant current region and start reduces as current flow reduces in constant voltage and cell balancing region upto 9000secs. After 9000secs the temperatures still reduces and remains stable at reaching ambient temperature.

BMS State:

For 3000 secs the battery module in drive mode(discharging mode). from 3000 secs to 4000secs the battery module in balancing standby mode. after 4000 secs upto 9000secs the battery module in charging and balancing mode. after 9000secs only post charge balancing mode operates upto end of simulation.

State of charge(SOC):

Initially the state of charge of battery module is in 80%. initially for 3000 secs SOC decreases to nearly 45% during drive mode. after 3000secs upto 4000 secs the SOC is constant at 45% as cell balancing occurs in standby mode. From 4000 secs to 9000secs module in charging mode.(4000 to 7000=CC mode,7000 to 9000=CV mode). after 9000 secs the SOC becomes constant approximately 94% in the output plot.

Cell balancing:

Initially upto 3000 secs balancing was not performing. After 3000 secs cell balancing starts with on cell 6 and cell 2 as they rised to 1 as shown above since its voltage is higher than all other cell voltages. After this stage charging and balancing,post charge balancing are occurs as Cell 6 and cell 2 remains in 1 state till end of simulation. At the end of the simulation all cell voltages try to balance equally as we seen earlier in single cell voltages plot.

Test case 2 selecting:

The test sequence is changes 1 to 2

OUTPUT:

Cell Voltages and Pack Current:

The cell voltages varies from maximum 3.9V and reduces to 3.05V as cutoff voltage upto 9000 secs.Entire capacity drained at 9000 secs. the pack current discharges upto maximum 70A and slight charging occurs during the overall drive. Some stand by mode conditions are in between the drive modes.

Cell temperature:

Cell 1 having lesser temperature variations from 288K to approximately 302.5K. Cell 6 is having higher temperature variation from 288K to approximately 322.5K.

Battery module state:

Entire plot is performing in driveing mode.

State of charge (SOC):

Here the SOC reduces from 80% to 0% in entire drive mode as shown in the above plot.

Cell balancing:

Here balancing of cells are not performed as shown as balance states are in 0 for entire drive mode.

Test case 3 selecting:

Test sequence is change to 2 to 3.

OUTPUT:

Cell voltages and pack current:

Initially cell 2 , voltages rises from 3.9V to maximum 4.2V and stabilised to 4.15V. Cell 1 increases from 3.85V to 4.1V and stabilised to 4.08V.The battery pack current increases from 0A to 30A and starts to reduce to 2.5 A gradually at end of full charging upto 2000 secs.

Cell temperature:

The cell temperatures continue to increase heat upto full charge and after that the temperatures get started to reduces to ambient temperature.

BMS State:

battery pack sequence is operating in charging mode as shown above graph.

State of charge (SOC):

The pack SOC gets fully charged at 2000secs and remains at full charge for rest of the sequence in above graph.

Cell balancing:

Here from 0sec the cell 6 and 2 starts to balance with respect to cell 1 and all cells are try to balancing at end of simulation.

2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?

Hint :(Download the file from the link and go to SOC estimation model-ECU block)

Coulomb counting:

SOC(State Of Charge): is defined as the rate of available capacity to its maximum capacity when a battery is fully charged and describes the remaining percentage of the battery capacity.

To find this SOC % we are using coulomb counting method.

The coulomb counting method measures the discharging current of the battery and integrates the discharging current over time in order to estimate SOC.

Which can be estimated from discharrging current(I(t)),previously estimated SOC values(SOC(t-1)).The relation can be given as,

SOC estimation in BMS model:

Here we see that there are three method for calculation of SOC estimation and these are:-

• Coulomb counting
• Unscented Kalman filter
• Extended laman filter.

the discharge current from battery given to gain with value as 1/3600 to convert current to Ampere hour

The value is divided by capacity due to temperature change. Then integrated with a gain value of 1 with sample time -1 as we are using discrete integrator.

The saturation limits are from 0 to 1. We will get the SOC values from 0 to 1 multiplied with 100 to get in percentage of SOC.