Week 7 State of charge estimation

TASK .

AIM:

To study 'Design and Test Lithium ion Battery Management Algorithms'MATLAB model.

OBJECTIVES:

1.To simulate the 3 test cases from the harness dashboard and study the results.

2.To study coulomb counting,and find how BMS implements coulomb counting  for SOC estimation ?

SIMULINK MODEL:

Body:

CASE 1.

We observed that driving is for 3000 seconds then followed by balancing for 1000 seconds followed by charging and balancing for 4000 seconds , then there is post charge balancing .

Case 2

The driving stage is for 0 to 10000 seconds . then the charging stage occurs . Case 3 Block Parameters : Subsystem X 4 State Request ( mask ) Test Sequence Variant Parameters Choose Test .

Case 3 

As there is only one stage that is the charging stage thus from 0 to 20000 seconds , only charging will take place .

RESULTS :

Case 1 ):

From the plot , we can see the vehicle is initially in a driving state , and charging was done to 90 % . After charging it reaches standby mode . It can be seen in the BMS state plot .

2 ) Battery temperature increases in charging . Battery voltage was increased to 4.2V while charging .

3 ) The current plot shows the value of current supplied to the vehicle while driving .

4 ) SOC decreased up to 50 % in driving conditions and it was increased while charging .

Case 2:

1 ) The cell voltage fluctuates from 3.9 volts to 3.2 volts repeatedly as seen in the graph of cell voltages.

 2 ) The battery pack current fluctuates from 0 to -60A and then rises to 10 A repeatedly as shown .

3 ) The cell temperature increases as there is a driving stage from 0 to 10000 seconds .

4 ) As the driving stage is there from 0 to 10000 seconds , thus BMS graph shows the BMS driving .

5 ) The SOC continues to reduce from 80 % to 0 % . the SOC does not increase because there is no charging and balancing stage .

6 ) The driving stage is till 10000 seconds as battery completely discharges at 9000 seconds ( as SOC is 0 % at 9000 seconds ) thus vehicle runs till 9000 seconds only . 

Case 3

1 ) Here we can see that the soc reached 100 % in charging and remained in standby mode . Cell temperature also decreased when standby mode was achieved .

2 ) From the 3 cases we observe that only charging takes place thus SOC was 80 % at first , then SOC increases to 100 % in 2000 seconds then SOC remains constant at 100 % .

Part - 2:

Coulomb counting:

It is a method that measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC , which is estimated from the discharging current , and previously estimated SOC values . SOC is calculated by the following equation : I ( t ) SOC ( t ) = SOC ( t - 1 ) + Qn But there are several factors that affect the accuracy of the 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 the 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 the battery . By the practice of experimental data , the corrected current is calculated by the following form : Ic ( t ) = k₂I ( t ) ² + k₁I ( t ) + kỵ , where are constant values obtained from the practice experimental data , In the modified Coulomb counting method , SOC is calculated by the following equation :SOC ( t ) = SOC ( t - 1 ) + -At . Ic ( t ) Qn The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method . But there are several factors that affect the accuracy of the Coulomb counting method including temperature , battery history , discharge current , and cycle life . In the model , the BMS implements coulomb counting for SOC estimation in the following way .

• This system does the estimation by the coulomb counting equation mentioned above . 
• The advantage of the coulomb counting method is its simplicity and low computational cost . 
• Temperature and current from the battery are given as inputs . The current signal is given to a gain block and temperature is given to capacity since it varies according to temperature . 
• These inputs are given for integration where the current will be integrated with respect to time . 
• This out is given to another block where it is divided by the capacity , Hence the SOC is estimated .