Week 7 State of charge estimation
Aim :-
SOC Estimation
Objective :-
1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results
2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
MATLAB and SIMULINK :-
1.Simulate the 3 test cases from harness dashboard and write a detailed report on the results.
Model 1
A: click on test sequence varient block
then select test case as 1
now click on BMS state request block.
we get the below chart.
from the above figure 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.
by running the simulation
After runnig the simulation for 1st test sequence all dash board lights shows the green signal that means there is no fault in system.
Results :-
1) The first plot shows the individual cell voltages. The voltage fluctuates between 3.2 and 3.7 volts between 0 to 3000 seconds. The nominal voltage of each cell is 3.7 volts and it decreases to 3.2 volts.
Fluctuation in cell voltage from 0 to 3000 seconds shows that the vehicle is in driving mode.
From 3000 to 4000 seconds we have balancing mode thus cell voltage gets balanced to 4 volts till 4000 seconds. From 4000 to 8000 seconds there is charging and balancing state thus cell voltage rises to 4.2 volts.
2) Entire battery pack current is shown in pack current graph as the current decreases while charging thus the current is -60A from 0 to 3000 seconds.
Now from 3000 to 4000 seconds there is cell balancing thus pack current reduce to 0 A and from 4000 to 8000 seconds there is chaging state thus current increases to 30 A.
Then occur post charge balancing thus pack current comes to 0A.
3) The third graph is cell temperature graph. The cell temperature increases in driving stage as vehicle is in drivng mode then during balancing stage cell temperature reduces and then at start of charging the temperature increases suddenly then latter at charging and post charge balancing the cell temperature shows the reducing behaviour.
4)The BMS state graph shows that durng 0 to 3000 seconds shows that the vehicle is in driving mode from 3000 to 4000 seconds we have standby mode from 4000 to 8000 seconds there is charnging state.
5)The SOC graph tells that during balancing state SOC remains constant at 45% during charging stage the SOC increases to 100% and then remains constant.
6)The cell balancing signal is shown in the balancecommand graph. During charging cell balancing remains constant for all the cell.
B: Now Select test case as
The diving stage is for to 5000 seconds then charging stage occurs.
From the dash board Fault state light goes red due to high temp. of cell (cell temp. light is also red). Cell temperature is ok upto 5000 second in driving state and after 5000 second it goes to charging mode. Here after 6000 seconds temperature reaches maximum limit and fault occurs.
Results :-
1) In above graph first two graph of cell voltage and pack current fluctuates for 5000 seconds due to it is in driving state. These fluctuates 3.1 to 3.9 V and currenr is -60A to 20A.
2)As see in cell temperature and SOC graph temperature goes high of each cell after driving state complete i,e after 5000 seconds and temperature goes 320 of cell no 6 and also other cell temp. increases then fualt state graph goes in BMS fault state. SOC goes down when driving state in transition from 0.8 to 0.3 and after that charging state activated.
While charging state in transition as we seen cell 1,2,3. temperature of cell 1 is less as compare to 2 and 3 then SOC of cell 1 os goes higher than other cell. It is due to temperature effect on SOC. Also high cell temperature is hampered on other factors.
C: Now select test case as 3
as there is only one state that is chanrging state
thus from 0 to 200000 seconds only charging will takes place.
Results :-
1)The cell voltage varies till 20000 seconds and then remains constant
2)at start the pack current increases to 30A and then reduce to 0A at 2000 seconds and then remain constant at 0A.
3)cell temperature varies as shown in the graph
4)as there is only charging state thus BMS graph shows BMS charging
5)as only charging takes place
thus SOC was 80% as first then SOC increases to 100% in 2000 seconds then SOC remains constant at 100%
2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
Columb Counting :-
The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC [20]. Coulomb counting method is done to estimate the , which is estimated from the discharging current, , and previously estimated SOC values, . 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 , and are constant values obtained from the practice experimental data.
In modified Coulomb counting method, SOC is calculated by the following equation:
he experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.
2: To compute the SOC of battery insert a plant block into the model.
BMS algorithm models protects reports the measurement from battery pack.
When we click on BMS ECU block we get
by clicking on the current power limit calc block we get the below block diagram
here the cell voltage and cell temperature is used to decide maximum allowable charging and discharging current limits.
When cell is at lowest SOC its voltages is low and thus it is important to prevent cell from delivering large amount of current because it will cause large voltage drop that could be petential below the cut off voltage specify by the cell manufacturer.
click on discharge current limit calculate block we get the following block diagram shown below
here by comparing the minimum cell voltage in the module against this lowest threshold voltage and then devided by maximum internal cell resistance value
We can compute the voltage based threshold
it is also important to limit the current delivery and current intake when temperature is too high or too low
Using the lockup table with rising and falling shape profile we can specify current threshold based temperature and thus can modulate allowable current delivery'
It is recommended to avoid low and high temperature during discharging and low temperature during charging.
then the above two threshold are then compared and minimum of this two becomes the current limit.
Knowing how much longer we can able to driver our car before we had to stop for recharge depends on the accureate estimation of battery SOC
In battery system we do not measure SOC directly we actually measure somthint else and then relate it with SOC. 3 methods are used for soc estimation but columb counting is mostly used.
click on SOC estimation block we get
By clicking on colulonb counting block we get the below block diagram
We get
in this coulomb counting method the entering and leaving currents of the cell are intergrated to track the safe chargable time.
Hete the two input signals current and temperature are taken . The cuttent signal goes to gain block and the temperature signal goes to current look up table'
Then as seen signal are multiplied and divided as shown and then output is given to discrete time integrator to track the safe chargeable time and then SOC is estimated. Advantage of coulomb counting method is accumulation of current sensor error and inability to recover from wrong initial condition because of lack of feedback from voltage measurement.
Referrence :-
https://www.hindawi.com/journals/isrn/2013/953792/