Week 7 State of charge estimation
Aim :
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 ?
Answer:
1.Simulate the 3 test cases from harness dashboard
As the simulation ends we got 6 graph results considering the BMS in an operation for the electric vehicle
1.The first graph indicates the cell voltage of all the cells involved in the battery pack we first observed that cell voltages changing as the current flowing in and out,at the begining of the simulation,the voltages are slightly different because we initialize the model with a slight assosiate of imbalance the voltages are deflecting from the range 3.1V to3.9V for the timeperiod of 0.3*10^4sec,and then there is balances state of all voltages of the respective cells and then there is a sudden hike in cell voltage till they reach 4.2V and at the end of the simulation,the values converge towards one another and trying to balance the voltages till the end still there is small difference in their voltages and the voltage line flows between 4 to 4.2volt because the system goes on stand by mode.
2.In charging state during the constant current phase current is debated because the maximum cells voltage is high enough to compared to prescribe 4.4V limit that an excessively high control current voltage beyond the threshould significantly limiting the life of the battery.
3.The temperature(starting from 289 C at end,all the cells have different temperatures)traces show the siginificantly discrepancy between the hottest and the coldest cell is the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour the cell no 6 is significantly hotter than the cell no 1 because it is thermally insulated in the one side.Even if the maximum temperature reached during the simulation is not concerned in terms of safety the temp difference at cell no 6 is higher temperature will eventually cost must faster degradation of the cell 6 compared to cell 1 leading to undesriable in cell condition.Therfore a need an active thermal managment system to keep thermal differences with in a few degree celcius.
4.The SOC is initial SOC of three different cells is at 80% as the system is putting on charging state the SOC hikes up to 100% for the yellow cell,ukf and the ekf initially having different charging rates but after 0.4*10^4 sec they have equal SOC of 91% and it maintains through out the simulation t is apparet that coloumb counting never counting never does because it has no way to realize it is wrong due to the absence of voltage information,both commonly filter algorithms on the other hand recover from the initial error.ekf and urf slightly outperforming coloumb counting.
5.The other scopes indicate the bms state at each of the balance command signals respectivelybms on charging state and battery command passing signal between 0to 1.
B.simultion of case 2
1.At the initial stage the voltage of the cells are deflecting in the same manner and dropping slightly in the range of 3 to 3.9 V it has the 4 fluctuation zines and in between each of these fluctuating zones,they are stable for the while and this phase lasts for 0.85*10^4 sec.
2.On the other hand the current graph has exactly flowed in the same mannner but in opposite direction when the voltage is getting drop current requried is high and current flows in the range of -70 A to 20A.
3.The temperature of all the cells initiating from 288 C and they get scattered all the cells have a different temprature)traces show the significant discrepancy between the hottest and the coldest cell the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour cell no 6 sognificantly hotter than the cell no 1 because it is thermally insulated on the other side.even if the maximum temprature reached during the simulation is not concerned in case of safety.
4.The cells are initailly at 80 % of SOC and they are falling at different time periods.To set their capability to recover it.It is apparent that columb counting never does because it has no way to realise it is wrong due to the absence of voltage information.Both the common filter algorithms on the other hand recover from the inital error with in the first hour of the simulated time.with the ekf and then apparent columb counting never does because it has no way to realize it is wrong due to the voltage information.both common filter algorithms on the other hand recover from the initial error with in the first hour of simulated time,with the elf slighty outperforming the ukf.
5.The other scopes indicate the bms state at each of the balance command signals respectively,where its on driving mode and command signals on zero lines.
Simulation 3
1.All the cells are initated with different voltages the where the 6 the one having the higher voltages of 4.2 V and the 1 st one having near about 4.1V and reaminig falling under the BMS range on charging state throughout the simulation.So the voltage graph move around the straight line because the voltage supply continous ,there have some deflections of voltage lines of different cells but has a very less amount of change in voltage at the end o fthe simulation BMS trying to balance the voltaeg verymuch.
2.The battery pack current is initiated with 20A and then it falls to zero current till it is complete simulation time of 0.2*10^4sec and then it maintains the constant current because it is put on charging state and it remains constant throughout the simulation.
3.All the cells initiate with 287C and then all the cells maintaing the straight lines but with different temperature range the end all the cells have a different temperature) traces show the significant discrepancy between the hottest and the coldest cell the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour cell no 6 is significantly is hotter than the cell no1 because it is thermally insulated on one side.Even if the maximum temperature is reached during the simulation is not concerned in terms of safety.The temperature difference at cell no 6 at higher temperature will eventually cost much faster degradation of cell 6 compared to cell 1 leading to undesirable and unevenness in cell condition.Therefore need an active thermal managment system to keep thermal difference with a few degree celcius.
4.The SOC is initial SOC of three different cells is at 80% as the system is put on charging state the SOC hikes up 100% for the yellow cell,ukf and ekf initially have different charging rates but after 0.4*10^4 sec they having equal state of charge of 91% and it maintains throughout the simulation.t is apparent that coloumb counting never does because it has no way to realize it is wrong due to absence of voltage information.both commonly filter algorithm on the other hand recover from the initial error.ekf and urf slightly outperforming then coulomb counting.
5.The other scope indicate the bms state at each of the balance command signals respectively,bms on charging state and battery command passing signal between 0 to 1.
2.What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?
Coloumb counting method measures the discharging current of the battery and integrates the discharging current over time to estimate SOC.Coloumb menthod is done to estimate the SOC which is estimated from the discharging current and previouslouy estimated SOC (1-SOC).It is calculated from the following the equations
SOC(t)=SOC(t-1)+I(t)/Qn.delta(t) ----(1)
But several factors effect the accuracy of the coloumb counting menthod including temperature,battery history,discharge current and battery life.
Modified Coloumb counting menthod :
This method is used to improve the coloumb counting method,This method uses corrected current to improve the accuracy of the estimation.
The corrected current is the function of the discharging current.There is a quadratic relationship between the corrected current and the discharging current of the battery,By the pratice of the experimental data the corrected current is calaculated
Ic(t) = K2I(t)^2+K1I(t)+K0 ------(2)
Where K1,K2 and K3 are the constants from the experimental data
In modified coloumb counting method SOC is calaculated
SOC(t)=SOC(t-1)+Ic(t)/Qn*delta(t) -----(3)
The experimental results shows that the modified coloumb counting method is has more accuracy when compared to the coloumb counting method
The above flowchart explains the algorithm used to manage the battery monitoring by switching from a battery operating mode to another from the transitions.
The estimation of the SOC of a Li-ion battery in this method is based on monitoring both the voltage,Vbat and the current Ibat.The operation mode of the battery is recognized by the direction of the current through the battery.
When the battery is in open circuit mode the Ibat =0 and the compensation of the self discahrge losses will be taken into account as explained in the subsection-C
The information needed to carry monitoring is the measurement of the battery voltage,the current flowing through it and the operating temperature,Coloumb counter is used to track the SOC when the battery is charged to discharged.
A.Charge Mode :
In this method the Coloumb counter is presented by the Qgain which represents the no of charge accumulated inthe 'T'
Qgained(t+ =Qgained(t)+delta(t) --- (4)
so the variation of the SOC gained in the same operating period is obtaied by relation 4
B.Discharge MOde :
In this mode the coloumb counter is presented by the Qlost,which represesnts the amount of charge losses
C.Self Discharge Mode :
At the battery storage periods,the percentage of the monthly self discharge is converted to the amount of discharged per hour,this amount is designed by the constant Q.Considering a 5% rate of self discharge per month is approximated to 0.0016A.So the quantity of the charges dissipitated in the open circuit period Qoc is calculated
Qoc(h+1)=Qoc(h)+Qper/hour ----(5)
h.hour of storage
This value will be added to the amount of charge lost Qlost during discharge mode and subracted from the amount of charge acculumated from Qgained in the charge mode as expressed in relations.This compensation of the self discharge is made at each open circuit and before switching to
Qlost =Qlost+Qoc --------(6)
Qtot =Qtot-Qoc ---------(7)
D.Temperature effect
The temperature variation continously effects the SOC of the Li-ion battery to improve the accuracy of the battery SOC a temperature coefficient 1 is introduced and multiplied by the value of the calculated SOC.The coeffocient varies depending on the operating temperature according to various intervals
0.5 T < -20C
a = 1 -20C<T<40C -----(8)
0.8 T>40C \
Recalibration:
When the loaded voltage Vbat drops under a limit Vilm (3V) during the dischrged mode the battery is considered totally discharged.When its empty,the battery can no longer be used and should be recharged.So after a cycle of charge and discharge,a recalibration should be made of Qmax which represents the maximum capacity that can be released by the battery,This recalibration allows rectifiying the indication of SOC by replacing Qrated by Qmax realtion also we calibrate SOH of the battery SOH defined is
SOH(nc) = Qmax(nc)/Qrated*(100%) ------(9)
nc=number of cycles
Morever the battery is fully charged.Then SOH should be updated by assiging the number of charges releasable form the battery to Qmax.The SOH indeed represents the aging degree of the battery,infact its a 100% maximum value whren the battery is new.When the battery is getting old and Qmax become more and more inferior than the Qrated,SOH value drops to indicate the health degradation of the battery.
1.Before we charge our vehicle we need an accurate estimation of the battery SOC,in the battery system we dont measure the SOC we measure different things like battery pack current cell voltage and cell which taking signals as input from the plant.
2.After entering the plant blcok we can see the stata machine,SOC estimation,and balancing logic block
3.In which we going to study SOC estimation.
4.In SOC estimation block we can see three different techiniques for the state of charge estimation,all three techiniquies have their own merits and limitations,the first method is coloumb counting and the other two are uncented(ukf) and extended kalman filter(ekf) here we are concern with coloumb counting method.
5.In coloumb counting method block
It takes to input signals such as current and the temperature and the current signal goes to gain block and the temp signal goes to lookup tables of battery time and current,and both the blocks are connected to the divided block and these signals are connected to discrete time integrator to track the charge and which finally helps to calculate the SOC.
Which consists of integrating a current and doesnt leave the cell for that here we using discrete time integrator to give the track of the cell charge of the time and the benefits of this is the simplicity and its low capitation cost and the drawbacks are an accumulation of current sensor error and it inability to recover from the wrong initial condition from the lack of feedback from voltage measurments.
Conclusion:
1.One of the main problems in coloumb counting is that it is necessary to calibrate the inital battery SOC,in several applications,the initialization rest and the initial SOC is estimated based in the voltage.
2.kalman filtering is an algorithm that provides estimation of some unkown variables given the measurments observed overtime.