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 ?
1)
MODEL:
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,
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 .
fluctuations 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 curent 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 charging state , thus current increases to 30A.
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 driving 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 during 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 charging state.
5)the SOC graph tells that,
during driving state , the SOC reduces from 80% to 45%
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.
B)
now select test case as 2.
the driving stage is for 0 to 10000 seconds.
then charging stage occur.
RESULTS:
1) the cell voltage fluctuates from 3.9 volts to 3.2 volts repeatedly as seen in 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 driving stage from 0 to 10000 seconds.
4)as driving stage is there from 0 to 10000 sconds ,thus BMS graph shows the BMS driving .
5) the SOC continue to reduce from 80% to 0% . the SOC do not increase beacuse there is no charging and balancing stage.
though the driving stage is till 10000 seconds but as battery completly discharges at 9000 seconds (as SOC is 0% at 9000 seconds) thus vehicle runs till 9000 seconds only.
C)
now select test case as 3.
as there is only one stage that is charging stage
thus from 0 to 20000 seconds , only charging will takes take.
1) the cell voltage varies till 20000 seconds and then remians constant
2) at start the pack current increses to 30A and then reduce to 0 A at 2000 seconds and then remains constant at 0 A.
3) cell tempearture varies as shown in the graph.
4) as there is only charging stage , thus BMS graph shows BMS charging .
5)as only charging takes place.
thus SOC was 80% at first , then SOC increases to 100% in 2000 seconds then SOC remains constant at 100%.
2a)
COULOMB COUNTING:
The SOC is one of the most important parameters for batteries. In general, the SOC of a battery is defined as the ratio of its current capacity (Q(t)) to the nominal capacity (Qn). The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows:
The various mathematical methods of estimation are classified according to methodology. The classification of these SOC estimation methods is divided into the following four categories.
(i)Direct measurement: this method uses physical battery properties, such as the voltage and impedance of the battery.
(ii)Book-keeping estimation: this method uses discharging current as the input and integrates the discharging current over time to calculate the SOC.
(iii)Adaptive systems: the adaptive systems are self-designing and can automatically adjust the SOC for different discharging conditions. Various new adaptive systems for SOC estimation have been developed.
(iv)Hybrid methods: the hybrid models benefit from the advantages of each SOC estimation method and allow a globally optimal estimation performance. The literature shows that the hybrid methods generally produce good estimation of SOC, compared to individual methods.
Book-keeping estimation:
Book-keeping estimation method uses battery discharging current data as input. This method permits to include some internal battery effects as self-discharge, capacity-loss, and discharging efficiency. Two kinds of book-keeping estimation methods have been employed: Coulomb counting method and modified Coulomb counting method.
1)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). 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 .
2)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 , k2, k1, k0 are constant values obtained from the practice experimental data.
In modified Coulomb counting method, SOC is calculated by the following equation:
The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.
2b)
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 voltage is low,and thus it is important to prevent cell from delivering large amount of current because it wil cause large voltage drop that could be potential below the cut off voltage specify by the cell manufacture.
click on dischargecurrentlimit calc 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 divided 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 s 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 drive our car before we had to stop for recharge depends on the accurate estimation of battery SOC.
In battery system , we do not measure SOC directly , we actually measure something else and then relate it with SOC.
3 methods are used for SOC estimation.
but coulomb counting is mostly used.
click on soc estimation block , we get,
by clicking on coulomb counting block , we get the below block diagram
in this coulomb counting method, the entering and leaving currents of the cell are integrated to track the safe chargable time.
here the two input signals current and temperature are taken
the curent signal goes to gain block and the temperature signal goes to current look up table.
then as seen , signals are multiplied and divided as shown and then output is given to discrete time integrator to track the safe chargable time and then SOC is estimated.
advantage of coulomb counting method is its simplicity and low computational cost.
the drawback 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.
CONCLUSION:
the various objectives given in the question are achieved and the various results are studied.
REFERENCES: