Week 7 State of charge estimation

Aim:- State of charge estimation

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

1. Simulate the 3 test cases from the 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? 

Q1. Simulate the 3 test cases from the harness dashboard and write a detailed report on the results

Model

 

 Test Case 1 

click on test sequence Variant block 

then select test case 1 

now click the BMS_StateRequest block 

we notice the below chart

from the above chart, we see that 

Driving is for 3000sec

Balancing is for 1000sec and 

PostChargeBalnceing is for 5000sec 

now we are running the simulation 

Result 

a) 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 a charging and balancing state.

thus cell voltage rises to 4.2 volts.

b) entire battery pack current is given in the 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 decreased to OA and from 4000 to 8000 seconds, there is the energising state, thus current raises to 30A

then occur post charge balancing, thus pack current comes to OA

c) the third graph is the cell temperature graph

the cell temperature increases in the driving stage as the vehicle is in driving mode than during the balancing stage, cell temperature reduces and then at the start of charging

the temperature increases suddenly then later at charging and post charge balancing, the cell temperature shows the reducing behaviour

d) 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 a standby mode

from 4000 to 8000 seconds, there is a charging state.

e) the SOC graph tells that,

during the driving state, the SOC reduces from[80% to 45%

during balancing state, SOC remains constant at 45%

during the charging stage, the SOC increases to 100 % and then remains constant.

f) the cell balancing signal is shown in the balance command graph.

 

 Test Case 2 

Now select test case as 2.

click on test sequence Variant block 

then select test case 2 

now click the BMS_StateRequest block 

we see the below chart

 

 

the Driving stage is for 0 to 10000 sec after that Charing stage is start 

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

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

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

iv)as the driving stage is there from 0 to 10000 seconds thus BMS graph shows the BMS driving

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

though the driving stage is till 10000econds as the battery completely discharges at 9000 seconds (as SOC is 0% at 9000 seconds) thus vehicle runs till 9000 seconds only

 Test Case 3 

Now select test case as 3.

click on test sequence Variant block 

then select test case 3

now click the BMS_StateRequest block 

we see the below chart

There is only one stage to that is the charging stage 

Thus from 0 to 20000 sec 

1) the cell voltage varies till 20000 seconds and then remains constant

2) At starting the pack current increases to 30A and then reduce to 0 A at 2000 seconds and then remains constant at O A.

3) cell temperature varies as shown in the graph.

4) as there is only a 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%.

Q2. What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation?

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 (On).

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.

Direct measurement this method uses physical battery properties such as the voltage and impedance of the battery

(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 of 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 a 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 the modified Coulomb counting method.

Coulomb counting method:

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

is done to estimate the SOC(1), which is estimated from the discharging current. I(t), and previously estimated SOC values 

SOC is calculated by the following equation

 

But several factors 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

where k2 k1 ko 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 the battery, insert a plant block into the model

BMS algorithm model Project reports the measurement from the battery pack 

when clicking the BMS ECU block we get

by clicking on the current power limit calc block we get the block diagram

here the cell voltage and cell temperature are used to decide maximum allowable charging and discharging current limits.

when the cell is at its lowest SOC. its voltage is low. and thus it is important to prevent the cell from delivering a large amount of current because it will cause a large voltage drop that could be

potential below the cut-off voltage specified by the cell manufacture

click on discharge current limit calculate block we get the following block diagram shown below;

 

hereby comparing the minimum cell voltage in the module against this lowest threshold voltage and then dividing by the 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 the 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

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 a battery system, we do not measure SOC directly, we measure something else and then relate it to SOC

2c. methods are used for SOC estimation.

but coulomb counting is mostly used

click on soc estimation block. we get

by clicking on the 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 chargeable time

here the two input signals current and temperature are taken

the current signal goes to gain block and the temperature signal goes to the current lookup table

then as seen signals 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 the coulomb counting method is its simplicity and low computational cost

the drawback of the coulomb counting method is an accumulation of current sensor error and inability to recover from the wrong initial condition because of lack of feedback from the voltage measurement

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