Week 7 State of charge estimation

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

ans.

The model is as follows

click on the test sequence varient block

then select test case 1

now click on BMS state request block

we get the below chart

from the above figure we observed that,

driving is for 3000seconds

then followed by balancing for 1000 seconds

followed by charging and balancing for 5000 seconds

then there is post charge balancing

by running the simulation

result is shown below

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 3.7V and it decreases to 3.2V

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 get balanced to 4 volts till 4000sec
From 4000 to 8000 s, there is charging in balancing state
thus cell voltage Rises to 4.2V

Entire battery pack current is shown in the pack current graph

As the current decreases while charging thus the current is -60A from 0 to 3000sec

now from 3000 to 4000 s, there is cell balancing, thus pack current reduced to 0A

and from 4000 to 8000 s, there is charging state the current increases to 30 A

then occur post charge balancing thus pack current comes to 0A

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 than latter at charging and post charging balancing the cell temperature shows the reducing behaviour

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

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 state the SOC increases to 100% and then remains constant

The cell balancing signal is shown in the balance, command graph

Now select test case 2

The result is

The cell voltage fluctuates from 3.9V to 3.2V repeatedly as seen in the graph of cell voltages

The battery pack current fluctuates from 0 to -60A and then Rises to 10A repeatedly as shown

This is a temperature increases as there is driving stage form 0 to 10000 seconds

As driving stage is there from 0 to 10000 seconds thus BMS graph shows the BMS driving

The SOC continue to reduce from 80% to 0% , the SOC do not increase because there is no charging and balancing stage

Though the driving stay is till 10000 seconds but as battery completely discharges at 9000 seconds thus vehicle Run till Al 9000 seconds only

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 take place

OoThe cell voltage varies till 20000 seconds and then remains constant

At start the back current increases to 30A and then reduces to 0A at 2000seconds and then remains constant at 0A

Cell temperature varies as shown in the graph

As there is only charging stage does BMS graph shows BMS charging

SOC was 80% at 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?

ans

Coulomb counting the SOC is one of the most important parameters for batteries in general the SOC of a battery e is defined as the ratio of its current capacity ,Q(t) to the nominal capacity(Q(n)). 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 models of estimation are classified according to the methodology. The classification of these SOC estimation methods is divided into to the following four categories:

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

Bookkeeping estimation: This method uses the charging current as the input and integrates the discharging current overtime to calculate the SOC

The adaptive systems: The adaptive systems and self designing and can automatically adjust the SOC for different discharging conditions various new adaptive systems for SOC estimation have been developed.

Hybrid methods: The hybrid models benefit from the advantages of each SOC estimation method and allow a globally optimum estimation performance the literature shows that the hybrid model is generally produce good estimation of SOC, compared to individual methods.

Bookkeeping estimation:
Bookkeeping estimation method uses battery discharging current data as input the method parameters to include some internal battery effects as self discharge capacity loss and discharging efficiency two kinds of bookkeeping estimation methods have been employed coulomb counting method and modified coulomb counting method

Coulomb counting method
The coulomb counting method measures the discharging current of a battery and integrate 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 it 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 method coulomb counting method uses the correct current to improve the accuracy of estimation the corrected current is the function of discharging current this is a quadratic relationship between the corrected current and discharging current of the 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 the 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.

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 limit.
When cell is at lowest SOC its voltage is low and does it is important to prevent cell from delivering large amount of current because it will cause large voltage drop that could be potential below the cut off voltage specified by the cell manufacturer

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

here by compairing 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 ton limit the current delivery and current intake when temperature is too high or too low using the lockup table with rising and falling shape we can specify current threshold based temperature and thus can modulate allowable current delivery. Its is recommended to avoid low and high temperature during discharging and low temperature during charging.

Then the above two threshold are compared this to become 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.

Three methods are used for SOC estimation
But coulomb counting is mostly used

Click on SOC estimation block, we get,

by clicking on colomb counting block we get the below block diagram

Hindi school I'm counting method entering and leaving currency of the cell are integrated to track the safe chargeable time here are the two input signals current and temperature at taken the current signal goes to gain block and the temperature signal goes to 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 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.