Week 7 State of charge estimation

Solution for Question 1:

In this case,we are going to run the simulation for the three test cases from the harness dashboard.

We can get the Model from MATLAB by using provided link.

The Model gives the Information about BATTERY MANAGEMENT SYSTEM (BMS) and the faults occuring in the Battery.

Based on the Model, various types of faults in BMS are

• Fault state
• High temperature
• Over Voltage
• Under Voltage
• Over Current
• Inverter
• Charger
• low temperature voltage sensor

Simulink Model:

First Case:

Test sequence variant is set at 1 and then the model is run for 20000 seconds.

The Output is obtained for voltage , temperature , SOC , pack current,BMS and Balancing command.

After simulation all the lamps show green light indicating no faults.

The Output obatined is

Output Graphs:

• Cell voltage and cell current are showing fluctuations at the starting, this is due to the driving action of car. The voltage lies between 3.1 V and 4.1 V. These stabilizes after some time.
• SOC at this time is decreasing.but when the voltage and current becomes constant SOC increases.
• Cell temperature rises duirng whole process of driving and charging.Also cells having different temperature due to location of cells.

Second Case:

Test sequence is set at 2

Driving state will be for 10000 seconds after that charging begins

Output Graphs:

Voltage and Current showing fluctuations

At the same time the SOC dropped to zero

the currenr state of the vehicle is driving. the balancing command has reached to zero.

The reasons for the same is because of the increase in temperature.

Cell temperature is exponentially increasing due to the improper packing of battery pack.

Third case:

Output Graphs:

• In the starting cell voltages are increasing but after sometime the fault occurs and voltage drops between 4.1V and 4.2V.
• Cell current is decreasing continously in the starting,after 20000seconds the fault occurs and pack current reaches sero value and remains in that state.
• Balancing command is high.
• SOC steadily increases in the beginning and at fault condition SOC reaches to 100%.The battery is in charging condition.
• After the initial increase the temperature drops.

Solution for Question 2:

State of Charge is defined as the ratio of the current capacity Q(t) to the nominal capacity Q(n).

Nominal capacity is the maximum amount of charge that can be stored in the battery.

SOC(t)=Q(t)/Q(n)

Different mathematical methods of SOC estimation includes

1.Direct measurement- Uses physical battery properties

2.Book keeping estimation- uses discharging current as the input and integrates the discharging current over the time to calculate SOC.

3.Adaptive systems-Self designing and can automatiaclly adjust SOC for different discharging conditions.

4.Hybrid methods

Coulomb counting is a method under Book keeping estimation.

It measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC.

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

SOC(t)=SOC(t-1)+(I(t)Q(n))△t

But the coulomb counting method has several factors that affects the accuracy.

Modified Coulomb counting method

In order to overcome the difficulties faced by the coulomb counting method .

This modified method improve the accuracy of estimation

The corrected current is the function of discharging current

Ic(t)=K2(I(t)^2)+K1I(t)+K0

K2.K1 and K0 are constant values.

In Modified coulomb counting method ,SOC is calculated by

SOC(t)=SOC(t-1)+((Ic(t)/Q(n))△t.

The experimental results shows that accuracy of the modified coulomb counting method is superior to the conventional coulomb counting method

Flowchart for the SOC estimation is as shown below

The coulomg counting method circuit is found in the BMS closed loop block>BMS ECU>SOC estimation>Coulomb counting

Here the input is currenr and temperature.

blocks like gain ,1-D lookup table , discrete time integrator are used.

The pack current is fed to the gain block having the value of (1/3600) to convert current into Ah.

The temperature is fed into1-D lookup table which then provides input to the divide block from the input temperature.

The output is then fed to the discrete time integrator to integrate the divided current and temperature.

The SOC can be estimated from the output of the discrete time integrator.