Project 2

Aim of the project :

By using MATLAB/simulink and the drive cycle from the attached excel sheet,

To calculate

1. The max heat generation of the battery
2. The SOC of the battery at 2 *104second of the battery operation

Heat generation in a battery :

Battery generate heat during charge and discharge due to enthalpy changes, electrochemical polarization and resistive heating inside the cell.

The heat generated during charge or discharge of a battery is separated into 2 ways, reversible heat and irreversible heat.

1. Joules heat.
2. Entropy heat (reversible heat).

Joules Heat :

The heat generated due to the I^2R losses as current flows through the internal resistance of the battery whether it is being charged or discharged is known as Joules Heat.

According to Ohm’s law :

V = I*R

Where V = voltage

I = current

R = resistance

When current flowing through a resistor, some part of energy is dissipated in the form of heat. This is known as joules heating.

Power (P) = V*I = I*R * I = I^2 * R

Joules Heat (H) =

`H = int_0^t P*dt`

`H = P*int_0^t dt`

`H = I^2 * R * t`

Entropy Heat

This is the heat generated due to the entropic change of battery.The rate of change of the batteries open circuit voltage with respect to the temperature is known as entropic heating coefficient. It is a function of state of charge and temperature and is often expressed in mV/K.

Total heat generated = Joules Heat + Entropy heat.

1. The max heat generation of the battery

Given data :

Maximum current = -0.896

Maximum current time = 2 min = 120 sec

Battery resistance = 0.5 mOhms

Entropy factor = 2

maximum joules heat =

`= I^2 * R * t + 2`

`= ((-0.896)^2 * 0.5 * 120) + 2`

`= (0.802*0.5*120) + 2`

`48mJ`

Maximum heat generated = maximum joules heat + Entropy factor

`48 +2`

`50mJ`

The maximum heat generated in the battery = 50 mJ

b.The SOC of the battery at 2 *104second of the battery operation

State of charge (SOC) :

The state of charge is measurement of amount of energy available in the battery which can be used. It is expressed in percentage.

The SoC can be measured in 2 ways. They are

• Voltage translation method.
• Coulomb counting method.

Voltage translation method

Open circuit voltage (OCV) is used to determine the SoC of the battery pack. SOC is estimated by comparing OCV and data sheet value of the battery.

This method is not effective for electric vehicles. This is used in consumer applications like battery banks, trimmer, torch lights, etc.

Coulomb counting method

Estimating the SOC is based on in and out flow of current in the battery pack. To find SOC we need 2 parameters.

• Rate of flow of current in and out of the battery.
• Capacity of the battery.

SOC (%) = (Total charge input / Maximum battery capacity) * 100

When SOC is 0.

If there is already some amount of SOC present in the battery then we need to consider this formula

`z(t) = z(0) - 1/(Cn) * int_0^t ni * i(t)*dt`

z(t) = SOC

z(0) = initial SOC

Cn = battery nominal capacity

t = time

ni = coulombs efficiency

i(t) = instantaneous cell current

Simulink model to find SOC For charging cycle

Simulink model to find SOC For discharge cycle

Parameters of the battery

Signal builder graph:

Total time for the simulation is taken as 2*104 = 208 sec.

The current signal is taken with time 208 sec with step size 1 sec by using a signal builder.The controlled current source is driven by this signal so that the current flow in and out accordingly.

Results and Conclusion

1. The max heat generation of the battery

The maximum heat generated = 50mJ.

1. The SOC of the battery at 2 *104second of the battery operation

Initially the battery SOC is taken as 75%.

For charging cycle SOC is increasing

For discharge cycle SOC is decreasing