Project 2

OBJECTIVE

To find the maximum amount of heat generated and the SOC estimation for a battery pack for a predefined current profile.

THEORY

Heat generation in battery packs:

Batteries generate heat during charging and discharging due to enthalpy changes, electrochemical polarization and resistive heating inside the cell. 

There are two heat sources for battery heat generation:

• Joule heat

        

        Joule heat = Power dissipation * time 

        where,

        Power dissipation = I^2 * R[battery resistance]

• Entropy heat 

        Entropy heat = Entropic factor * Joule heat

Total Heat Generation = Joule Heat + Entropic Heat

SOC Estimation in Li-ion batteries:

The SoC refers to the State of Charge of the battery, which is the capacity percent left in the battery which can be used.

SOC can be estimated in different ways out of which few are listed below:

• Voltage translation method(VT)

        It is measued by Open Circut Voltge of the battery and compared it with the data sheet of the battery and SoC is            estimated. 

Pros	Cons
Easy to implement	High error rate in reading
Comparitively cheaper	Not effective for EV application due to the dynamic voltage/torque profile.

• Coulomb counting method(CC)

        SOC is estimated based on the rate of flow of current in and out of the battery pack.

        Z(t) = Z(0) - 1/Cn ∫ Ni*i(t) dt    with time limits of 0 to t

        where, Cn = Battery Nominal Capacity

                   Z(t) = State of charge

                   i(t) = Instantaneous cell current

                     t = time 

                    Ni = coulomb efficiency

                   Z(0) = initial state of charge

Pros	Cons
Comparitively better accuracy than VT method	Increase in error rate as the nominal capacity of the battery pack changes w.r.t time, temperature and life cycle

•  Combination of CC and VT:

         In this method the error rate w.r.t time, temperature and life cycle in CC method is calibrated with VT method.

Pros	Cons
Comparitively better accuracy	If the caliberation is done periodically, then the error rate would be the same as that of CC method

SIMULINK MODEL

The current values are saved in an excel sheet and loaded in to drive cycle cource block which gives the battery current profile as output.

For SOC estimation i have used the coulomb counting method:

Z(t) = Z(0) - 1/Cn ∫ Ni*i(t) dt

Here i have assumed:

The battery nominal capacity, Cn= 5Ah 

Coulomb efficiency, Ni = 90%

Battery initial SOC, Z(0)= 50%

Calculating maximum total heat generation in battery pack:

To find the maximum current value from the given current profile, i have defined a matlab function module as below

 

Simulation:

The model was simulated for 2*104 seconds.

CONCLUSION:

From the simulation result, We can see that the SOC of the battery has been increased from 50% to 51% at 2*104 seconds of the battery operation as the current is -ve through out the drive cycle (charging current). 

Also the total maximum heat energy generated in the battery pack is 0.3313 Joules.