Project 2

1. Using MATLAB/simulink and the drive cycle from the attached excel sheet, find-

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

Time	Time Step	Battery Current
00:00.4	0.1	-0.9632
00:00.5	0.2	-0.952
00:00.6	0.3	-0.9072
00:00.7	0.4	-0.9632
00:00.8	0.5	-1.0304
00:00.9	0.6	-0.9632
00:01.0	0.7	-1.0304
00:01.1	0.8	-1.008
00:01.2	0.9	-0.9856
00:01.3	1	-0.9632
00:01.4	1.1	-0.9184
00:01.5	1.2	-0.9296
00:01.6	1.3	-0.9296
00:01.7	1.4	-0.9296
00:01.8	1.5	-0.9184
00:01.9	1.6	-0.9408
00:02.0	1.7	-0.896
00:02.1	1.8	-0.9072
00:02.2	1.9	-0.9072
00:02.3	2	-0.9408
00:02.4	2.1	-0.9296
00:02.5	2.2	-0.9296
00:02.6	2.3	-0.9968
00:02.7	2.4	-0.9968
00:02.8	2.5	-0.9408
00:02.9	2.6	-0.9408

Battery resistance

2 milli ohm

Consider the battery resistance is 0.5 mOhm, delta time is 0.1 and entropic factor is 2.

Answer:

Aim:

To find the maximum heat generation of the battery and SOC of the battery at 2 *104second of the battery operation using MATLAB/Simulink.

Heat Generation and SOC Estimation:

Batteries generate heat during charge and discharge due to enthalpy changes, electrochemical polarization and resistive heating inside the cell. Temperature variation inside the batteries can lead to uneven temperature distribution which creates uneven charge/discharge behaviour within the battery pack. So, thermal runaway may occur in a cell when heats are not properly controlled and possible to cause fire explosion. For this reason, the need for battery thermal management is vital for electric and hybrid vehicles in order to keep the vehicle at its optimum performance. Heat generation occurs in 3 stages. In stage 1; Overheating, Stage 2; Heat accumulation and gas release process, Stage 3; Combustion and explosion.

Heat generation Calculation;

There are 2 heat sources for battery heat generation.

• Joule heat
• Entropy heat

Heat generated = Joules heat + Entropic heat

For finding Joule heat;

According to Ohms law, V = I*R

When a current is flowing through a resistance, there is heat dissipated in the resistor. This heat disspiation is called Joule heating, also known as Ohmic heating.

Power, P= V*I = I*IR = I²*R

Heat, `H=int(P.dt) = int_0^t(I*IRt) = I²*Rt`

Joules heat, H = I²*Rt

where, P is power

V is voltage

I is current

R is resistance

H is heat

t is Time.

For finding Entropy heat;

The heat generation due to entropy change inside a battery occurs when electrochemical reactions are performed. The entropy heat is reversible heat resulting from changes in open circuit voltage with respect to temperature at two electrodes.

Calculation:

Given data;

Resistance = 2 mOhm

Considering the magnitude of the current, Maximum current = 1.0304 A

Total time = 129 seconds

Joules heat = I²Rt = (1.0304)²* 2*10^-3 * 129 = 0.2739 J

Entropy heat = 2*Joules heat = 2*0.2739 = 0.5478 J

Maximum Heat generated = Joules heat + Entropic heat = 0.2739 J+ 0.5478 J = 0.8217 J.

SOC Estmation Techniques;

• Voltage Translation method
• Coulomb counting method
• Combination of Voltage Translation and Coulomb counting method

Voltage Translation method;

The battery voltage (open circuit voltage) is used to determine the state of charge of the battery pack. The voltage translation method is used in applications like consumer electronics (battery bank, trimmer, torch light) and small energy storage systems. The LCD and LED are used to determine SOC. The errors in voltage translation methods are voltage fluctuation, noise in the system and cell chemistry. So, it is not used in electric vehicles.

Coulomb counting method;

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

Case1: Estimating SOC that before charging SOC is 0%.

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

Case2: Estimating SOC that battery has already 50% charge.

`Z(t) = Z(0)-(1/(Cn)*int_0^tni*i(t)*dt)`

Where, Z(0) is initial SOC

Cn is battery nominal capacity

Z(t) is state of charge

i(t) is instantaneous cell current

t is time

ni is coulomb efficiency.

However, for this method there is increase error rate with respect to temperature and life cycle.

Sign convention of current, + for discharge and - for charging.

Combination of Voltage Translation and Coulomb counting method;

This method eliminates and reduces error rate. The method include Voltage-based measurement and Coulomb counting. In Coulomb counting, error rate will increase with respec to temperature and life cycle of battery and affects the capacity pf battery, and increases error in SOC measurement. This error rate increment is caliberated with Voltage translation method.

Simulink Model for SOC calculation:

The blocks used are:

• Signal Builder: Create and generate interchangeable groups of signals whose waveforms are piecewise linear
• Controlled Current source: Converts the Simulink input signal into an equivalent current source. The generated current is driven by the input signal of the block.
• Resistance; Series RLC brank with resistance value 0.5 mOhm
• Battery: Implements a generic battery model for most popular battery types. Temperature and aging (due to cycling) effects can be specified for Lithium-Ion battery type.
• Bus selector: This block accepts a bus as input which can be created from a Bus Creator, Bus Selector or a block that defines its output using a bus object.
• Scope
• Powergui: The powergui block allows you to choose one of these methods to solve your circuit. Continuous, which uses a variable-step solver from Simulink. Discretization of the electrical system for a solution at fixed time steps. Continuous or discrete phasor solution.

Signal Builder:

After 129 secs, the value will be constant, as we have to run the drive cycle for 208 seconds.

For the signal builder, we have provided the time values and current values as,

Time in secs	Battery Current
4	-0.9632
5	-0.952
6	-0.9072
7	-0.9632
8	-1.0304
9	-0.9632
60	-1.0304
61	-1.008
62	-0.9856
63	-0.9632
64	-0.9184
65	-0.9296
66	-0.9296
67	-0.9296
68	-0.9184
69	-0.9408
120	-0.896
121	-0.9072
122	-0.9072
123	-0.9408
124	-0.9296
125	-0.9296
126	-0.9968
127	-0.9968
128	-0.9408
129	-0.9408

Resistance:

Battery:

Results:

The SOC% remains to be constant and 100%, as it was fully charged condition at the start. Now, if we are considering 50% battery SOC, then there will be an increase in SOC as the charging is done. The negative values in the battery current indicates that the battery is charging. The Current and voltage will have variations in their value according to the charging and discharging.

Considering the 50% SOC, we get the plot as:

SOC seems to be increasing, as the charging is done.