Project 2 Thermal modeling of battery pack

Ans.)

The lithium-ion batteries are widely used as energy sources in different hybrid and electric vehicle. The operation characteristics of the battery in such vehicle are strongly dependent on the operation temperature and therefore, with purpose to improve these characteristics a good thermal control system should be utilized.

The temperature of a lithium battery cell is important for its performance, efficiency, safety, and capacity and is influenced by the environmental temperature and by the charging and discharging process itself. Therefore, a thermal model of the battery is used in order to calculate and estimate the cell temperature.

Lithium-ion battery pack with fault was simulated. A battery pack consisting of multiple series-connected cells in an efficient manner. It also shows how a fault can be introduced into one of the cells to see the impact on battery performance and cell temperatures. For efficiency, identical series-connected cells are not just simply modelled by connecting cell models in series. Instead a single cell is used, and the terminal voltage scaled up by the number of cells. The fault is represented by changing the parameters for the cell 5 fault subsystem. reducing both capacity and open-circuit voltage, and increasing the resistance values.

Model :-

Matlab simulink model for thermal modelling is given as :-

Cell 01 to 04 Subsystem :-

Cell subsystem basically consist of a Thermal Model and Lithium Cell 1RC block which calculate the temperature and thermal effect for cells and overall calculate for the battery pack.

The block which are inside the cell subsystem are PS Gain block which multiplies the input physical signal by a constant i.e. y = u.* gain. Another block are voltage sensor and current voltage source.

The voltage sensor block represents an ideal voltage sensor, that is, a device that converts voltage measured between any electrical connections into a physical signal proportional to the voltage.

Current voltage source represents an ideal voltage source that is powerful enough to maintain the specified voltage at its output regardless of the current passing through it. The output voltage is V = Vs, where Vs is the numerical value presented at the physical signal port.

Thermal Model Subsystem :-

The important block which is inside the thermal model subsystem are:

Thermal Reference :-

This block represents a reference point in a thermal network where the temperature is equal to absolute zero.

Controlled heat flow rate source :-

This block represents an ideal energy source in a thermal network that can maintain a controlled heat flow rate regardless of the temperature difference. The heat flow rate is set by the physical signal port S. A positive heat flow rate flows from port A to port B.

Thermal Mass :-

This block models internal energy storage in a thermal network. The rate of temperature increase is proportional to the heat flow rate into the material and inversely proportional to the mass and specific heat of the material.

Temperature Sensor :-

This block measures temperature in a thermal network. There is no heat flow through the sensor. The physical signal port T reports the temperature difference across the sensor. The measurement is positive when the temperature at port A is greater than the temperature at port B.

Lithium Cell 1RC Subsystem :-

This block basically consists of capacitance value table which depends on an external physical signal inputs SOC and T. It is assumed that the capacitance value is slowly varying with time, and hence the equation i = C*dv/dt holds.

Resistance value (R) table which depends on an external physical signal inputs SOC and T.

Voltage value (EM) which implements the cell's main branch voltage source, and determines values for capacity (C) and state of charge (SOC). The defining equations depend on cell temperature T.

Thermal behaviour and fault setup :-

Lithium-ion cell one RC- Branch equivalent circuit detail for 10 cells.

For cell 01 to 04 :-

Number of series connected cells are 4.

Initial temperature is 300 k.

For cell 05 (faulty cell) :-

In this cell i have done some changes in electrical system of cell.so that we can see the fault.

For cell 06 to 10 :-

Number of series connected cells are 5.

Initial temperature is 300 k.

The other block which are used in lithium ion battery pack are as :-

Temperature Source :-

This block represents an ideal energy source in a thermal network that can maintain a constant absolute temperature at the port regardless of the heat flow rate.

This block is used for ambient temperature and which set as 300 K.

Convective heat transfer :-

This block models heat transfer in a thermal network by convection due to fluid motion. The rate of heat transfer is proportional to the temperature difference, heat transfer coefficient, and surface area in contact with the fluid.

This block is used for cell 01 to 0

This block is used for cell 05

This block is used for cell 06 to 10

Conductive heat transfer :-

This block models heat transfer in a thermal network by conduction through a layer of material. The rate of heat transfer is governed by Fourier's law and is proportional to the temperature difference, material thermal conductivity, area normal to the heat flow direction, and inversely proportional to the layer thickness.

The conduction is done from the 2 set of blocks, 1 block is used for 04-05 and other is used for cell 05-06.

For SOC calculation Gain block, Integrator and Sum block with PS simulink convertor was used.

Results :-

Simulation based on different cell temperature :-

Test 1:-

Cells Tempearature :-

Calculated SOC :-

1. The Ambient temperature provided by the Temperature source is 300 K

2. The Cell pack 1-4 reaches the maximum temperature of 332.8K, Cell pack 5 which is the faulty cell reaches the maximum temperature of 343.8 K and the Cell pack 6-10 reaches the maximum temperature of 331.8 K

3. The SOC of the model starts from 0 and goes up to 0.95 in 300 secs and again discharges to O in 300 secs and the cycle keeps on repeating.

4. The Life cycle of the battery pack is 6 cycles per hour.

Test 2:-

Cells Tempearature :-

Calculated SOC :-

1. The Ambient temperature provided by the Temperature source is 320 K

2. The Cell pack 1-4 reaches the maximum temperature of 352.3K, Cell pack 5 which is the faulty cell reaches the maximum temperature of 363.4 K and the Cell pack 6-10 reaches the maximum temperature of 352.5 K

3. The SOC of the model starts from 0 and goes up to 0.95 in 296.1 secs and again discharges to O in 303 secs and the cycle keeps on repeating.

4. The Life cycle of the battery pack is 6 cycles per hour.

As we increase the ambient temperature, the temperature of faulty cell 05 is more increasing.