Project 2 Thermal modeling of battery pack

Aim: For a 10 cell series lithium ion battery model, simulate the thermal effects and compare life cycle performance at various temperatures, charge & discharge rates using MATLAB.

Theory: The lithium ion battery are the most commonly used battery in different hybrid and electric vehicle. We also know that the operational performance of this types of battery mainly depends on the operating temperature of the battery, So it become more important to control the battery temperature with a proper cooling system.Four different cooling systems:

• Phase Change Material (PCM)
• Fin Cooling
• Air Cooling
• Liquid Cooling (both Direct and Indirect)

1. Phase change material absorbs heat energy by changing state from solid to liquid. While changing phase, the material can absorb large amounts of heat with little change in temperature. Phase change material cooling systems can meet the cooling requirements of the battery pack, however, the volume change that occurs during a phase change restricts its application. Also, phase change material can only absorb heat generated, not transfer it away, which means that it won’t be able to reduce overall temperature as well as other systems. Although not favorable for use in vehicles, phase change materials can be useful for improving thermal performance in building by reducing internal temperature fluctuations and reducing peak cooling loads.

2. Cooling fins increase surface area to increase the rate of heat transfer. Heat is transferred from the battery pack to the fin through conduction, and from the fin to the air through convection. Fins have high thermal conductivity and can achieve cooling goals, but they add a lot of additional weight to the pack. The use of fins has found a lot of success in electronics, and traditionally they have been used as an additional cooling system on internal combustion engine vehicles. Using fins to cool the electric car battery has fallen out of favor since the additional weight of the fins outweighs the cooling benefits.

3. Air cooling uses the principle of convection to transfer heat away from the battery pack. As air runs over the surface, it will carry away the heat emitted by the pack. Air cooling is simple and easy, but not very efficient and relatively crude compared to liquid cooling. Air cooling is used in earlier versions of electric cars, such as the Nissan leaf. As electric cars are now being used more commonly, safety issues have arisen with purely air-cooled battery packs, particularly in hot climates. Other car manufacturers, such as Tesla, insist that liquid cooling is the safest method.

4. Liquid coolants have higher heat conductivity and heat capacity (ability to store heat in the form of energy in its bonds) than air, and therefore performs very effectively and own advantages like compact structure and ease of arrangement. Out of these options, liquid coolants will deliver the best performance for maintaining a battery pack in the correct temperature range and uniformity. Liquid cooling systems have their own share of safety issues related to leaking and disposal, as glycol can be dangerous for the environment if handled improperly. These systems are currently used by Tesla, Jaguar, and BMW, to name a few.

The working temperature of the lithium ion cell is important because it influence the performance, efficiency, safety and capacity and by the charging and discharging itself. Hence thermal model of battery are made to calculate and estimate the cell temperature for safe use of the battery pack.

For this we have a default matlab model made with name ' Lithium ion battery pack with fault '. which shows 20 cell in series with one of the cell as faulty to check the temperature variation overall performance of the battery pack. To make a cell faulty its capacity is reduced. The default model is shown below:

In the above model we have made some changes and simulated it to get the desired output. The revised model is shown below:

In this above model 10 cells are connected in series to find thermal effect and life cycle performance related to temperature, charging and discharging and also conduction and convection blocks are used to show how heat transfer takes place in battery pack.

The 10 cells are connected as a combination of three packs i.e. first pack consist of 4 number of cells in series the second pack has only one cell which is faulty. The last pack has 5 cells in total connected in series.

The heat transfer is observed in the pack through conduction and convection block which are mode of heat transfer.

T subsystem is connected at every battery pack to plot the temperature graph for each battery pack.

A external ambient temperature source is present along with temperature sensor to display the ambient temperature

The battery pack is connected to AC current source to observe cyclic charging and discharging rate graph.

A current sensor is added to sence the SOC of the battery pack. and all the other result are their is simscape result menu.

Cell 01 to 04 subsystem:

Cell subsystem consist of thermal model and lithium cell 1RC block which calculate the temperature and thermal effect of cells and overall calculation of battery pack.

1RC Subsystem:

This block basically consist of capacitance value which depends on external physical signal input SOC and T. It is assumed that the capacitancevalue is slowly varing with time, hence the equation 1 = c*dv/dt holds.

Resistance value table which depands on an external physical signal input i.e. SOC and T.

Voltage value which implements the cell main branch voltage source and determine values of capacity and SOC. The defining equation depends on cell temperature.

Blocks used in the above model is explained and configured below:

1. Ambient Temperature Source: The Temperature Source block represents an ideal source of thermal energy that is powerful enough to maintain specified temperature at its outlet regardless of the heat flow consumed by the system.

On double clicking on the block configuration block will open where we can enter the temperature that we need to maintain. The configuration block is shown below:

2. Temperature Sensor: The Temperature Sensor block represents an ideal temperature sensor, that is, a device that determines the temperature differential measured between two points without drawing any heat.

Connections A and B are thermal conserving ports that connect to the two points where temperature is being monitored. Port T is a physical signal port that outputs the temperature differential value.

The block positive direction is from port A to port B. The measured temperature is determined as T = T_A – T_B

3. Thermal reference: The Thermal Reference block represents a thermal reference point, that is, a point with an absolute zero temperature, with respect to which all the temperatures in the system are determined.

4. Convective Heat Transfer: The Convective Heat Transfer block represents a heat transfer by convection between two bodies by means of fluid motion. The transfer is governed by the Newton law of cooling:

where:

5. Conductive Heat Transfer: The Conductive Heat Transfer block represents a heat transfer by conduction between two layers of the same material. The transfer is governed by the Fourier law:

where:

Convection:

Cell 01 to 04:

Cell number 5:

Cell 06 to 10:

Test 3: Ambient temperature = 299.1 K , Cell from 01 to 04 is set to 310K , Cell 05 is set at 320K , Cell 06 to 10 set to 330K. For Cells to set the temperature we double click on the cell we get configuration parameters and at bottom we see initial cell temperature there we can change the temperature of cells.

Temperature Graph:

SOC:

Conduction:

From cell 04 to 05:

From cell 10 to 11:

Convection:

For cell 01 to 04:

For cell 05:

For cell 06 to 10:

Test 4: Ambient temperature = 299.1 K , Cell from 01 to 04 is set to 350K , Cell 05 is set at 325K , Cell 06 to 10 set to 300K. For Cells to set the temperature we double click on the cell we get configuration parameters and at bottom we see initial cell temperature there we can change the temperature of cells.

Temperature graph:

SOC:

Condection:

For cell 04 to 05:

For cell 10 to 11:

convection:

For cell 01 to 04:

For cell 05:

For cell 06 to 10:

Test 5: Ambient temperature = 299.1 K , Cell from 01 to 04 is set to 310K , Cell 05 is set at 350K , Cell 06 to 10 set to 310K. For Cells to set the temperature we double click on the cell we get configuration parameters and at bottom we see initial cell temperature there we can change the temperature of cells.

Temperature graph:

SOC:

Condection:

For cell 04 to 05:

For cell 10 to 11:

convection:

For cell 01 to 04:

For cell 05:

For cell 06 to 10:

Here we have completed all the 5 test cases. In all the test case we see that what ever may be the starting temperature of batteries the perfect cell comes to 315 K temperature and gets stable and the faulty one will achieve more temperature than perfect cell i.e 225 K.

The SOC in all the test case is same i.e. from 0% to some where around 53% nd is repeated till 7200 sec.

For all conduction graph we see that for faulty cell the heat transfer coefficient and the thermal conductivity value decreases hence the heat transfer also decreases causing higher temperature of the faulty cell. Where as for all perfect cell the value of the heat transfer coefficient and the thermal conductivity is increasing which helps increase the heat transfer rate and hence keep the cell temperature low.