Project 2 Thermal modeling of battery pack

Project-2

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:

A Lithium-ion (Li-ion) battery is a type of rechargeable battery used in electric vehicles and a number of portable electronics. They have a higher energy density than typical lead-acid or nickel-cadmium rechargeable batteries. This means that battery manufacturers can save space, reducing the overall size of the battery pack.

Lithium is also the lightest of all metals. However, lithium-ion (Li-ion) batteries contain no lithium metal, they contain ions. For those wondering what an ion is, an ion is an atom or molecule with an electric charge caused by the loss or gain of one or more electrons.

Lithium-ion batteries are also safer than many alternatives and battery manufacturers have to ensure that safety measures are in place to protect consumers in the unlikely event of a battery failure. For instance, manufacturers equip electric vehicles with charging safeguards to protect the batteries during repeated rapid charging sessions in a short period of time.

A built- in MATLAB model “Lithium-ion battery pack with fault” is shown below.

As we can see in the model, the 20 lithium ion cells are connected in series. The 10th cell in the battery pack is considered faulty.

The faulty cell means that the temperature of that particular cell is not maintained properly and thus this faulty cell will effects the performance of overall battery pack.

A fault can be introduced into one of the cells to see the impact on battery performance and cell temperature. For efficiency identical series-connected cells are not just simply modeled by connecting cell models in series. Instead a single cell is used and terminal voltage scaled up by the number of cells.

The fault is represented by changing the parameters for the cell 10, reducing both capacity and open circuit voltage, and increasing the resistance values.

Now if we open the cell 01 to 09 block,

We can see that the cell is working to its full potential.

Now if we open the cell 10 block,

We can see that,

• Capacity is multiplied by 0.95 this means that capacity is reduced by 5%.
• The open-circuit voltage is multiplied by 0.9, this means that the open-circuit voltage is reduced by 10%.
• The cell resistance is multiplied by 5, this means that cell resistance is increased by 5 times.

As the resistance increases, the temperature of the cell also increases. This all changes are done to show that the cell 10 is in faulty condition.

As we can see in the model , there is convective heat transfer from the battery pack to the surrounding environment and there is conduction heat transfer between the cells of the battery pack.

As in the question, it is mentioned that ‘10 cell series lithium ion battery’ thus we have to make certain changes in the model.

1. Thus change the 9 cells to 4 cells.

2. Change the 10th cell as 5th cell and let this cell be faulty.

3. Change 10 cells to 5 cells.

Thus we got in total 10 lithium ion cell and 5th cell is faulty

TO ESTIMATE SOC:

1. Current sensor block:

The current sensor block is used in the mode. This current sensor will measure the current through the battery pack.

2. PS-Simulink converter:

The output of the current sensor is given to PS-simulink converter to convert physical signal to simulink format.

3. Gain block:

As we are using the coulomb counting method, a gain block is used here.

4. Discrete time integrator block:

The discrete time integrator block is used to integrate the current flowing through the battery pack circuit.

5. Constant:

The constant block of value 0 is used; the value 0 means the initial SOC is 0.

6. Sum block:

The output of the discrete time integrator block is subtracted from the constant 0 block and given to the scope.

Model for the SOC calculation:

To measure the temperature of cells:

1. Create subsystem:

Here in the subsystem, we have used a temperature sensor block.

Where A point is connected to the temperature port and B point is connected to thermal reference.

The T-port of temperature sensor block is given to ps-simulink converter and then output of the PS- simulink converter is given to the GOTO block.

2. The 3 different blocks are then used in the model of respective Goto blocks.

3. These 3 from blocks are connected to 3 input mux and the output of mux is connected to scope.

Convective heat transfer:

There is convective heat transfer from the battery pack to surrounding.

As there as 4 cells(01 to 04), thus area is cell area*4.

As the 5th cell is faulty and there is only one cell, then taken cell area only.

As there as 5 cells (06 to 10), thus area is cell area*5.

Conductive heat transfer:

There is conductive heat transfer between the cells of the battery pack.

The default parameters are taken in the conductive heat transfer block.

To measure the ambient temperature:

A temperature sensor block is taken, the port A is connected to ambient temperature B port is connected to thermal reference and the T port is connected to PS-simulink block and output of the PS-simulink block is given to scope.

The complete model looks like:

Here in this model cyclic charge discharge profile is considered.

From the parameters , we can see that the peak amplitude of the ac current source is 50 amp and we have considered phase shift as 0 and frequency is considered as 1/600 Hz.

Here in this model, the load is considered in the form of alternating current which is provided by the ac current source block.

AC current source positive terminal is connected to the battery pack and the negative terminal is connected to the electrical reference.

Simulation results:

Run simulation for 7200 seconds.

1. The ambient temperature is found to be 293.1k.

2. Temperature of cells:

Thus from simulation results, we can say that the faulty cell have higher temperature than the normal cells.

The normal cell temperature is between 295k-325k and the faulty cell temperature has a maximum temperature of 337k.

3. State of charge (SOC):

We found that the SOC graph is fluctuating in the entire 7200 second time period.The SOC rises from 0 to 0.95 in 0 to 318 seconds and then falls from 0.95 to 0 in 318 to 600 seconds.

From the graph, we can say that there are 6 charging-discharging cycles in 3600 hours. Thus we can say that the life cycle of the battery pack is 6 cycles per hour. This life cycle can be increased if we remove the faulty cell.

Conclusion:

Thus we can say that, if the faulty cell is there in the battery pack, the life cycle of the battery pack reduces.

The temperature of faulty cell is much higher than normal working cells and because of this higher temperature, the efficiency of the battery pack gets reduced.