Project 5

Thermal Modelling of Battery Pack

Aim:

• To create is 10 cell series lithium-ion battery model
• To simulate the thermal effects and compare life cycle performance at various temperatures, charge and discharge rates using MATLAB

Theory and Explanation:

Battery Pack:

A battery pack is a set of any number of identical individual cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity or power density. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging, Interconnects are also found in batteries as they are the part which connects each cell, as per our requirement battery connection configuration made. If we want more voltage than the individual one cell then serries connection is made and if we want more current capacity then connect cells in parallel manner. And it looks similar to the figure below.

 

The Simulink simulation for 10 lithium-ion batteries connected in series without thermal effect is done in MATLAB 2022a.

 

Subsystem Information (10 lithium-ion batteries):

 

Parameters for Lithium-Ion battery:

Aging Parameters for Lithium-Ion battery:

 

Simulation Result:

We can see from simulation result that when the C-rate is 1C i.e., battery charges completely and discharges completely to complete one life cycle. As we can see it from figure shown below after one complete charge and discharge cycle battery life reduces.

 

The Simulink simulation for 10 lithium-ion batteries connected in series with thermal effect is done in MATLAB 2022a.

 

The above figure shows the 10 series configuration of the lithium-ion battery pack. These are all battery cells connected in series. This battery pack charges or discharges by using the controlled current source and square wave generator. For this cell this cell enables the temperature effect and aging effect option for observing and estimating the temperature and signal effects. So, in this model I can estimate the age of the cell, SOC of the battery pack, battery pack temperature, ambient temperature, battery pack current by the using a bus selector from any one cell of the battery pack, because this battery pack is made from connecting the cells in series. So current temperature, SOC and age of the cell is similar to other cells. The battery pack voltage is calculated by using a voltage measurement block, and this measurement block is connected across the battery pack i.e., first cell positive terminal connected to positive terminal of voltage measurement block and last cell negative terminal connected to negative terminal of voltage measurement block. Also, by using constant blocks, it provides ambient temperature to all cells.

Signal Generator:

This signal generator block can produce on of four different waveforms: sine, square, sawtooth and random. We can express signal parameters in hertz or radians per second. For this model square waveform with 5.4 as amplitude and 1 / (3600*2) Hz frequency. This signal is provided to the current controlled source.

 

Controlled Current Source:

The controlled current source block converts the Simulink input signal into an equivalent current source. The generated current is driven by the input signal of the block. The positive current direction is as shown by the arrow in the black icon. We can initialize the controlled current source block input must be connected to a signal starting as a sinusoidal or DC waveform corresponding to the initial values. For this model default values have been used.

Generic Battery:

The battery block implements a generic dynamic model represents most popular types of rechargeable batteries. For this model enables the temperature effects and aging effects options, so cell nominal voltage is 7.2 V capacity 5.3 A, initial SOC 100%, in aging section aging model aging sample time is 20 sec, aging characteristics at cell ambient temperature of cell 25°C and l the other parameter are default values. So now our battery pack voltage is 7.2V and capacity is 5.4 A.

So, all these blocks are connected as per shown in the above model figure. And this model runs for 10 hours so we can charge and discharge the battery pack multiple times.

Simulation Result:

Case 1: Ambient temperature 25°C and charge-discharge current 5.4Ah

The above figure shows the results, in this case ambient temperature is similar to the cell ambient temperature and similarly the charge-discharge current also similar to cell maximum current capacity i.e., cells charge-discharge at 1C rate. Therefore, the battery pack SOC completely reduces or packs full charge within the 1 hour. Thus in 2 hours the battery pack completed 1 life cycle and if we see in the age plot the age of the battery pack increased by exactly 1 age. It is because the ambient temperature and aging characteristics at cell ambient temperature is the same. So in the 10 hours the battery pack used 5 life cycles and the age of the battery pack increases to 5. If we observe the cell temperature at starting it gradually increases and settles at 52.2 °C and fluctuates within 52-53°C. When the battery starts charging, temperature starts increasing upto 53°C and when battery starts discharging temperature starts reducing upto 52°C

Case 2: Ambient temperature 45°C and charge-discharge current 5.4Ah

In this case the battery pack SOC and voltage is same as case 1 but this ambient temperature adverse effect replicates in the age of cycle and of course cell temperature also increases. In two hours, the battery uses 1 life cycle but at the same time the age of the battery increases approximately 1.5 instead of 1. After completing the 5 life cycle, the battery age is approximately 7.5 and the cell temperature is 75°C

Case 3: Ambient temperature 10°C and charge-discharge current 5.4Ah

In this case battery SOC is same and in the age of the battery favorable shows. After completing the one life cycle of the battery, the age of the battery increases only 0.75. But in the temperature plot, huge spikes are present because of the significant difference between ambient temperature and cell temperature. The cell temperature varies between 38-70°C.

Case 4: Ambient temperature 25°C and charge-discharge current 3Ah

In this model ambient temperature is kept the same like case 1 and charge – discharge current is reduced to 3A i.e., 0.56C rate. Due to the reduction in the C-rate, now our battery does not discharge 45%. Because of this reason, the voltage of battery pack plots also changes and after completing the two hours simulation the age of the battery is below 0.5 and reaching one age of the battery requires more than 5 hours. In this case charge-discharge rate decreases, therefore cell temperature is also less i.e., 49°C as compared to case 1.

Case 5: Ambient temperature 25°C and charge-discharge current 1Ah

Here the scenario is similar to case 4, in this case the battery pack do not discharge beyond 82%. This effect replicates in the age of battery, cell temperature and battery pack voltage. After completing the 2 hours of simulation, the age of the battery only increases to 0.1. in the battery pack voltage plot voltage spike amplitude is increased as compared to the case 4. Because the C-rate is very small, the cell temperature is 49°C and it is stable and does not fluctuate widely.

Case 6: Ambient temperature 40°C and charge-discharge current 3Ah

This case is different from other cases. The charge-discharge current 3A i.e., 0.56C rate. Because our C rate is lower than 1C, our battery pack does not discharge beyond 45%. Therefore reaching 1 age of the battery requires more than 3 hours, in the battery pack voltage spikes are present and cell temperature is stable at 55°C.

Conclusion:

• When ambient temperature increases then obviously cell temperature is increased also this ambient temperature adverse effect reflects on the age of the battery pack and vice versa.
• Similarly, when we decrease C rate then it shows favorable and unfavorable effects on the age, SOC, temperature and voltage of the battery pack.
• The age of the battery increases slowly, SOC not able to charge or discharge completely, cell temperature is less and in the voltage spikes are present.