Project 2 Thermal modeling of battery pack

Aim:- Create 10 cell series lithium ion battery model, simulate the thermal effects and compare life cycle performance.

Objectives:-

1] Simulate the model and observe the thermal effects at various temperatures, charge and discharge rates.

2] Simulate the model and observe the life cycle performance of battery pack.

Battery Pack:-

A battery pack is a set of any number of (preferably) identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density.

Components of battery packs include the individual batteries or cells, and the Interconnects which provide electrical conductivity between them. 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, though batteries are most often only arranged in series strings.

When a pack contains groups of cells in parallel there are differing wiring configurations which take into consideration the electrical balance of the circuit. battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged, bringing the whole pack back into balance. Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cells to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.

MATLAB/Simulink Model:-

The above figure shows the 1P10S 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 enable the temperature effect and aging effect option for observing and estimating the temperature and signal effects. So in this model I estimate the age of the cell, SOC of the battery pack, battery pack temperature, ambient temperature, battery pack current by 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.

Used Blocks And their Parameters:-

1] Sine Wave:-

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

Parameters:-

2] 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 block icon. We can initialize the Controlled Current Source block with a specific AC or DC current. If you want to start the simulation in steady state, the block input must be connected to a signal starting as a sinusoidal or DC waveform corresponding to the initial values. For this model I used the default value.

3] Generic Battery:-

The Battery block implements a generic dynamic model that represents most popular types of rechargeable batteries. For this model enable the temperature effects and aging effects options. So cell nominal voltage is 7.2 V, capacity 5.4 Ah, initial SOC 100%, in aging section aging model aging sample time is 20 sec, aging characteristic at cell ambient temperature of cell 25°C and nominal charge and discharge current take as Ia variable (I take variable because every simulating time with different current value, it is tedious process to change the all cells charge-discharge current) which is similar to the input current signal. And all other parameters are default values. So now our battery pack voltage is 72 V and capacity is 5.4 Ah. So all these blocks are connected as per shown in the above model figure. And this model runs for 10 hour, so we can charge and discharge the battery pack multiple times.

Parameters:-

Results:-

So this simulink model simulates multiple times at various different ambient temperatures and different charge-discharge current rate. The ambient temperature varies by changing the ambient temperature constant block and charge-discharge value varies by using the MATLAB command window as shown in below figure.

Case 1] Ambient temperature: 25°C and Charge-Discharge Current: 5.4 A

The above figure shows the results, in this case ambient temperature is similar to the cell ambient temperature and similarly the charge-discharge current is 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 plote, the age of the battery pack increases by exactly 1.5 age, it is because the ambient temperature and aging characteristic 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 upto 5. If we observe the cell temperature at starting it gradually increases and settles at 26°C and tehn it goes continues upto nearly 1.5 sec then again increase same tgings happen in each cycle and the temperature will be increase. The final iincreasing temp is 31 degree celscius. When the battery starts charging, temperature starts increasing upto 25°C and when battery starts discharging, temperature starts increasing upto 30 degree celcius in some level. but when the battery charge then it comes to 26 degree in each cycle.

Case 2] Ambient temperature: 45°C and Charge-Discharge Current: 5.4 A

In this case we change the ambient temperature to 45°C and charge-discharge C-rate keeps the same i.e. 1C. Due to this reason battery pack SOC and voltage is same 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 2.15 instead of 1.5 After completing the 5 life cycle, the battery age is approximately 10 And the cell temperature reaches 53°C.

Case 3] Ambient temperature: 10°C and Charge-Discharge Current: 5.4 A

Now in this case ambient temperature decreases upto 10°C and charge-discharge rate is the same i.e. 1C. Because of these two reasons battery SOC is same and in the age of the battery favourable result shows. After completing the one life cycle of the battery, the age of the battery decrease to 1. 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 0-180°C.

Case 4] Ambient temperature: 25°C and Charge-Discharge Current: 3 A

In this model ambient temperature is kept the same such 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 charge above 55%. 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 0.75 and reaching one age of the battery requires more than 5 hours. In this case charge-discharge rate decreases, therefore cell temperature is nearly same as compared to case 1.

Case 5] Ambient temperature: 25°C and Charge-Discharge Current: 1 A

This case scenario is nearly similar to the case 4 scenario, in this case we change the charge-discharge current to 1A i.e. 0.18C rate. That is why these battery packs do not charge above 17%. 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 25 degree celcius and it is normaly fluctuate.

Case 6] Ambient temperature: 40°C and Charge-Discharge Current: 3 A

This case is different from other cases because in this case we changed both parameters. The ambient temperature is 40°C and charge-discharge current is 3 A i.e. 0.56C rate. Because our C rate is lower than the 1C, our battery pack does not charge above 55%. Therefore reaching 1 age of the battery requires more than 3 hour, in the battery pack voltage spikes are present and cell temperature is stable at 47°C.

Conclusion:-

From this different case study I conclude that:

1. When we 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.
2. Similarly when we decrease C rate then shows the favorable and unfavorable effects on the age, SOC, temperature and voltage of the battery pack.
3. The age of the battery increases slowly, SOC not able to charge or discharge completely, cell temperature is less and in the voltages spikes are present.

Thankyou

Dharmesh Bonde...