Project 2 Thermal modeling of battery pack
Objective : To study the behavior of EV battery cooling system under three conditions :
- FTP75 drivecycle
- Continuous constant current charge cycle
- Pulse charging current cycle
Description :
This study shows an Electric Vehicle (EV) battery cooling system. The battery packs are located on top of a cold plate which consists of cooling channels to direct the cooling liquid flow below the battery packs. The heat absorbed by the cooling liquid is transported to the Heating-Cooling Unit.
The Heating-Cooling Unit consists of three branches to switch operating modes to cool and heat the battery. The Heater represents an electrical heater for fast heating of the batteries under low temperature conditions. The Radiator uses air-cooling and/or heating when the batteries are operated stably. The Refrigerant system is used for cooling the overheated batteries. The refrigeration cycle is represented by the amount of heat flow extracted from the cooling liquid.
The system is simulated under either FTP-75 drive cycle or fast charge scenarios with different environment temperatures.
Modelling :
Following is the complete model for the study.
It consists of a battery connected to a cold plate via thermo sensors.
A heat exchanger pump, that can circulate the fluid to cool/heat the liquid into heating-cooling unit.
A heat-cool unit to vary the temperature of the liquid and typically maintain it in the range of 22-25 degree celcius.
Environment to act as a heat sink and release the extra energy to the environment.
- Battery :
It consists of 4 battery packs modules each with 20 cells connected in series with pack voltage of 72V/pack and capacity of 28Ah/pack.
Hence the nominal voltage of complete pack is 288V and 28Ah.
Thermo sensors are connected to the battery pack modules to sense the temperature.
- Heating – cooling unit :
There are three types of mechanisms used for cold, normal and hot conditions.
The heater gets activated when the battery temperatures fall below the operational limit of the pack, and the battery is heated, generally this case occurs in cold snowy regions.
The Radiator is used to circulate the liquid during the normal temperature conditions of around 25 degree celcius, and during the normal driving conditions.
The refrigerant system is activated during generation of excess heat and cool down the battery packs, this can typically be seen during aggressive driving, fast charging or pulsating charging.
Simulation :
Case 1 :
FTP75 drive cycle : A FTP 75 drive cycle is generated using the signal block and the current generated in turn is fed to the battery pack. And the system is simulated, for the response.
Result :
As we can see as the vehicle is driven, the temperature of the system rises, due to which the pump gets activated and the refrigerant cooling system is used to cool down the liquid and lower the temperature of battery pack.
As soon as the temperature is lowered, refrigerant is cutoff, and pump is stopped until the temperature rises again and so on.
Following is the heat energy distribution and cooling distribution :
Case 2 :
Continuous current charge cycle :
A continuous constant current of 100A is applied to the system and response is checked.
Here we can see that, during a constant current, the pump is continuously on to dissipate the heat by circulating refrigerant liquid.
Following is the heat energy distribution and cooling distribution :
Case 3 : Pulse charge current cycle.
Here a pulse current from 150 to 100 has been induced to check the system response.
Here, there are fluctuations in the operation of the refrigerant action, being a pulse signal.
Following is the heat energy distribution and cooling distribution :
Conclusion :
The cooling system can cool the pack dring a specific current consumption of 100A, when the current is drawn more than 100A, the heat generated is more than the heat dessipated. Hence the battery pack heats and cannot maintain the optimal temperature of 22-25 degree celcius.
Hence, its necessary to limit the C rating during discharging and avoiding fast charging / chargers of high power to keep the current drawn in limit.