Pulse charge-discharge experiments show that at －40°C ambient temperature, the heated battery pack can charge or discharge at high current and offer almost 80 % power. Table 3. Comparative analysis of different external heating methods. 3.1.5. Comparative analysis of different external heating methods
A hot environmental temperature can trigger the battery to accelerate the redox chemical reactions, which directly causes an abundance of heat generation (thermal event). In the long run, the phenomenon can cause thermal runaway in the battery. For subtropical climates, EVs require a heating system to ensure the battery does not freeze.
Then, the air is conducted in the battery pack for the thermal management; Active technique: part of the exhausted air is brought to the inlet and mixed with new fluid from the atmosphere. Then, the heat exchanger cools down or heats the fluid to reach the optimal temperature for battery pack management.
The operating process involves the liquid (e.g., silicone oil) heated by the heater flows between the cells by employing the pump, facilitating the transfer of heat from the liquid to the battery. The inlet temperature, heating time, and external ambient temperature of the battery heating system all have an effect on the heat balance performance.
It was shown that for the ambient and initial cell temperature of −30°C, a single heating system based on MHPA could heat the battery pack to 0°C in 20 min, with a uniform temperature distribution in the battery pack, a maximum temperature difference of less than 3.03°C, and a good temperature rise rate.
The batteries can be then warmed up to a chargeable temperature by the HVAC system through ventilating warm air to the pack. In the battery preheating system, heating efficiency plays a crucial role in determining the heating performance.