Battery System Thermal Management
Battery thermal management mainly includes cooling, heating, and temperature equalization. Cooling and heating functions primarily adjust for the potential impact of external ambient temperature on the battery. Temperature equalization reduces temperature differences within the battery pack, preventing rapid battery degradation caused by overheating in certain areas.
Generally, power battery cooling methods are mainly divided into three categories: air cooling, liquid cooling, and direct cooling. Air cooling utilizes natural air or cooling air from the passenger compartment to achieve heat exchange and cooling over the battery surface. Liquid cooling typically uses independent coolant piping to heat or cool the power battery; this is currently the mainstream cooling method, used in Tesla and Volt batteries. Direct cooling systems eliminate the need for separate cooling piping for the power battery, directly using refrigerant to cool it.
1. Air Cooling System
Early power batteries, due to their smaller capacity and energy density, often used air cooling. Air cooling is divided into two main categories: natural air cooling and forced air cooling (using a fan), utilizing natural air or cool air from the passenger compartment to cool the battery.
Currently, the 48V batteries in 48V mild hybrid vehicles are generally located in the passenger compartment and cooled by air. Air-cooling systems are relatively simple in structure, technologically mature, and low in cost. However, due to the limited heat removal capacity of the air, their heat exchange efficiency is low, resulting in poor internal temperature uniformity and difficulty in precisely controlling battery temperature. Therefore, air-cooling systems are generally suitable for applications with short driving ranges and lightweight vehicles.
2. Liquid Cooling Systems
Liquid cooling involves using a coolant to exchange heat with the battery. Coolants are divided into two types: those that can directly contact the battery cells (silicone oil, castor oil, etc.) and those that contact the cells through water channels (water and ethylene glycol, etc.); currently, a mixture of water and ethylene glycol is more commonly used. Liquid cooling systems typically include a chiller coupled to the refrigeration cycle, using the refrigerant to remove heat from the battery. The core components are the compressor, chiller, and water pump. The compressor, as the power source for refrigeration, determines the heat exchange capacity of the entire system. The chiller facilitates the exchange of heat between the refrigerant and coolant, and the amount of heat exchange directly determines the coolant temperature. The water pump determines the flow rate of the coolant within the pipes; a faster flow rate results in better heat exchange performance, and vice versa.
Liquid cooling systems offer greater flexibility. Cooling channels can be installed between battery modules (currently the mainstream approach), cooling plates can be used at the bottom of the battery, or the cells or modules can be immersed in coolant. The advantages of liquid cooling systems include high heat transfer coefficients, fast flow rates, good temperature uniformity, and precise temperature control. Disadvantages include system complexity, high sealing requirements, the cooling system accounting for a significant portion of the battery pack's weight, and relatively high cost.
