The temperature of the power battery has a great influence on its performance, life and safety. At low temperature, the internal resistance of lithium-ion batteries will increase and the capacity will decrease. In extreme cases, the electrolyte will freeze and the battery cannot be discharged. The low-temperature performance of the battery system will be greatly affected, resulting in the power output performance of electric vehicles. Decrease in fade and driving range. When charging new energy vehicles under low temperature conditions, the general BMS first heats the battery to a suitable temperature before charging. If it is not handled properly, it will lead to instantaneous voltage overcharge, resulting in internal short circuit, and further smoke, fire or even explosion may occur. The low-temperature charging safety problem of electric vehicle battery system restricts the promotion of electric vehicles in cold regions to a large extent.
Battery thermal management is one of the important functions in BMS, mainly to keep the battery pack working in an appropriate temperature range at all times, so as to maintain the best working condition of the battery pack. The thermal management of the battery mainly includes the functions of cooling, heating and temperature equalization. The cooling and heating functions are mainly adjusted for the possible impact of the external ambient temperature on the battery. Temperature equalization is used to reduce the temperature difference inside the battery pack and prevent rapid decay caused by overheating of a certain part of the battery. As shown in Table 1, generally we expect the battery to work in the temperature range of 20~35 degree , which can achieve the best power output and input of the vehicle, the maximum available energy, and the longest cycle life.
Generally speaking, the cooling modes of power batteries are mainly divided into three categories: air cooling, liquid cooling and direct cooling. The air-cooling mode uses natural wind or cooling air in the passenger compartment to flow through the surface of the battery to achieve heat exchange and cooling. Liquid cooling generally uses an independent coolant pipeline to heat or cool the power battery, and this method is currently the mainstream of cooling.
1. Air cooling system:
In the early power batteries, due to their small capacity and energy density, many power batteries were cooled by air cooling. Air cooling is divided into two categories: natural air cooling and forced air cooling (using fans). The battery is cooled by natural wind or cold air in the cab. The structure of the air cooling system is relatively simple, the technology is relatively mature, and the cost is low. However, due to the limited heat taken away by the air, its heat exchange efficiency is low, the internal temperature uniformity of the battery is not good, and it is difficult to achieve a more accurate control of the battery temperature. Therefore, the air-cooling system is generally suitable for situations where the mileage is short and the weight of the vehicle is light.
2. Liquid cooling system
