As the main power source of new energy vehicles, the importance of power batteries to new energy vehicles is self-evident. In the actual vehicle use process, the battery will face complex and changeable working conditions. In order to improve the cruising range, the vehicle needs to arrange as many cells as possible in a certain space, so the space for the battery pack on the vehicle is very limited. Batteries generate large amounts of heat during vehicle operation and accumulate in relatively small spaces over time. Due to the dense stacking of cells in the battery pack, it also makes it more difficult to dissipate heat in the middle area to a certain extent, which aggravates the temperature inconsistency between cells. As a result, the charging and discharging efficiency of the battery will be reduced, and the power of the battery will be affected; It will cause thermal runaway, affecting the safety and life of the system.
The temperature of the power battery has a great influence on its performance, life and safety. At low temperature, the internal resistance of the lithium-ion battery will increase and the capacity will become smaller. 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 the electric vehicle. Attenuation and reduced driving range. When charging a new energy vehicle under low temperature conditions, the BMS generally heats the battery to a suitable temperature before charging. If not handled properly, it will cause instantaneous voltage overcharge, resulting in an internal short circuit, which may further lead to smoke, fire or even explosion. The safety of low-temperature charging of electric vehicle battery systems has largely restricted the promotion of electric vehicles in cold regions.
Battery thermal management is one of the important functions in BMS, mainly to keep the battery pack working within a suitable temperature range, so as to maintain the best working condition of the battery pack. The thermal management of the battery mainly includes functions such as cooling, heating, and temperature equalization. The cooling and heating functions are mainly adjusted according to the possible influence 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. Usually we expect the battery to work in the temperature range of 20~35℃, 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 the cooling air in the passenger compartment to flow through the surface of the battery to achieve the effect of heat exchange and cooling. Liquid cooling generally uses an independent coolant pipeline to heat or cool the power battery. At present, this method is the mainstream of cooling. For example, Tesla and volt use this cooling method. The direct cooling system saves the cooling pipeline of the power battery and directly uses the refrigerant to cool the power battery.
