Why do car batteries need heating film?

A series pure electric drive vehicle has two problems in low temperature environment (for example,-20℃). One is that at low temperature, the battery discharge ability is extremely poor, even if it is fully charged, the driving ability of the vehicle will be greatly reduced; The other, low-temperature charging, has been considered as the fastest way to reduce the life of power batteries. At low temperature, the conductivity of charged ions in the battery is poor, and the ability of charge transfer and embedding in the negative electrode material structure is also poor. If normal current is charged at low temperature, a large number of lithium ions can not be embedded on the surface of the negative electrode, and electrons are obtained in various ways and deposited on the surface of the electrode, forming lithium accumulation; Lithium metal is very prone to the phenomenon of uneven crystal growth. When dendrites grow to a sufficient scale, they may pierce the diaphragm, resulting in direct communication between the anode and cathode to form an internal short circuit. In short, the consequences of starting an electric vehicle at low temperature are very serious, so it is necessary to heat the power battery before starting it.