The same batteries will have completely different lifespans under different operating conditions. The main factors affecting battery life are: high temperature (accelerating internal side reactions); low temperature (easy to reduce metal ions, easy to deposit lithium, easy to destroy the crystal structure of active materials); high SOC or overcharge (electrolyte decomposition, electrolyte and positive electrode) side reactions, lithium ion deposition); low SOC, low or overdischarge (the anode copper current collector is easy to corrode, and the crystal structure of the active material is easy to collapse); high charge-discharge rate (the crystal structure of the active material is easy to fatigue damage, high rate causes The temperature rises, which accelerates internal side reactions). Generally speaking, batteries have a reasonable operating window. The main purpose of BMS and TMS is to make the battery work in a long-life and high-performance working area, and to prevent the battery from working in a dangerous area, an alarm and measures should be taken in time.

(1) Influence of temperature

Temperature is one of the most important factors affecting battery life. Both high and low temperatures will accelerate battery degradation. Generally speaking, for most commercial lithium-ion batteries, the suitable operating temperature range is 15~35℃. The main reactions and side reactions of various reaction rates inside the battery are related to temperature. The higher the temperature, the faster the side reaction rate. Additionally, if the battery exceeds a certain temperature, self-heating may be further triggered, resulting in a thermal runaway of the battery. At low temperature, polarization increases due to the increase in internal resistance, which may cause additional side reactions. In particular, low-temperature charging may lead to lithium deposition, causing rapid battery decay and even safety issues. Embrittlement of materials at low temperatures can also affect battery life. Therefore, ensuring that the battery works within a suitable temperature range is the key to improving battery life.
The battery temperature is determined by many factors, including ambient temperature, battery heat capacity, battery thermal conductivity, battery heat generation, TMS heating and cooling system, etc.

Ambient temperature has a big impact on battery life. For the power battery in most electric vehicles, the most common state is actually the storage state, which corresponds to the parked state of the vehicle. At this point, all the vehicle's electrical systems are turned off, and the temperature of the battery is basically determined by the ambient temperature. The key factors that affect battery schedule life are temperature and SOC. Storage state in areas with high ambient temperature, the capacity loss is large. In addition, the cycle life of a battery is also related to temperature. Ambient temperature is determined by a combination of factors such as climate, weather and season, which may be related to the geographic location of the vehicle. It is generally accepted that the lower the latitude, the higher the temperature. The data shows that the battery capacity loss rate of American Leaf cars in low-latitude regions is significantly higher than that in high-latitude regions. In high latitudes, since winter temperatures can be below 0°C, it is necessary to employ heating systems to prevent lithium deposition caused by low-temperature charging, which may lead to safety and durability issues for lithium-ion batteries.

During the charging and discharging process of the battery, a large amount of ohmic heat will be generated. The battery temperature change caused by this part depends on the thermal characteristics of the battery (heat capacity, thermal conductivity, etc.), resistance (internal resistance of the battery and resistance of wires, bus bars, solder joints) and the intensity of the current flowing through the battery. Through rational battery and system design, the thermal characteristics and resistance of the battery can be improved. However, the current can be affected by many factors, especially the design of the vehicle. In BEVs, the battery discharge rate is usually low, and the battery temperature rises slowly; while in hybrid electric vehicles, the battery charge and discharge rates are respectively higher, and the battery temperature rises faster. The road conditions of the vehicle and the driving habits of the driver directly determine the working state of the battery; under harsh working conditions, the current will be more extreme, causing the battery temperature to rise significantly. And a reliable BMS can reasonably estimate the battery SOP to consider safety and life issues, limiting the current through the battery. Also, the charging system can have a big impact on battery temperature. For example, the charging rate of 350 kW ultra-high-speed charging in the future will be much higher than the discharge rate while driving. During the charging process, the temperature of the battery will increase seriously, which will affect the battery life.

In addition, the design of TMS (including low-temperature heating function, high-temperature cooling function and thermal insulation measures) can ensure that the battery works within an appropriate temperature range. Depending on the cooling medium, cooling systems are generally classified into air cooling (including natural convection and forced convection, typically used for BEVs with lower battery temperature rise), liquid cooling (usually used for HEVs due to higher thermal conductivity), and phase change cool down. The heating system can be divided into internal heating and external heating. External heating methods include heating plate, heating film, Peltier heating, etc. The external heating method is easy to realize, but the energy loss is large and the temperature uniformity of the battery is poor. The indirect heating method is to heat the battery by heating the medium, which can make the battery heat evenly. Internal heating methods include built-in nickel heating method, AC heating method, trapezoidal internal heating method, etc. These methods can uniformly heat the battery with low heat loss and high efficiency. Electric vehicles use reliable TMS, which can effectively maintain battery temperature and prolong battery life. For secondary batteries working in energy storage stations, the temperature is usually well controlled due to the use of high-performance air conditioners.