Lithium-ion power battery is an electrochemical battery based on Li+ concentration difference. The level of ambient temperature during operation directly affects the activity of positive and negative electrode materials and electrolyte, and has an important impact on its life. The electrical performance and service life of the same lithium-ion power battery at different operating temperatures are very different. Generally, the lithium-ion power battery can exert its maximum efficiency at a room temperature of about 25 °C. When the ambient temperature is too low, the activity of the electrolyte is affected, and the internal resistance increases significantly, resulting in difficult battery charging, reduced power, reduced usable capacity, and impaired battery life. When used at high temperature, the heat dissipation of the battery system will be affected. When the internal temperature of the battery exceeds the limit temperature, the internal chemical balance will be destroyed, resulting in corrosion and aging of battery materials, seriously aggravating the battery life decay process, and causing the battery to fail prematurely.
Figure 1 shows the relationship between the capacity retention rate and the number of cycles of a lithium-ion power battery cell at different temperatures. In the cyclic charge-discharge test, the discharge system is: 1C constant current discharge to voltage 2.5V; charging system: 1C constant current charge to 3.7V, transfer to constant voltage 3.7V to charge until the current drops to 1/30C, stop charging, and complete the charging process. After charging, let it stand for 1h, and then re-charge and discharge test. The remaining capacity of the monomer was measured every 20 charge-discharge cycles completed.
It can be seen from Figure 1 that when the high temperature is 40~60°C, the battery decays faster as the temperature increases, especially when the battery is at an ambient temperature of 60°C, the battery discharge capacity decays to 80.63 after 20 charge-discharge cycles. %. When the low temperature is -10~10℃, with the decrease of the ambient temperature, the battery attenuation speed is accelerated, and the battery capacity attenuation speed is significantly accelerated in the -10℃ environment. Moreover, it can be seen that the high temperature environment has a greater impact on the life attenuation than the low temperature environment, which is more detrimental to the life of the battery. The temperature of the battery increases rapidly in the high temperature environment. When the charge and discharge test is carried out in the environment of 40°C, the temperature of the battery increases by 20°C after 7 cycles of charge and discharge.
In order to protect the lithium-ion battery and improve the service life of the battery, the ambient temperature of the lithium-ion power battery should be controlled within the range of 0~40 °C, and it is forbidden to work in a high temperature environment above 50 °C. In order to further ensure that the battery capacity decay rate is within a certain range, the working temperature of the battery should preferably be controlled at 0~25℃.
When the lithium-ion power battery cells are connected in series and parallel to form a power battery pack, the temperature field of the power battery pack is not a simple superposition of the temperature field of the single cell, and the temperature distribution of the battery pack is not as uniform as that of the battery cells. The stability is also not as good as the monomer. The non-uniformity of the temperature distribution of the battery pack leads to inconsistent cell activity at different positions inside the battery pack, thereby aggravating the expansion of the inconsistency. Therefore, the power battery system needs to design a special thermal management system to ensure that the battery works in an appropriate temperature range and the uniformity of the battery temperature distribution.