Interpretation of key technologies of 3.7 volt 18650 lithium battery management system

Electric vehicle power batteries require high power density, high energy density, long life, and environmental protection. Lithium batteries have the above advantages and are therefore widely used in electric vehicles. Today, let’s talk about these lithium batteries and the systems that manage them.

Common battery types and their application requirements

Lithium batteries for vehicles mainly include the following types (some).

The battery discharge temperature is -20~55℃. Charging temperature is 0~45℃. If the negative electrode material is used, the charging temperature can reach -30°C. Usually the voltage range of lithium batteries is 1.5V~4.2V (C/NCA, C/NCM, C/LMO is 2.5V~4.2V, LTO/C/LMO is 1.5V~2.7V, C/LFP is 2.0V~3.7V).

Usually at a temperature of 90~120°C, the SEI film begins to undergo exothermic decomposition (Figure 1). Some electrolytes will even decompose at lower temperatures; when the temperature exceeds 120°C, the SEI film cannot protect the carbon negative electrode from reacting with the organic electrolyte to produce gas; when the temperature exceeds 130°C, the separator begins to melt and cut off the battery reaction. When the temperature gets higher, the cathode material begins to decompose. When the temperature exceeds 200°C, the electrolyte begins to decompose and produce flammable gas. The decomposed combustible gas will react violently with oxygen and cause thermal runaway. Charging temperatures below 0°C will cause the deposition of metallic lithium on the surface of the carbon negative electrode, thus reducing the cycle life of the battery. Under extreme conditions of low temperature, the negative electrode of the battery may be punctured, resulting in a short circuit. If the voltage is too low or the battery is over-discharged, the phase change will cause the battery lattice to collapse and affect the battery's performance. It may even cause the negative electrode current collector to dissolve in the electrolyte. Extreme overdischarge can also lead to the reduction of electrolytes and the generation of flammable gases, thus posing potential safety risks. High voltage and overcharging can destroy the positive electrode composition and cause a lot of heat generation. It will also cause metal lithium to deposit on the surface of the negative electrode, accelerate capacity fading, cause an internal short circuit in the battery, and cause safety issues. The electrolyte begins to decompose when the battery voltage is around 4.5V.

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