How Lithium Batteries Deal with Inconsistencies

　　Inconsistencies in the performance of the electric core are formed in the production process and deepened in the use process. The electric cells in the same battery pack are always weak when they are weak, and become weak when they accelerate. The dispersion degree of parameters between individual electric cores increases with the deepening of aging degree.

　　At present, engineers should consider three aspects when dealing with the inconsistency of single electric core. Single battery sorting, thermal management after grouping, and the battery management system provides equalization function in case of a small amount of inconsistency.

　　1) Sorting

　　In theory, different batches of cells should not be used together. Even if the cells of the same batch are selected, the cells with relatively concentrated parameters should be placed in one battery pack and the same battery pack.

　　The purpose of sorting is to select the electric cores with similar parameters. Separation methods, which have been studied for many years, are mainly divided into static separation and dynamic separation.

　　Static sorting: screen the characteristic parameters such as open circuit voltage, internal resistance and capacity of the cell, select target parameters, introduce statistical algorithms, set screening criteria, and finally divide the cell of the same batch into several groups.

　　Dynamic screening is to screen the characteristics of the cell during the charging and discharging process. Some choose the constant current and constant voltage charging process, some choose the pulse impact charging and discharging process, and some compare the relationship between their own charging and discharging curves.

　　Dynamic and static sorting is combined. Static screening is used for preliminary grouping, and dynamic screening is carried out on this basis. In this way, more groups are divided, and the screening accuracy is higher, but the cost will rise accordingly.

　　This is a small demonstration of the importance of a power lithium battery production scale. Large scale shipment enables manufacturers to carry out more refined sorting and obtain battery packs with similar performance. If the output is too small and there are too many groups, a batch cannot be equipped with a battery pack, and no good method can be used.

　　2) Thermal management

　　Aiming at the problem that the internal resistance of the electric core is inconsistent and the heat generated is different. The thermal management system can adjust the temperature difference of the whole battery pack to keep it in a small range. The electric core that generates more heat still has a higher temperature rise, but it will not open a gap with other electric cores, so there will be no obvious difference in the deterioration level.

　　3) Equilibrium

　　Inconsistency of individual electric cores. The terminal voltage of some electric cores is always ahead of that of other electric cores and reaches the control threshold first, leading to the reduction of the entire system capacity. In order to solve this problem, the battery management system BMS has designed the equalization function.

　　A certain battery cell is the first to reach the charging cut-off voltage, while the voltage of the other battery cells lags behind significantly. The BMS starts the charging equalization function, or connects the resistor to discharge part of the power of the high voltage battery cell, or transfers the energy to the low voltage battery cell. In this way, the cut-off condition for charging is removed, the charging process starts again, and the battery pack is charged with more power.

　　Thermistors. Some electric cores are equipped with thermistors. Once over-current occurs, the resistance increases sharply after reaching a certain temperature, and the current in the circuit decreases, preventing the temperature from rising further.

　　Fuse and electric core are equipped with fuse with over-current fusing function. Once over-current risk occurs, the circuit will be disconnected to avoid malignant accidents.

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