Consistency of lithium battery

　　The lithium battery can not be made into a large one, so we have to organize many small cells, and everyone can work together and cooperate sincerely, so that we can also fly with the electric vehicle. At this time, we need to face the problem of consistency.

　　Our daily experience is that when two dry cells are connected, the positive and negative poles, the flashlight can emit light. Who cares if it is consistent or inconsistent. However, the large-scale application of lithium batteries is not so simple.

　　The inconsistency of lithium battery parameters mainly refers to the inconsistency of capacity, internal resistance and open circuit voltage. The following problems may occur when inconsistent electric core strings are used together.

　　1) Capacity loss: the battery pack is composed of single cells, and the capacity conforms to the "cask principle". The capacity of the worst cell determines the capacity of the entire battery pack.

　　To prevent overcharge and overdischarge of the battery, the logic of the battery management system is set as follows: when discharging, when the lowest single voltage reaches the discharge cut-off voltage, the entire battery pack stops discharging; When charging, stop charging when the highest monomer voltage touches the charging cut-off voltage.

　　Take two batteries in series for example. One battery has a capacity of 1C, while the other has a capacity of only 0.9C. In a series connection, two batteries pass the same amount of current.

　　When charging, the battery with small capacity must be fully charged first, and the system will not continue to charge when the charging cut-off condition is reached. When discharging, the battery with small capacity must first discharge all available energy, and the system will stop discharging immediately.

　　In this way, the small capacity electric core is always full, while the large capacity electric core always uses part of its capacity. A part of the capacity of the entire battery pack is idle

　　2) Life loss. Similarly, the life of the battery pack is determined by the cell with the shortest life. It is very likely that the electric core with the shortest life is the one with small capacity. The small capacity electric cell is fully charged and discharged every time. If the output is too strong, it is likely to reach the key point of life first. As the life of the electric core ends, a group of electric cores welded together will die.

　　3) With the increase of internal resistance, different internal resistance will flow the same current, and the electric core with large internal resistance will generate more heat. If the battery temperature is too high, the degradation speed will be accelerated, and the internal resistance will further increase. The internal resistance and temperature rise form a pair of negative feedback, which accelerates the deterioration of the high internal resistance core.

　　The above three parameters are not completely independent. The inner resistance of the deeply aged core is relatively large, and the capacity attenuation is more. Explain them separately, just to clarify their respective influence directions.

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