The life of the battery pack has increased by more than five times. Why is the balance so cow?

68 Published by admin Dec 04,2018

lithium-ion batteries factory

In order to meet the energy and voltage requirements of lithium-ion batteries, hundreds or even thousands of single cells need to be combined into a battery pack by series and parallel. In theory, these single cells should have identical characteristics. However, in fact, due to fluctuations in process parameters related to manufacturing and production processes, even the same batch of lithium-ion batteries still have some differences in performance (such as capacity, internal resistance and decay rate, etc.), although before the grouping We will filter and match, and we still can't guarantee 100% consistency. Therefore, after the group is set, these differences will accumulate with the increase of the number of cycles during the use, resulting in the performance difference between the cells expanding. At the same time, due to the large number of batteries in the battery pack, there must be a certain temperature gradient inside the battery pack during use. The existence of the temperature gradient will also lead to inconsistency in the internal resistance of the battery and inconsistency in the current distribution, resulting in the decline of the single cell. Inconsistent speed, these factors will cause the battery pack's cycle performance to be much lower than the single battery The cycle life, such as the bus running on the Beijing bus demonstration line, in the absence of equalizer protection, although the battery life can reach more than 1000 times, but after only 150 cycles after the battery pack is formed, serious The capacity is attenuated. It is found that the capacity of some single cells is less than 80% of the rated capacity. This is mainly because the small difference in the coulombic efficiency, decay rate and internal resistance of the single cell continues to accumulate in the cycle. Eventually, some of the cells failed to decay too quickly.

Inconsistencies between single-cell lithium-ion batteries mainly include temperature, voltage, SoC, capacity, and internal resistance. If we take time factor into account, the inconsistency of lithium-ion batteries should also include self-discharge, coulombic efficiency, The capacity decline rate, etc., Long Zhou et al. of Shanghai University of Technology divided these inconsistencies into three categories: the first category is the initial factors, such as battery capacity, internal resistance, etc., they determine the basic capabilities of lithium-ion batteries; 2) The second is the current factors, such as capacity, voltage, SoC, etc. These indicators determine the current capabilities of lithium-ion batteries; 3) the third is time accumulation factors, such as capacity decay rate, internal resistance increase rate and charge and discharge coulombic efficiency, These factors determine the future capabilities of lithium-ion batteries. Once the lithium-ion batteries are grouped, the "initial state" and "current state" of the battery pack have been determined. What we need to solve is the effect of the "time accumulation factor" on the performance of the battery pack.

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