Inconsistency analysis of single battery

　　1 Definition of Inconsistency: Inconsistency of lithium-ion battery pack refers to the difference of voltage, capacity, internal resistance, life, temperature effect, self discharge rate and other parameters after the battery pack is composed of single batteries of the same specification and model.

　　There are some differences in the initial performance of single cells after they are manufactured. With the use of batteries, these performance differences continue to accumulate. At the same time, the use environment of each single battery in the battery pack is not completely the same, which also leads to the gradual amplification of the inconsistency of the single battery, thus accelerating the decline of battery performance, and ultimately causing premature failure of the battery pack.

　　2. Inconsistency performance The inconsistency of lithium ion battery is mainly manifested in two aspects: the difference of cell performance parameters (battery capacity, internal resistance, self discharge rate, etc.) and the difference of battery state of charge (SOC).

　　Dai Haifeng and other researchers found that the distribution of capacity difference between cells is close to the Weir distribution, and the dispersion of internal resistance is more significant than the capacity, and the internal resistance of the same batch of batteries generally meets the normal distribution law, and the self discharge rate also presents a similar normal distribution. SOC represents the state of charge of the battery, which is the ratio of the remaining capacity of the battery to the rated capacity. Decompetition and others believe that the capacity decay rate of the battery is different due to the inconsistency of the battery, leading to the difference in the maximum usable capacity between batteries. The SOC change rate of the battery with small capacity is faster than that of the battery with large capacity, and the cut-off voltage is reached faster during charging and discharging.

　　3 Cause of inconsistency There are many reasons for the inconsistency of lithium ion batteries, mainly in the manufacturing process and use process. Each link in the manufacturing process, such as the uniformity of the slurry during batching, the control of the surface density and surface tension during coating, will cause the difference in the performance of the single cell.

　　Luo Yu et al. studied the influence of lithium ion battery production process on battery consistency, focusing on the influence of lithium ion battery production process with water-based binder system on battery consistency. During the use of the battery, Xie Jiao and others believed that the connection mode, structural parts/devices, operating conditions and environment would affect the consistency of the battery pack. Because the energy consumed at each connection point is inconsistent, the performance and aging rate of each component or structure are also inconsistent, so the impact on the battery is also inconsistent. In addition, due to the different positions, temperatures and performance attenuation of each cell in the battery, the inconsistency of the cell will be amplified.

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