Hazards of Inconsistency in Lithium Batteries

The consistency of lithium batteries mainly refers to the consistency of capacity, internal resistance, and open circuit voltage. Inconsistent battery cell strings used together can cause the following problems:

1) Capacity loss refers to the formation of a battery pack consisting of individual cells, which conforms to the "barrel principle" in terms of capacity. The capacity of the worst cell determines the capacity of the entire battery pack.

To prevent overcharging and discharging of the battery, the logic of the battery management system is set as follows: during discharge, when the lowest individual voltage reaches the discharge cut-off voltage, the entire battery pack stops discharging; When charging, stop charging when the highest unit voltage touches the charging cutoff voltage.

Take two batteries connected in series as an 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 through the same amount of current.

When charging, batteries with small capacity must first be fully charged, reaching the charging cut-off condition, and the system will no longer continue to charge. When discharging, batteries with small capacity will inevitably discharge all available energy first, and the system will immediately stop discharging.

In this way, cells with small capacity are always fully charged and discharged, while cells with large capacity are always using a portion of their capacity. A portion of the entire battery pack's capacity is always idle.

2) Life loss, similarly, the lifespan of a battery pack is determined by the battery cell with the shortest lifespan. It is highly possible that the battery cell with the shortest lifespan is the one with a small capacity. Small capacity battery cells are always fully charged and discharged, with excessive output, which is likely to reach the key point of their lifespan first. When the lifespan of a battery cell ends, a group of battery cells welded together will also die with it.

3) As the internal resistance increases, different internal resistances flow through the same current, and cells with high internal resistance generate relatively more heat. If the battery temperature is too high, it will accelerate the degradation rate and further increase the internal resistance. Internal resistance and temperature rise form a pair of negative feedback, accelerating the degradation of high internal resistance cells.

The above three parameters are not completely independent. Cells with deep aging have higher internal resistance and more capacity decay. The inconsistency in battery cell performance is formed during the production process and deepened during use. The cells within the same battery pack remain weak and accelerate to weaken. The degree of dispersion of parameters between individual cells increases with the deepening of aging.