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.
Currently, engineers mainly consider three aspects when dealing with inconsistencies in individual battery cells. Single battery sorting, thermal management after grouping, and balancing function provided by the battery management system in case of a small amount of inconsistency.
Different batches of battery cells should not be used together in theory. Even cells from the same batch need to be screened and 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 cells with similar parameters. Sorting methods have been studied for many years, mainly divided into two categories: static sorting and dynamic sorting.
Static sorting involves selecting characteristic parameters such as open circuit voltage, internal resistance, and capacity of the cells, selecting target parameters, introducing statistical algorithms, setting screening criteria, and finally dividing the cells of the same batch into several groups.
Dynamic screening refers to the screening of the characteristics exhibited by the battery cells 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.
Combining dynamic and static sorting, static screening is used for preliminary grouping, and dynamic screening is carried out on this basis. This results in more groups being divided and higher screening accuracy, but the cost will also increase accordingly.
This reflects the importance of the scale of lithium battery production. Large scale shipment allows manufacturers to conduct more refined sorting and obtain battery packs with closer performance. If the production is too small and there are too many groups, a batch cannot be equipped with a battery pack, and even the best method cannot be applied.
2) Thermal management
In response to the issue of inconsistent internal resistance and heat generation in battery cells. The addition of a thermal management system can adjust the temperature difference of the entire battery pack to maintain it within a small range. Cells that generate more heat still have a higher temperature rise, but they will not widen the gap with other cells, and there will be no significant difference in degradation level.
Inconsistency in the individual cells results in some cell terminal voltages always exceeding those of other cells and reaching the control threshold first, resulting in a decrease in the overall system capacity. To address this issue, the battery management system BMS has designed a balancing function.
A certain battery cell reaches the charging cut-off voltage first, while the voltage of the other battery cells lags significantly. The BMS starts the charging equalization function, or connects a resistor to release some of the electricity from the high voltage battery cell, or transfers the energy to the low voltage battery cell. In this way, the charging cut-off condition is lifted, the charging process resumes, and the battery pack is charged with more electricity.
Until now, the inconsistency of battery cells remains an important area of research within the industry. No matter how high the energy density of the battery cell is, encountering inconsistencies to disrupt the situation will greatly reduce the capacity of the battery pack.
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