MISMATCHED CELLS


Mismatched cells are often found in brand-new battery packs as well as those that have aged. The reasons for unevenly matched cells are poor quality control at the cell manufacturing level and inadequate cell matching when assembling the batteries. If not too far off, the cells in a new pack tend to adapt to each other after a few charge/discharge cycles in a similar way players adjust in a winning sports team.

The weak cells hold less capacity and are discharged quicker that the strong ones. This imbalance causes cell reversal on the weak cells, especially if the battery pack is discharged below one volt per cell at full load. Likewise, the weak cells reach full charge first and go into heat-generating overcharge while the stronger cells still accept charge and remain cool. In both situations, the weak cells are at a disadvantage, making them weaker and contributing still further to the cell-mismatch condition. A analogy can be made with high school bullies that pound on the weaker kids, an activity that does not solve the issue but makes it worse.

Similar to other products on the market, the buyer has a choice between quality and price. High quality cells are more consistent and their capacities better matched than their lower quality counterparts. During the life span, high quality cells degrade at about the same rate, helping to maintain the matching condition. With lower quality cells, the aging process may not be as uniform, contributing to premature battery failure as a result of cell mismatch.

The capacities between the cells in a battery pack should be within +/- 2.5%. Tighter tolerances are required for high-current, high voltage batteries (15 volts and up) and low-temperature applications. There is a strong relationship between well-balanced cells and the longevity of a battery, especially at high-load currents and low-temperature applications.

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