What's mean Battery Cell Mismatch ? Cell mismatch can be found in brand-new as well as aged battery packs. Poor quality control at the cell manufacturing level and inadequate cell matching when assembling the batteries cause unevenly matched cells. If only slightly off, the cells in a new pack adapt to each other after a few charge/discharge cycles, like players in a winning sports team.
A weak cell holds less capacity and is discharged more quickly than the strong one. This imbalance causes cell reversal on the weak cell if the battery is discharged below 1V/cell. The weak cell reaches full charge first and goes into heat-generating overcharge while the stronger cell still accepts charge and remains cool. In both situations, the weak cell is at a disadvantage, making it weaker and contributing to a more acute cell mismatch condition. An analogy can be made with a high school bully who picks on the weaker kid.
High quality cells are more consistent in capacity than lower quality counterparts. During their life span, high quality cells degrade at about the same rate, helping to maintain the matching. Manufacturers of power tools choose high quality cells because of their durability under heavy load conditions and temperature extremes. Lower-cost cells have been tried, but early failure and consequent replacement is costlier than the initial investment.
The capacity matching between the cells in a battery pack should be within +/- 2.5 percent. Tighter tolerances are required on batteries with high cell counts that also must generate high load currents and are operating under adverse temperatures. There is a strong correlation between well-balanced cells and the longevity of a battery.
