14.2 Low Capacity Cells
Even with modern manufacturing techniques, the capacity of a cell cannot be accurately predicted. As part of the manufacturing process, each cell is measured and segregated into categories according to their inherent capacity levels. The high capacity A cells are commonly sold for special applications at premium prices; the large mid-range B cells are used for commercial and industrial applications such as mobile communications; and the low-end C cells are mostly sold in supermarkets at bargain prices. Cycling will not significantly improve the capacity of the low-end cell. When purchasing rechargeable batteries at a reduced price, the buyer should be aware of the different capacity and quality levels offered.
As part of quality control, the battery assembler should spot-check each batch of cells to examine cell uniformity in terms of voltage, capacity and internal resistance. Failing to observe these simple rules will often result in premature battery failures. When buying quality cells from a well-known manufacturer, battery assemblers are able to relax the matching requirements somewhat.
14.3 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.
Lithium-based cells have tighter matching tolerances than their nickel-based cousins. Tight matching of all cells in a pack is especially important on lithium-based chemistries. All cells must reach the end-of-discharge voltage threshold at the same time. The full-charge point must be attained in unison by all cells. If the cells are allowed to get out of match, the weaker cell will be discharged to a lower voltage point before the cut-off occurs. On charge, this weak cell will attain the full-charge status before the others, causing the voltage to go higher than on the stronger cells. This larger voltage swing will put undue strain on the weak cell.
Each cell in a lithium-based pack is electronically monitored to assure proper cell matching during the battery’s life. An electronic circuit is added to some packs that compensate the differences in cell voltages. This is done by connecting a shunt across each cell string to consume the excess energy of the cells which are more energetic. The low-voltage cut-off occurs when the weakest cell reaches the end-of-discharge point.
The Li-ion battery is controlled down to the cell level to assure safety at all times. Because this chemistry is still relatively new and unpredictable under extreme conditions, manufacturers do not want to take undue risks. There have been a few failures but such irregularities are often kept a secret. This chemistry is considered very safe, considering the large number of Li-ion batteries that are in use.