what's battery Cell Mismatch(1)?

what's battery Cell Mismatch(1)?

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.

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