How to Revive Dead Ni-Cad Batteries Using a High Voltage Power Source
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Explanation (via http://ysuusy.com/easybatteryfix.html):
"Over time the crystals within a NiCd cell begin to combine and get larger. This causes them to loose capacitance and also break through the inner seals of the cell. Surging the NiCd cell using a higher voltage causes the crystals within the NiCd cell to shatter and become smaller. These smaller crystals have a greater surface area and therefore contain a greater capacitance for energy."