Shorts, how they occur.
A short develops in a Ni-Cd when conductive particulates bridge the separator
or the separator itself deteriorates to the point where it allows the positive
and negative plates to touch. Rarely does the short occur all at once but
rather building up a very small conductance path termed "soft shorts".
In a charged cell the energy in the cell will blow away any short as it
tries to develop. You've heard about "zapping" cells. The cell actually
zaps itself before the short can develop. Only in cases of severe overcharge
at high rates when the cell heats up significantly, can the separator melt
down to the point where the plates contact each other (hard short). In
this case the energy in the cell then dumps and we have what is referred
to as a hot steamer, the electrolyte boils, nylon in the separator melts
down and is forced by the steam through the vent. On some occasions the
vent is clogged by the molten nylon separator and becomes inoperative causing
the cell to rapidly disassemble. So under normal circumstances a cell maintained
at some state of charge is much less likely to short than a cell that is
completely discharged. It should be noted however that the self discharge
increases rapidly in cells where there is a short building (high resistance
-soft short) due to separator deterioration and/or cadmium migration. One
other shorting mechanism is a manufacturing defect where the positive or
negative collector tab bridges the opposite plate. These usually fall out
before the cells are shipped or assembled into batteries.
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cls 12/96