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Nickel Cadmium and Nickel Metal Hydride cells are generally
accepted as being gas tight. It is for this reason that the cells can be
charged in any relative position as no electrolyte will leak from the cells.
However, no manufacturer that we know of will guarantee
this feature for the entire expected life of the cells! An over-pressure
may be produced during the charge or discharge cycles, which may open the
integral pressure relief valve.
This means that electrolyte or hydrogen gas may escape from the cells. The
electrolyte is an aggressive alkaline and it conducts electricity, this
may result in damage of the cells due to electrolytic corrosion.
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The leaking hydrogen gas will combine
with oxygen in the atmosphere and produce a highly explosive mixture. The
dangers resulting from such a reaction are obvious to all. A cell the size
of a standard D battery can produce up to 25 litres (approx. 1 cu. ft.)
of gas. The consequences of this in a small enclosed housing are easy to
imagine.
For this reason, it is important that the cell packs are removed from the
housing during the charging cycle.
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