The
Nickel Metal hydride(NiMH)
accumulator is a
type of
rechargeable battery. The NiMH was developed
in the mid 1970's, but wasn't commonly available before the
early 90's. At that time a
concern for the
negative environmental
impact of the
cadmium used in the NiCd accumulator started
growing. This and the need for more compact power sources
spurned the production of the NiMH cell. A HiMH cell
consists of a
positive electrode made from a fused
nickel granulate, while the negative is made from an
alloy of
manganese,
nickel,
magnesium,
cobalt,
aluminium an some
rare earth metals. Between the electrodes
an alkaline
electrolyte and a
polymer separator is
added(The manufacturers is of course
reluctant to reveal the
exact alloys and electrolytes used)
The
advantages of the NiMH over the NiCd, in addition
to being more environmentally friendly, is that it has a
higher
energy density and a
negligible memory effect.
The higher energy density means that a NiMH battery of the
same size as a NiCd, will have a longer usage time. The
low memory effect means you don't have to discharge the
accumulator completely before recharging. But the NiMH cell
needs a more accurate charging monitor system than the NiCd accumulator.