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.