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nickel-metal hydride battery

A nickel-metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery that uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd batteries, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size NiCd. However, compared to the lithium-ion battery, the volumetric energy density is lower and self-discharge is higher.

Common AA batteries (penlight-size) have nominal charge capacities (C) ranging from 1100 mA·h to 2700 mA·h at 1.2 V, usually measured at a discharge rate of 0.2×C per hour. Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in one hour), it does not differ significantly from the nominal capacity.

The specific energy density for NiMH material is approximately 70 W·h/kg (250 kJ/kg), with a volumetric energy density of about 300 W·h/L (360 MJ/m³).

It is common to refer to most NiMH products as batteries, even though the word battery refers to the grouping of multiple cells. As a result sizes AA, AAA, C, and D are technically cells, whereas the 9V size is a real battery.

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