Nickel-metal hydride battery

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A nickel-metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel-cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size nickel-cadmium battery. Compared to the lithium-ion cell, the volumetric energy density is similar but self-discharge is higher. (Compare: low self-discharge NiMH battery)

Common AA cells (penlight-size) NiMH batteries have nominal charge capacities (C) ranging from 1100 mA·h to 3100 mA·h at 1.2 V, measured at rate that discharges the cell in five hours. 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 for NiMH material is approximately 70 W·h/kg (250 kJ/kg), compared to 40–60 W·h/kg for the more common Ni-Cd, or 100-160 W·h/kg for Li-ion. NiMH has a volumetric energy density of about 300 W·h/L (1080 MJ/m³), significantly better than nickel-cadmium at 50–150 Wh/L, and about the same as Li-ion at 250-360 W·h/L.

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