Nickel–metal hydride battery, abbreviated NiMH or Ni-MH, is a type of rechargeable battery. It is very similar to the nickel–cadmium cell(NiCd). NiMH use positive electrodes of nickel oxyhydroxide (NiOOH), like the NiCd, but the negative electrodes use a hydrogen-absorbing alloyinstead of cadmium, being in essence a practical application of nickel–hydrogen battery chemistry. A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and their energy density approaches that of a lithium-ion cell.
The typical specific energy for small NiMH cells is about 100 Wh/kg, and for larger NiMH cells about 75 Wh/kg (270 kJ/kg). This is significantly better than the typical 40–60 Wh/kg for NiCd, and similar to the 100–160 Wh/kg for lithium-ion batteries. NiMH has a volumetric energy density of about 300 Wh/L (1080 MJ/m3), significantly better than NiCd at 50–150 Wh/L, and about the same as lithium-ion at 250-360 Wh/L.
NiMH batteries have replaced NiCd for many roles, notably small rechargeable batteries. NiMH batteries are very common for AA (penlight-size) batteries, which have nominal charge capacities (C) ranging from 1100 mAh to 2800 mAh at 1.2 V, measured at the 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. NiMH batteries normally operate at 1.2 V per cell, somewhat lower than conventional 1.5 V cells, but will operate most devices designed for that voltage.
About 22% of portable rechargeable batteries sold in Japan in 2010 were NiMH. In Switzerland in 2009, the equivalent statistic was approximately 60%. This percentage has fallen over time due to the increase in manufacture of lithium-ion batteries: in 2000, almost half of all portable rechargeable batteries sold in Japan were NiMH. By 2011, NiMH only represented about 22% of secondary batteries.
The significant disadvantage of NiMH batteries is the high rate of self-discharge; NiMH batteries lose up to 20% of their charge on the first day and up to 4% per day of storage after that. In 2005, a low self-discharge (LSD) variant was developed. LSD NiMH batteries significantly lower self-discharge, but at the cost of lowering capacity by about 20%.