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  • A nickel–metal hydride battery, abbreviated NiMH or Ni–MH, is a type of rechargeable battery. Its chemical reactions are somewhat similar to the nickel–cadmium cell. NiMH use positive electrodes of nickel oxyhydroxide, like the NiCd, but the negative electrodes use a hydrogen-absorbing alloy instead 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 W·h/kg, and for larger NiMH cells about 75 W·h/kg. This is significantly better than the typical 40–60 W·h/kg for NiCd, and similar to the 100–160 W·h/kg for lithium-ion batteries. NiMH has a volumetric energy density of about 300 W·h/L, significantly better than NiCd at 50–150 W·h/L, and about the same as lithium-ion at 250–360 W·h/L. NiMH batteries have replaced NiCd for many roles, notably small rechargeable batteries. NiMH batteries are very common for AA batteries, which have nominal charge capacities of 1.1–2.8 A·h at 1.2 V, measured at the rate that discharges the cell in five hours.