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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. An 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.