A nickel metal hydride battery, abbreviated NiMH or Ni–MH, is a type of rechargeable battery. The chemical reaction at the positive electrode is similar to that of the nickel–cadmium cell (NiCd), with both using nickel oxide hydroxide (NiOOH). However, the negative electrodes use a hydrogen-absorbing alloy instead of cadmium. A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and its energy density can approach that of a lithium-ion battery.
NiMH cells are often used in digital cameras and other high-drain devices, where over the duration of single-charge use they outperform primary (such as alkaline) batteries.
NiMH cells are advantageous for high-current-drain applications, largely due to their lower internal resistance. Typical alkaline AA-size batteries, which offer approximately 2600 mAh capacity at low current demand (25 mA), provide only 1300 mAh capacity with a 500 mA load. Digital cameras with LCDs and flashlights can draw over 1000 mA, quickly depleting them. NiMH cells can deliver these current levels without similar loss of capacity.
Devices that were designed to operate using primary alkaline chemistry (or zinc–carbon/chloride) cells may not function with NiMH cells. However, most devices compensate for the voltage drop of an alkaline battery as it discharges down to about 1 volt. Low internal resistance allows NiMH cells to deliver a nearly constant voltage until they are almost completely discharged. Thus battery-level indicators designed to read alkaline cells overstate the remaining charge when used with NiMH cells, as the voltage of alkaline cells decreases steadily during most of the discharge cycle.
Lithium-ion batteries have a higher specific energy than nickel metal hydride batteries, but they are significantly more expensive. They also produce a higher voltage (3.2-3.7V nominal), and are thus not a drop-in replacement for alkaline batteries without circuitry to reduce voltage.
As of 2005, nickel metal hydride batteries constituted three percent of the battery market.
NiMH batteries have replaced NiCd for many roles, notably small rechargeable batteries. NiMH batteries are commonly available in AA (penlight-size) batteries. These have nominal charge capacities (C) of 1.1–2.8 Ah at 1.2 V, measured at the rate that discharges the cell in 5 hours. Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in 1 hour), it does not differ significantly from the nominal capacity. NiMH batteries nominally operate at 1.2 V per cell, somewhat lower than conventional 1.5 V cells, but can operate many devices designed for that voltage.