The chemical reaction at the positive electrode is similar to that of the nickel–cadmium cell (NiCd), with both using nickel oxide hydroxide (NiOOH).

[6] They are typically used as a substitute for similarly shaped non-rechargeable alkaline batteries, as they feature a slightly lower but generally compatible cell voltage and are less prone to leaking.

[7][8] Work on NiMH batteries began at the Battelle-Geneva Research Center following the technology's invention in 1967.

Patent applications were filed in European countries (priority: Switzerland), the United States, and Japan.

Research carried out by Philips Laboratories and France's CNRS developed new high-energy hybrid alloys incorporating rare-earth metals for the negative electrode.

In 1987, Willems and Buschow demonstrated a successful battery based on this approach (using a mixture of La0.8Nd0.2Ni2.5Co2.4Si0.1), which kept 84% of its charge capacity after 4000 charge-discharge cycles.

The most common is AB5, where A is a rare-earth mixture of lanthanum, cerium, neodymium, praseodymium, and B is nickel, cobalt, manganese, or aluminium.

Some cells use higher-capacity negative electrode materials based on AB2 compounds, where A is titanium or vanadium, and B is zirconium or nickel, modified with chromium, cobalt, iron, or manganese.

A similar approach is suggested by Energizer,[20] which indicates that self-catalysis can recombine gas formed at the electrodes for charge rates up to C/10.

This is the approach taken in emergency lighting applications, where the design remains essentially the same as in older NiCd units, except for an increase in the trickle-charging resistor value.

[22] To prevent cell damage, fast chargers must terminate their charge cycle before overcharging occurs.

However, the voltage drop is much less pronounced for NiMH and can be non-existent at low charge rates, which can make the approach unreliable.

[23] Another option is to monitor the change of voltage with respect to time and stop when this becomes zero, but this risks premature cutoffs.

[citation needed] A resettable fuse in series with the cell, particularly of the bimetallic strip type, increases safety.

[23] Modern NiMH cells contain catalysts to handle gases produced by over-charging: However, this only works with overcharging currents of up to 0.1 C (that is, nominal capacity divided by ten hours).

One inherent risk with NiMH chemistry is that overcharging causes hydrogen gas to form, potentially rupturing the cell.

[20] Voltage depression (often mistakenly attributed to the memory effect) from repeated partial discharge can occur, but is reversible with a few full discharge/charge cycles.

Under a light load (0.5 amperes), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts.

Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and perform an auto-shutdown, but devices such as flashlights and some toys do not.

Irreversible damage from polarity reversal is a particular danger, even when a low voltage-threshold cutout is employed, when the cells vary in temperature.

Separators keep the two electrodes apart to slow electrical discharge while allowing the transport of ionic charge carriers that close the circuit during the passage of current.

[38] 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 compared to alkaline batteries, largely due to their lower internal resistance.

Low internal resistance allows NiMH cells to deliver a nearly constant voltage until they are almost completely discharged.

By the late 1990s, NiMH batteries were being used successfully in many fully electric vehicles, such as the General Motors EV1 and Dodge Caravan EPIC minivan.

General Motors shut down production of the EV1, citing lack of battery availability as a chief obstacle.