Nickel-MH cells are becoming common for most portable equipment such as camcorders, laptop computers and other domestic appliances. They do not retain their charge as well as Ni-Cd and cannot be operated at as low temperatures. However, the recognized environmental problem associated with the Ni-Cd batteries has shifted the the market in favor of NiMH. Compared with lead-acid batteries, the Ah efficiency is poor, typically requiring 140% to 160% of the discharged Ah to restore full capacity, compared with 120% for lead-acid. There is continuing improvement in this system, although it may not be able to compete with lithium ion in the future.
Research of the NiMH system started in the seventies as a means for hydrogen storage for a nickel Hydrogen battery. The metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980's that were stable enough for use in a cell. Since the late eighties, the NiMH has steadily improved, mainly in terms of energy density. engineers have indicated that the NiMH has a potential of yet higher energy densities.
In the case of overcharging, the nickel hydr cathode becomes fully charged and begins generating oxygen, which recombines with hydrogen at the anode to form water and heat. At low charge rates, the NiMH battery can keep up with the oxygen generation and recombination cycle. However, at high charge rates, oxygen can be produced faster than the anode can recombine it, resulting in internal cell pressure buildup. Once the pressure reaches a certain value, the valves will open and vent the oxygen to the outside of the battery. The advanced charging systems developed for electric vehicle applications monitor the battery voltage to prevent the generation of gases during battery charging.