The Nickel-Metal Hydride (NiMH) battery
Research of the NiMH system started in the 1970s as a means of discovering how to store hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used for satellite applications. They are bulky, contain high-pressure steel canisters and cost thousands of dollars per cell.
In the early experimental days of the NiMH 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 1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily improved.
The success of the NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to NiCd. There is potential for yet higher capacities, but not without some negative side effects.
The NiMH is less durable than the NiCd. Cycling under heavy load and storage at high temperature reduces the service life. The NiMH suffers from high self-discharge, which is considerably greater than that of the NiCd.
The NiMH has been replacing the NiCd in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than NiCd batteries. This is due to environmental concerns about careless disposal of the spent battery.
Experts agree that the NiMH has greatly improved over the years, but limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with the NiCd battery. It is widely accepted that NiMH is an interim step to lithium battery technology.
|Source: The Battery University|