Research on the Nickel-metal-hydride system started in the 1970s as a means of storing hydrogen for the nickel hydrogen battery. The success of Nickel-metal-hydride has been driven by its high energy density and the use of environmentally friendly metals. The modern Nickel-metal-hydride offers up to 40 percent higher energy density compared to Nickel-cadmium. The industrial production of Ni-Mh batteries was begun in 1990.

ADVANTAGES:

30 percent higher capacity over standard Nickel-cadmium. Nickel-metal-hydride has potential for yet higher energy densities. Less prone to memory than Nickel-cadmium - fewer exercise cycles are required. Environmentally friendly - contains only mild toxins; profitable for recycling. Sealing possibility

LIMITATIONS:

Limited service life - if repeatedly deep cycled, the performance starts to deteriorate after 200 to 300 cycles. Limited discharge current - although Nickel-metal-hydride is capable of delivering high discharge currents, heavy load reduces the battery's cycle life. Best results are achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of the rated capacity). More complex charge algorithm needed - Nickel-metal-hydride generates more heat during charge and requires a longer charge time than the Nickel-cadmium. Trickle charge settings are critical because the battery cannot absorb overcharge. High self-discharge - typically 50 percent higher than Nickel-cadmium. New chemical additives have improved self-discharge but at the expense of lower energy density. Performance degrades if stored at elevated temperatures - Nickel-metal-hydride should be stored in a cool place and at a state-of-charge of about 40 percent. High maintenance - battery requires regular full discharge to prevent crystalline formation. Nickel-cadmium should be exercised once a month, Nickel-metal-hydride once in every 3 months.

In compliance with basic characteristics (capacity, weight and value) Ni-Mh accumulators are between Ni-Cd and Li-Ion accumulators.

Main application - small size accumulators for portable and domestic equipment, are used for industry (stationary accumulators and so on.) more seldom.