Nickel-metal hydride batteries: from concept to characteristics

Since the development of Nickel-Metal Hydride (NiMH) batteries in the late eighties of the previous century, these batteries have become widely accepted in our present-day portable society. In the very early days of the current century this battery type has become even more popular in portable electronics than any other battery system. This is, among other things, due to their high storage capacity, good rate capability and environmental friendliness. During battery operation hydrogen shuttles from one electrode to the other. /4s such, this battery system can be considered as the first commercial success towards the so-called hydrogen era, which is generally accepted about to begin. This justifies reviewing this nowadays mature battery system. The basic electrochemical principles underlying the energy storage reactions and the various side reactions, occurring during (overcharging, (over)discharging and under open-circuit conditions will be discussed. In the second part of the paper the most important battery characteristics, such as charge/discharge capabilities, self-discharge behaviour and battery life are reviewed.