This is a rechargeable battery very similar to the nickel-cadmium (NiCd) battery. The difference is that the Nickel-metal hydride battery makes use of a different alloy as its negative electrode as opposed to using cadmium. The positive is similar to the nickel-cadmium which used the nickel oxyhydroxide. The aptitude of these battery types is usually 2 or 3 times that of the nickel cadmium cell of the same capacity. The NiMH is used as an electric vehicle battery on a number of plug-in cars. Some of the cars that employ the use of nickel-metal hydride batteries are GM EV1, ford ranger and the Honda Plus among others. Hybrid cars that use NiMH are Honda Civic, Toyota Prius and the Honda Insight among others. Consumer electronics that use rechargeable batteries also employ NiMH technology. Some of these include Alstom Citadis tram and the prototype humanoid robot ASIMO that has been designed by the Honda Company.
When charging NiMH, ensure that maximum efficiency is achieved by having the charging current sufficient. The charging efficiency is typically 66%. This means that for every 150 amp hours that is put into the battery, then the output will be 100 amp hours. As the charging speed increases, the number of output hours decreases further and further. To avoid over-charging the battery which can cause damage, use a temperature meter to detect when the battery is over-charged. When the battery is nearing to full charge, oxygen is usually formed at the electrodes and is then recombined at the catalyst. This chemical reaction results in the production of heat. A thermistor can then be used to determine the temperature increase and this is actually the safest way to determine when a battery is fully charged, especially during a fast charge.
Nickel-Metal hydride batteries are usually charged with a charge that is 10% of the rated battery capacity measured per hour. That means that a battery of 100 milli-amperes per hour will need about 15 hours to safely charge it to its maximum potential without the risk of over-charging. Using this method will usually not need a person to use an end-of-charge sensor to determine if the maximum charge has been attained since it has been reliable. The absence of cadmium in nickel-metal hydride batteries makes them more environmentally friendly than NiCd batteries. In addition, their low internal resistance means they can be used for high-current drain applications.
