NiMH Batteries
Functions, charging, and discharging of NiMH batteries
A NiMH battery, which is the abbreviation of nickel-metal hydride, is a rechargeable battery, having similarities with Nickel Cadmium (Ni-Cd) batteries, except that the NiMH batteries have a hydrogen-absorbing alloy for the anode. On the other hand, Ni-Cd has Cadmium as its anode. NiMH batteries have nickel as its cathode, similar to Ni-Cd. NIMH has two to three times the capacity when compared to a Ni-Cd battery of the same size, and unlike Ni-Cd, the memory effect in NIMH batteries is not significant. When compared with a lithium-ion battery, the NiMH batteries have lower volumetric energy density, and have a higher self-discharge rate.
NiMH batteries are used in hybrid vehicles such as the Toyota Prius or Honda Insight/Civic, and consumer electronics. It is also planned to be used in on the Alstom Citadis low floor tram ordered for Nice and France . The battery will also be used in the humanoid prototype robot ASIMO, designed by Honda. In moderate drain devices, like, the digital cameras, flashlights, and other consumer electronics standard NiMH batteries perform better. NiMH batteries, with its low internal resistance have the advantage in high current drain, making it ideal to be used in cordless power tools and RC cars.
A NiMH battery has a metallic anode, which is an inter-metallic compound. There have been several different compounds which have been developed for the application as an anode, and out of these the compounds presently in use, fall in two categories. AB5 is the most commonly used compound, where A is an earth mixture of lanthanum, cerium, neodymium, praseodymium, and B is made up of nickel, cobalt, manganese, and aluminum. The other is AB2 compound, which is very rarely used in batteries and there are very few of them using this material. In AB2, A is titanium and/or vanadium, and B is zirconium or nickel, in which chromium, cobalt, iron, and/or manganese have been added to extend its life performance.
You should always use a smart battery charger, when fast charging a NiMH battery, as otherwise it may get overcharged. Overcharging a NiMH battery can cause damage to the cells and dangerous conditions may occur, with gases forming in the cells. Modern NiMH batteries contain catalysts, which absorbs the gas immediately and prevents the harming effect. In discharging NiMH batteries, you should take care that the battery is not completely depleted, and one or more cells, connected in series in the battery, do not become completely discharged. This will take the cells into polarity reversal. NiMH batteries would be required to be charged by low duty cycle pulses of high current for long term maintenance of the battery. Brand new NiMH batteries, or batteries which have not been used for a long time, would need "reforming" for reaching its complete charged condition. In order to reform, you would need to put the battery through charge and discharge cycles before you can operate them on their specified capacity.
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