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Nickel metal hydride
A Nickel metal hydride (or NiMH) battery is a type of rechargeable battery similar to a nickel-cadmium (NiCad) battery but without the expensive and environmentally unfriendly metal cadmium. This is why they are sometimes called the most environmentally friendly battery type. NiMH batteries tend to have a higher capacity than NiCads and suffer far less from the memory effect. However, when compared with lithium ion batteries they have a lower energy density and a higher self-discharge rate. They are being investigated for use in electric vehicles. Also NiMH batteries perform better with high drain electronic devices like digital cameras and other common electronic devices, while for high torque power tools and other devices that require fast discharge rates, NiCd can outperform NiMh.
When fast-charging, it is advisable to charge the NiMH batteries with intelligent (microprocessor controlled) chargers to prevent over-charging with large currents, which could damage the battery. Modern NiMH batteries contain catalysts to immediately deal with gases developed as a result of over-charging without being harmed. (2 H2 + O2 ---catalyst--> 2 H2O) This however only works with over-charging currents of up to C/10 h (nominal capacity divided by 10 hours). As a result of this reaction, the batteries will heat up considerably, marking the end of the charging process. Some quick chargers (one hour or less) include fans to keep the batteries cool.
NiMH can thus be charged in most simple fixed (low) current chargers with or without timers, since permanent over-charging is permissible with currents up to C/10 h. In fact, this is what happens in cheap cordless phone base stations and the cheapest battery chargers.
Permanent trickle charging (small current overcharging) can cause battery deterioration. According to the Panasonic NiMH charging Manual (link below), the trickle charge rate should be limited to 0.033×C per hour to 0.05×C per hour for a maximum of 20 hours to avoid damaging the batteries.
Common penlight-size (AA) batteries have nominal capacities C ranging from 1100 mA·h to 2500 mA·h (4.0 C to 9.0 C) at 1.2 V; the true useful capacities can be significantly lower and depend on the discharge rate. NiMH batteries have an alkaline electrolyte. The specific energy density for NiMH material is approximately 60 W·h/kg (220 kJ/kg), with a volumetric energy density of about 100 W·h/L (360 MJ/m³).
Nickel metal hydride batteries have a high self-discharge rate of approximately 30% per month and more. This is higher than that of NiCd batteries, which is around 20% per month. The self-discharge rate is highest for full batteries and drops off somewhat for lower charges. The rate is strongly affected by the temperature at which the batteries are stored. Recommended long time storage charge is around 40%.
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