A Nickel metal hydride (or NiMH) battery is a type of rechargeable battery similar to a nickel-cadmium battery but without the expensive and environmentally unfriendly cadmium. NiMMH batteries tend to have a higher capacity than NiCd (nickel cadmium) batteries and suffer far less from the memory effect. They are being investigated for use in electric vehicles.
Specific energy density: ~60Wh/kg
Volumetric energy density: ~100Wh/L
When fast-charging, it is advisable to charge the NiMH batteries with intelligent (micro-processor controlled) chargers, as over-charging with large currents should be prevented. 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/10h (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. 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/10h. In fact, this is what happens in cheap cordless phone base stations and the cheapest battery chargers.
According to the Panasonic NI-MH charging Manual see http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiMH_ChargeMethods.pdf the trickle charge rate should be limited to 0.033C to 0.05C for a maximum of 20 hours to avoid damaging the batteries.
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. Recommended long time storage charge is around 40%.