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 catalytics to immediately deal with gases developed as a result of over-charging without being harmed. (2 H2 + O2 ---catalytic--> 2 H2O) This however only works with over-charging currents of up to C/10h (nominal capacity devided 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 the 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 cheapest battery chargers.
The common penlite-size (AA) batteries have capacities ranging from 1100 mAh to 2000 mAh at 1.2V.
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%.