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Specific energy density: ~60 Wh/kg
Volumetric energy density: ~100 Wh/L
When fast-charging, it is advisable to charge the NiMH batteries with intelligent ( microprocessor 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/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. 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.
Permanent trickle charging (small current overcharging) can cause battery deterioration. According to the Panasonic NI-MH charging Manual (link below), the trickle charge rate should be limited to 0.033 C/h to 0.05 C/h for a maximum of 20 hours to avoid damaging the batteries.
The common penlight-size (AA) batteries have nominal capacities ranging from 1100 mAh to 2300 mAh at 1.2 V; the true useful capacities can be significantly lower and depend on the discharge rate. NiMH batteries have an alkaline electrolyteAn electrolyte is a substance which dissociates free ions when dissolved (or molten), to produce an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutes . They are sometimes refer.
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%.