Some useful informations about battery charger.

How long will it take a charger to charge batteries?
It's pretty easy to estimate how long it will take. Simply divide the capacity of the battery by the charge rate of the charger, then increase the amount of time by about 20% to allow for a certain amount of inefficiency. As an example, a battery with a capacity of 1600 mAh will require about 4 hours to be fully charged by a charger with a charge rate of 500 mA. (1600 mAh/500 mA x120%). Incidentally, this example would apply to a standard AA NiMH battery and a typical "rapid charger". Keep in mind that a battery that is only partially discharged will be recharged in less time.
What's the difference between a NiMH battery charger and a NiCd battery charger.
The biggest differences are in the charge rate (how fast the charger can charge batteries) and the charge control (how the charge determines when to stop the charge). Many of the inexpensive NiMH battery chargers are simply NiCd chargers that have been modified slightly. Typically a 5 hour NiCd charger has a switch that allows the charge time to be increased from five hours to eight hours. Thus a 5 hour NiCd charger becomes an 8 hour NiMh charger. As we mentioned above, we do not recommend this type of charger design. While a timer type charger is less expensive to manufacture than a smart charger, it can lead to overcharging and battery damage if batteries are frequently charged before they have been discharged (that is, the batteries are used for a short time and then fully charged again).
NiMH smart chargers have actually been designed to detect when a NiMH battery is fully charged and then shut off or go into a trickle charge mode. Because of the more complex circuitry, this type of charger costs more to make, but should lead to greater battery life. Some of these chargers only cost slightly more that the "dumb" chargers. We strongly recommend investing in a smart charger for your NiMH or NiCd batteries.