Choosing the best charger for your rechargeable nickel-metal hydride (NiMH) batteries begins with selecting the feature set most appropriate to your needs. Pick a NiMH battery charger that supports the size and type of batteries you will be using. You will also need to decide between inexpensive timer-based models and more costly chargers equipped with microprocessors to monitor charge conditions. Be cautious of potentially misleading marketing labels, and compare charge times only when the same capacity battery is used.
Your first step in selecting a NiMH battery charger is identifying necessary features. Some important things to consider include what size batteries — AA or AAA for example — you need to charge, how many you’d like to charge at once, and how long you're be willing to wait for the charge to complete. If you already have a collection of rechargeable batteries, check whether they use NiMH or nickel-cadmium (NiCd) chemistry because many types of battery chargers support only one. A portable NiMH battery charger with different kinds of power adaptors may be useful if you are a frequent traveler.
The cost of a NiMH battery charger can vary significantly based on features and quality. Many of the least expensive devices use timers that pump a steady amount of electric current into a battery for a given amount of time and then revert to a trickle charge mode. These types of charging units are adequate when used properly, but can damage or shorten the life of a battery if used improperly. You will need to make sure all batteries are fully drained before charging because timer-based units can overcharge a cell that still has partial charge. Be sure not to leave batteries in a charger for extended periods of time; trickle charging for long periods can shorten battery useful life.
More expensive NiMH battery chargers feature microprocessors that monitor a battery’s charge level, temperature, or a combination of the two. These units, often branded as smart chargers, stop charging a battery when the cell is fully charged or when it exceeds a certain temperature. The most advanced microprocessor-based units can test and recondition batteries that may have been damaged by other chargers.
No matter which type of charger you select, beware of marketing terms such as quick charging, fast charging, and rapid charging. There is no industry-wide meaning to these labels, so one "rapid" NiMH battery charger may actually take more time to charge a battery than a quick charger offered by different manufacturer. Also keep in mind that batteries of higher capacities take longer to charge than batteries with lower capacities. Charging times can only be compared across two or more units when the same capacity battery is used.