Capacity, Types

• Compare the different types/sizes of batteries the chargers can charge. It's best to have the widest variety of options available, though many people don't need to charge any C or nine-volt batteries. Also consider the battery quantity different NiMH chargers can hold at one time.
  
  

Charging Time

• Check how long the charger takes to charge a battery. Don't compare apples and oranges; it will certainly take longer to charge a D battery than a AA cell. It's also good if the charger has a charge timer function; this helps prevent overcharging, which can cause battery damage.
  
  

Other Features

• The ability to also charge NiCd (Nickel Cadmium) batteries remains useful if you already own rechargeables of this type, though otherwise unnecessary. Some NiCd/NiMH chargers have a discharge/conditioner feature. According to RadioShack, this function is only important for Ni-Cd batteries and unnecessary for NiMH batteries.