Here are a few aspects of chargers to look for and compare before making your purchase.

1. Minus Delta V
Chargers that test for -dV monitor the voltage over the cycle of the charge. When a battery is fully charged there is a drop in voltage (hence 'minus delta v'). When this is detected the charger will either switch off or switch to a trickle charge, depending on the specifications of the charger.

2. Temperature Monitor
Some chargers monitor the temperature of the batteries as they are being charged. This is a basic safety measure to prevent overcharging and is typically included in fast rate chargers.

3. Safety Timer
Some of the lower priced chargers are built with a simple timed charge. For instance, a charger may say that it charges AA NiMH batteries in 6 hours...so it charges the batteries for 6 hours and then shuts off. If a battery has a higher capacity than the example (i.e., 2000mAh versus 1600mAh), and the battery is fully discharged, you are likely not going to have a fully charged battery at the end of the cycle. This type of charger is good for non-critical use of rechargeable batteries.

4. Trickle Charge
Most chargers will switch to a trickle charge once the charger determines that the battery is done charging, or once the charger's timer completes. The purpose of this trickle charge is to keep the battery topped off to the optimum capacity. This is standard in most chargers, but is still something you should look for.

5. Channels
The most common number of channels you will see in AA & AAA chargers is 2 or 4 channels. A 4 place 2 channel charger treats each set of 2 batteries as a single unit. It is important to charge and discharge each 'pair' of batteries together, since the voltage is measured as a total voltage for the pair and so you can easily over charge a battery if you attempt to charge two batteries at different states of discharge. A 4 channel charger will charge each cell independently for complete charging.

6. Conditioner
Some chargers are built to charge both Nicad and NiMH batteries. You will find that some of these chargers have a 'conditioner' designed to remove the 'memory effect' in NiCad batteries. This conditioner basically discharges and recharges the cells a couple of times to remove the voltage depression that NiCad batteries sometimes develop.

Determining your need
When comparing the differences in chargers it's important to consider what you need a charger for. If you are a die-hard photographer that is looking to squeeze out the very last drop of energy out of each cell for the most pictures, then you might want to consider purchasing the very best charger, like the MH-C401FS charger. This charger has everything you need for getting the most out of your NiMH cells.

If you are a casual user you can look for something a little less intense, like the PowerIzer flip-down charger, or the PRO99 charger.