The "Lenmar MSC815 2000mAh 8-15 Minute Mach 1 Lightning Battery Chargers" charges 4 AA batteries in about 15 minutes. It has been reported to work great. You can get those at Fry's and on-line (for example, at Amazon.com) for about $30.
There are several unique features of this charger:
- It includes a fan to cool the batteries during charging. Which I think is why they can charge in 15 minutes.
- It has individual charging circuit / monitoring for each battery so that you can replace just one and not have other batteries in the charger get "overcharged".
- It has LEDs for each battery indicated whether it is charging or charged.
With a 15 minute charger, you can get by with two sets of batteries. One in the robot and one in the charger as the robot's batteries hopefully last longer than 15 minutes.
Lenmar makes several different chargers that can take 15 minutes, 1 hour or 8 hours to charge. If you're buying a different model check the charging time first.
Regarding NiMH battery life
NiMH batteries do "wear out". When battery life on a robot does not appear to last very long, one of the batteries is likely bad. Measure the voltage of the discharged batteries and if one of them is much lower than the others, consider it bad and recycle the offending battery.
It has been reported that there isn't much difference between 2000 mAH and 2500 mAH batteries.