I think as far a power goes you cant go wrong with these [link=http://store.yahoo.com/batterynexus/ne21mahnireb.html]2100 mah batteries[/link]. Milliamp-hour indicates the battery’s charge capacity. The more capacity, the longer a battery will last in a given application. More power to the servo for a longer time. The duratrax hump pack only has 600mah. It's about the same as if you ran some normal 4 x aa alkaline.