Couple questions for ya, how old is the battery, what kind of charger and has it ever been cycled before. Nimh batteries work to their full potential after fully cycled. They also recommend every say 20 charges or so, to cycle the battery again. With that said, my Venom Pro Duo charger has a cycle function, to which (depending on its state) will either fully discharge then fully charge or fully charge, discharge and then charge. You can even select how many cycles you want it to do. Its a way of conditioning your battery. I always do it with my NiMh packs.
So to your question, if you don't have a cycle option on your charger, fully discharge your pack. It can take some time depending on how much is in it and your discharge rate. Once fully discharged, then fully charge it. I use a 5000mah Nimh pack on my Raminator as the reverse and throttle servos stock are not high voltage...thus they could get toasted on a 2s lipo. No shame in a Nimh pack as long as it works for your application.