If you let your batteries sit for more than a few hours it's best to "repeak" them before you fly. All you do is throw them back on the charger and let them recharge fully. Nimh cells tend to loose charge if left alone. You will get more starting power out of the battery if you put it back on the charger for 5-15 minutes before the flight.

If you are using a peak charger it should get it back to fully charged on its own. I've found that if I leave my nimh batteries in a fully charged state and then go to repeak them at the field after 2-3 days it takes 15-20 minutes to repeak them.

It's also better to fly nimh batteries when they are warm and right off the charger.