If you have NiMh or NiCd, if you dont cycle them, they will wear down without use. I lost a good 6V set in the same way.

Two things are good to do: one, cycle the battery, two, keep a meter in the airplane. Use VoltWatch for these batteries. Once you put in the battery, deflect all your servos to the max possible extent. If the voltage drops, keep an eye on it for some time. If it goes into yellow, you need to recharge.

Note, voltwatch does not work relaibly with LiFe battries.

Ameyam