If it is NiMH, your pack may need to be cycled, especially with that amount of down time. Do you have a charger cycler? That is by far the best option

If not, you'll probably need a multimeter, and a load of some sort. I use a 24vdc motor. You need to fully charge the pack, and then discharge it around 2 to 3c, so for your pack that should be 1.3 to about 2 amps. Discharge it to 1v per cell (so if it's 8 cell, 8 volts, and 7 cell, 7 volts) under load (0.9v per cell max if you are really watching it). This means while it is hooked to the motor or the light or whatever. You've got to be very careful not to over discharge the batteries, because that will kill them. And also an even load is much better than the pulsed load of say your heli.

If you don't have a cycler or a multimeter, there might be someone else that has some good advice too. I've just never had to improvise that way, so I don't know what to use.

A full cycle is a charge then a discharge. Cycle it probably 3 to 10 times. If you have a nice charger you can see how much is going back into the pack, and see if you are gaining any mAh when you cycle.

If your pack does not read atleast 520mAh when charging (if it's the 650mAh pack), then it's probably time to recycle that pack, and get a new one.

Hope this helps a bit....

you could also tell for sure if it is the battery by just going and buying a new one.