Well the stock pack and charger that came with this heli are it's Achilles heel. I hate to say it, but they're crap.

It's unlikely anything is wrong with the heli. I'd call Hobbico and tell them your problem. See what they say. I'm leaning toward a defective charger unless the pack is getting really hot or has some other tell-tale sign.

The best advice I can give is switch to Li-Po's as soon as you can. Believe me it's a whole new heli.