I have upgraded my batttery in my 9C to a 2000 Mah NMH . I can fly all week end plus a couple of night after work. Pretty much charge once a week and not worry about it. I'm not sure about the warranty thing but it would not surprise me, I've hears of similar things with other electronic items. My radio was already out of warranty so that part was not a concern for me. Do it and rnjoy the flying time!
Peter