The current output of the battery is the 'C' rating of the battery multiplied by the capacity in mAh/1000, i.e. if it's a 25C continuous 3200 mAh Li Po then the max continuous current output would be 25 x (3200/1000) = 80A
I'm not clever, I've just done some research because I wondered the same thing myself. See this link
http://www.flyingsites.co.uk/features/lipo.htm
Cheers
Chris