If you want to hook in a larger battery, take into account the fllowing-
1. These little cameras are packed full of static sensitive components- they are easily dispatched just by handling, especially if you have nylon based carpet! Therefore once you open up the case, I would recommend covering the components with sticky tape beofre proceeding further.
2. The onboard battery is about 200-250mAH capacity lipo. attached to this battery is a tiny PCB with a circuit to cut off the camera when the voltage falls too low, so you don't knacker the battery. You need this board for use with your bigger battery.
3. I would recommend using a different charger with the new battery. The onboard charger is rather crude.
A small battery from a mobile phone would be ideal (even one that's past its best for the phone), should keep these cameras going for several hours.
Lots of information on modifying these cameras at http://www.chucklohr.com/808
and on RCGroups.com forum (aerial photography section).