If you are the do-it-yourself kind of person, then this project is just right for you. Instructables member Kention3 has posted how he juiced up his cell phone with an AA battery set after his original battery died out. The changeover works on a phone that takes batteries of 3-3.7 volts; however any battery from 6-18v will work. I’m not sure whether this project will work for my iPhone, because when it’s battery dies out on me, the only option left will be to chuck it into the sea! Kention3 says that 2700mAh Ni-Cd rechargeable give about four times the battery life than the 880mAh standard battery; however he also warns that after the mod, you should not hook on the phone to a charger, lest it gets damaged.
Via Instructables