Cellphone battery on iPod touch

So one of my friends gave me an iPod touch that needed a new battery. But I didn't want to pay about $15.00 for a new one, considering that I could screw the thing up trying to solder it in. So I got the crazy idea to modify and use the working battery from my old cellphone. But it didn't fit inside the case because it was too thick. So I successfully connected it to the old battery's controller circuit and hooked up the "new" battery. I had to glue it to the outside of the case. Now, everybody wonders what that big black thing is on the back of the ipod. But hey, it works and charges just fine!