I was planning to use my old iPhone 3GS for weekend project but I discovered that the screen was popped out and the back was bulging, I took it apart and this
Battery iPhone 3G/3GS iPhone 3G - SOSav English
ZERO CYCLE Dr.Gadget Lithium Polymer 1220mAh Battery For iPhone 3GS with Opening Tools | Lazada
iPhone 3GS Battery Problems - Read This Before You Replace It! — navigate