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
Original Battery replacement for iPhone 3GS
Amazon.com: Winnerplusa Battery kit for iPhone 3GS 3G Include The Tool Sets 2 in1 : Cell Phones & Accessories
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iPhone 3GS Battery
iPhone 3GS battery appears to have expanded. | MacRumors Forums
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