20 - 30 minuten
My wife gave our 7 year old son her iPhone 3GS after she upgraded to the 4S in November. For the past few months it was working fine. One evening my son and I noticed that the screen was suddenly loose from body of the phone. I removed the screen and saw the problem - the battery was rapidly expanding, causing the system board to bend and dislodge the screen entirely. As I removed the system board, screws began popping off from the pressure of the battery. The battery itself never actually ruptured, but I shudder to think what could have happened if the boy were playing with it and it did.
The install was a piece of cake - the 3GS is very easy to fix when it comes to replacing a simple battery. The numbered connectors also make it a no-brainer. The biggest issue I ran into was reconnecting the number "3" ribbon - I actually cracked part of the black casing after mistakenly thinking it would popup.
The only problem post-fix is that the external sound is hit or miss. Sometimes it works, sometimes it doesn't. I attribute that problem to the bending of the system board during the expansion of the battery though. From what I have read the number "4" connector controls the external speakers, but that cable is fine. Also, the device charges and I believe the number "4" connection also controls that function.
My advice to anyone who is having battery problems with the 3GS (be it expanding battery or even a problem holding the charge) would be to upgrade the battery and extend the life of the 3GS. There is no reason this device can't be passed on and used as either a phone or glorified iPod touch. One caveat - do not attempt to use a 3G battery - we had a damaged one laying around - even though the battery matches closely, the phone will simply enter a boot loop while charging.