20 - 30 minuten
Got my son's old iPhone 3GS unlocked to use with another carrier but it would shut itself off after a few minutes. My son confirmed that it's been a problem for quite a while and that's why he upgraded to a 5S. I saw someone else's post about this kind of problem so I quickly ordered a new battery in the hopes that that would fix the problem.
As usual, the repair guide was easy to follow and gave enough details to make sure things would work after installing the new battery. With care and patience, the installation went smoothly without any problems. I was a bit hesitant when removing the battery from the back of the case. I didn't want to totally mangle the battery. After installing the new battery and giving a full charge, the old 3GS is working like new.
The battery is stuck to the back of the case by double-sided rectangular strip of sticky tape. Work gently but firmly; going a little at a time to loosen things. I didn't try using heat to loosen things so I can't say how much easier it would have made things.