20 - 30 minutos
My iPhone 3GS (purchased on launch day in June 2009) was beginning to suffer from battery power dropouts (from 30 or 40% down to 0% in the blink of an eye) and an Apple Genius diagnosed that the battery needed replacement, which the Apple Store would happily do for $75. I had a better (cheaper) idea. iFixit had supplied me with iPod repair parts in the past, so you guys were the first vendor I thought of.
The Installing iPhone 3GS Battery guide is excellent. The photos are great and the steps are clearly explained. The most time-consuming part of the entire task was getting the logic board back into place -- I had to make several attempts until I was finally successful.
During the logic board reinsertion, one metal pin I hadn't noticed before (on the logic board near the 2.9 mm Philips screw in the Step 12 photo) was somehow bent outward, and I wasn't quite sure what to do with it. After pondering a while, I concluded that the pin was probably supposed to point down (towards the back of the iPhone), so I re-bent it to its proper orientation. Since reassembly, the iPhone has been working great! 3GS battery replacers would do well to watch out for this pin.