iPhone 3GS battery and volume button replacement
20 - 30 Minuten
A bad battery was causing phone to shut off / reboot randomly with remaining charge reading between 50 - 85% especially when running any app that used location services (Google maps etc.). The external portion of volume button (silver / chrome) fell off and was eventually lost after be glued back on a few times.
It was fairly simple to replace the battery. The ifixit guide was very helpful / detailed. I took the advice of another user who suggested taking the silver/chrome portion off the replacement button and gluing it on as opposed to completely disassembling the phone (down to the bezel) which probably saved 45 mins. All in all the repair took about an hour (10-15 min of which was used looking for a misplaced screw)
Dedicate a nice clean and clear (light colored) area for the repair. Include a container to hold your screws (as I found out can be moved pretty far with just a hard breath) some have suggested a cupcake pan. A suction cup is the easiest way to remove the screen and you can pick up at a dollar store. I also used some alcohol and q-tips to remove the grime around the bezel / screen edge. Separating the button cover from the base is done by using a small screwdriver / x-acto knife. You can use a toothpick to apply the glue to the center of button base making sure to avoid getting too close to the button edges (to prevent gluing the base to the case screwing up the button function / movement). Best advice, take your time don't rush it especially when trying to remove the mother board, it takes a bit of finesse.