I replaced both the battery and power button on a 3GS that is old and has been opened up several times (by me and my predecessor). The battery starting dying after I opened it up last time to replace the digitizer. I think because of ESD damage, but really not sure. As far as the process itself, it was probably the dirtiest (literally and figuratively) repair I've ever done. The inside of the phone was very dusty and dirty which didn't help. Getting the logic board back in was a pain! Really don't have any advise to give to those who are struggling with that as well, don't give up! You'll get there eventually. I definitely could have been more gentle with the camera, golden ground tab and the cables, but in my experience, electronics are a lot more durable and strong than we sometimes think. I did use a anti-static wristband this time though (iFixit sells them, but so does Radioshack), just to make sure I wouldn't fry anything again. Good luck!