Battery life was going down the tubes on my iPhone 3gs. Phone would just shut completely off at 50%.
The repair went smooth. The first time i replaced the battery, the phone would not turn on unless plugged into power source. I thought i had ruined my phone. But, i put the old battery back in and it worked fine. Contacted iFixit and they sent out a replacement that same day. Installed the replacement now the iPhone works like it's supposed to! Could not have been any easier.
Take your time and follow the guide