The phone would randomly turn off, reboot and sometimes turn of and refuse to turn on until I plug it on the computer.
It would even say "activation required" when restarting.
I could charge it only using the pc and sometimes it would seat at 30 or 40% for hours.
The repair was very easy after watching the respective video from ifixit.com
Always buy the plastic opening tools or isesamo. I didn't but I was lucky because I found a set at a local hardware store.