Power won't stay on.

Plenty of things can cause the issue, and unfortunately most of this requires trial and error.

if your device is able to boot while plugged in, and battery removed, its simply a battery problem. however, you could also have a bad charge port, that is not giving current to the device, so it is not charging. You could have a problem with a board level component as well. some devices have a failsafe, that if it does not sense a fan, it will prevent itself from booting.

these are some of the more comon reasons devices wont start.