Chosen Solution
I’ve received an iPhone 6 to repair, it wasn’t turning on so I opened it up. I took the battery out and tested it and it has completely shorted. The LCD has also been tested and isn’t displaying anything. I put a new battery in and connected an LCD and the iPhone powers up, but the logic board gets really hot. If connected to a power source, it shuts down after a few seconds on the lock screen. No visible water damage, and no visible broken components on the front side of the logic board.
Can you confirm that the battery and LCD are known-good (tested in another device)? If so, then your logic board has been damaged. It would be interesting to know a bit more on the history of the device(impacts, bending, cheap chargers or Lightning cables) so that we can give you more guidance. Let us know if you are confident of taking measurements on the logic board and if you have access to schematics. Otherwise, you will need to find a repair shop that specializes in micro-soldering repairs.
It seems you may have to, replace the logic board, the defective battery would have caused damage to it (electrical in nature). First do a wash with a little isoprithyl alcohol.
