iPhone won't turn on and Heats when charging

I bought an iPhone as a fun project off eBay to fix. The iPhone has no water damage (seller claim) and the sensors are all white (no red).

I started by installing a new battery and plugged it to the charger. The phone didn't turn on but started heating. The more I waited the more the heat started increasing until I smelled a burn so I disconnected it.

Any ideas where to start?