iPhone repeatedly flashes apple logo w/ white background, won't boot

I replaced a screen on a 6s that the customer hasn't used in 4-6 months. Upon disassembly I noticed many screws missing and the metal covering for the ribbon cables of screen to lcd was missing.

After the repair, only when on a charger, the screen will show the black apple logo on a white background. This will stay on 1-5 seconds before going black. After a few seconds it will repeat. The issue is still present when battery is disconnected and chargers in. Can't get it past this and itunes is not detecting the phone.

The battery does look old, and has white crust near the edge, as if there was an abrasion. I've suggested a new battery to the customer already but told him that I'm unsure it will fix his issue. I have a battery ordered from ifixit so I'll post updates within the next few days.

Any ideas?