Phone turns off when plugged in

Showing 100% battery but shuts off when unplugged?

I have an iPhone 4s that has been water damaged. The only issue is that it will not use the battery at all. While plugged in it turns on and shows 100%, it immediately shuts off when unplugged. Everything else works.

I have disassembled the phone and cleaned all visible corrosion on the main board and flex connectors (there was very little corrosion near the power management unit, aka PMU).

I have checked all visible solder joints with a microscope and all components appear to be fine, with the exception of one capacitor near the PMU.

I have tried 3 different replacement batteries that were tested in other iPhones, the batteries are good, and charged to 4.0 Volts. I have also tried replacement charging ports, no change.

I have tested all of the small components near the battery connector with a multimeter and they are also fine. The battery terminal solder joints are fine.

When the battery is connected, the voltage across the board + and - terminals reads 0.0 Volts.

I need help finding which components/chips might need to be reballed or replaced to fix this issue. What IC's are related to this issue?

I have searched for hours for a schematic or PCB photo to show which IC is the culprit, and came up empty handed.