NP900X3E Power/Charging Issue

Hi

I've got a really tough case here I haven't been able to solve. I originally purchased an NP900X3E labelled as 'battery won't hold a charge' thinking it would be a simple DC jack or new battery swap.

I have an older NP900X3C with a broken screen but HDMI out works, so I've been able to test the battery, charger, and DC jack and confirm they are all working on the X3C.

Originally the laptop would boot to the Samsung screen then immediately turn off. After much trial and error, when I put the motherboard of the X3E in the chassis of the X3C (with the X3C daughter boards and X3E screen assembly) it booted for longer but still turned off suddenly/without warning.

Figuring I haven't got much else to lose I reflowed the motherboard in my oven at 200 C for 8 minutes, let it cool then reassembled everything into the X3E case (original daughter boards, screen). I reapplied thermal paste etc during assembly.

This appeared to fix the problem as the laptop booted past the Samsung screen to Windows 10 and worked perfectly for 30 minutes, but then it resumed the original problem of powering off without warning. I left the laptop plugged in to the charger for 3 hours and tried to boot it however now there is no response from when I hit the power button.

I've tried removing hitting the battery button and reconnecting the power supply, but it doesn't start. I believe this is a sign that there is no power flowing from the PSU to the battery to reconnect the circuit (judging from other forum posts).

I'm hoping that there is a simple IC or fuse I can replace on the motherboard to repair this laptop.

If anyone has any suggestions, service manuals or board schematics or ideas I'm open to trying them. I'd hate to dispose of such a nice laptop because of one little component.