S4 phone says it's charging, but charge decreases, eventually dies

**UPDATE: check comment below**

Hi everyone!

I have a Samsung Galaxy S4. I have recently replaced the motherboard due to water damage. I replaced it from a used phone i brought cheap with a broken screen.

It looked like it worked, the phone turned on fine. Next, i tried charging it while FULLY turned off - it displayed the charging icon, however instead of the percentage going up, it went down, eventually reaching 0% and dying. This was with the water damaged phone's original USB logic board.

So i replaced the USB logic board - i transplanted the board from the used phone i had used to replace the motherboard with, thinking that the water also damaged the original logic board. The 'new' logic board appeared to have worked at first - it charged up to 8%, but then went down again. The USB pin's actually appear to be a bit damaged on the used phone, as charging only starts when i move the usb cable around a bit.

I tried with 4 different batteries (including a brand new one).

One thing is that the phone appears to get very hot around the motherboard left side area, both sides (motherboard and screen) - the phone's logic board, battery don't seem to be affected. I ordered a brand new original logic board - i hope that my original USB logic board and the used phone's logic board are both faulty - otherwise this would mean there is a problem with the motherboard's power IC.

If so, i think buying an external battery charger could work.

Now, the phone is completely dead as it drained the battery, but when i plug it in it still appears to be 'charging', as i can feel the heat from the motherboard, but no display or anything. I suspect this may be because the battery is charging, but at the same time draining, so it doesn't have enough power to actually turn on the display.

Also, the 'new' USB logic board doesn't appear to get AS hot as the original one.

USB function worked on both phones (MTP).

What do you guys think? Motherboard fault or logic board? Just it seems so unlucky that both logic boards have the exact same battery draining problem.