failed to fix it, anyone has an idea of what could be the problem?

hi there

2 months ago my sony xz stopped working. It had a low charge and went suddenly to blackscreen. When i tried to charge, the red light would turn on, but de device didnt really charge, i couldn´t start it. After a while also the red light wouldnt turn on.

my first thought was that the battery got in deep sleep mode. so i ordered a new battery.

When changing the battery i tested both of them for voltage. Strangely the old one had 4.19, the new had 3.85.

with the new battery inside there was no change of the situation, no charging, no red light.

i thought it might be a problem between charging port and battery plug on motherboard. so having screen and battery removed, i tried to plug in and see if there is any voltage on the battery plug of the motherboard. Since its so small and i dont know where to put plus and minus from my voltage meter, i tried to look for voltage using NCV mode (non-contact-voltage-mode) and it shows me a signal when i hold the meter over the plug.

So i assume that it is working.

so what could be the problem? Could it be that both batterys are in deep sleep mode?

or is it more likely the motherboard itself? or is my way of checking voltage via NCV a bad method?

Im extremely thankfull for any idea!