iPhone 5S shows low battery black screen even when it's not connected.

I am repairing friends phone. Since I don't know the history of it and it was bought second hand I can't say if it was water damaged or not. Phone itself was roughly beaten and display changed. It was working ok and charging awhile then one moment stopped receiving charge.

After I have transplanted the motherboard and home button to another housing (being almost mint but with dead motherboard) I was able to charge the phone — after sitting on low battery screen for half an hour it booted.

Then it again stopped taking charge in few days. After some troubleshooting we discovered that applying pressure to lower right corner of the screen start the charging and decided it was due too loose connector.

I didn't repaired it instantly and in a while friend reported that method of applying pressure no longer works.

Now phone is in the state that it is showing Low Battery Charging black screen even when it is not connected at all. This one

And lighting connector icon is flickering slightly about every second.

I can't say if it is really charging or not. It sure is not booting. Hard reset doesn't help. I can't put it in DFU.It is not showing in System Profile USB section. Disconnecting battery doesn't help.

I have multimeter if that is needed. Anyone has any ideas how to troubleshoot it further?

P.S. My girlfriend is now using iPhone 3GS I was able to jumpstart after it was seemingly dead after fake charger. I wonder if this problem is of the same kind.