HTC Desire 820: Battery Draining Rapidly, Charging Issue & Possible Short Circuit Cause

Recently, I got from a friend HTC Desire 820 with a damaged display after falling. The display with touch was replaced as well as the battery that was not holding. The problem with it is that the battery is charging but when the rear cover is fitted, the battery is discharged. I will add that the phone without back cover lasted 2 days and the battery was kept. Today, I wanted to assemble it and the phone with 80% battery discharged in a few seconds.
The battery has a voltage of ~ 4.2V. I am also surprised that the connected USB cable on the pins of the motherboard + and - does not display any voltage and yet the indicator shows that the phone is charging. I suspect that somewhere there is a short circuit I would ask for help in locating or directing it, which may be the reason for such behavior of the phone.

Replaced battery new or used ??

At the auction they wrote that the new, original from what I remember cost about 50 PLN.
Now I don't want to land through the phone anymore, I have an external charger connected to the battery pins, I will leave it until the morning we will see if it gets up, from what I read about HTC, charging systems like to fall, but I do not know if in this model also because I did not find such a mention .

Often this is an android bug, upload a new rom and it should help.

Now the phone is totally dead. The red LED flashes, the screen shows an empty battery. This condition does not change even after a few hours of charging. Charging with an external charger does not work. The battery if charged after plugging into the phone behaves the same.

The phone was flooded maybe?

USB socket good ?? Didn't they release pins from the socket after this fall ??

The phone apparently was not flooded, I checked the socket is good because the voltage is spread across the board. I suspect that the power system is to blame because I read that HTC liked to rain.