Battery drains when phone is turned off

I have a used Samsung Galaxy S3 Mini GT-I8190 . When I shut down the phone the battery still drains. The longer I leave the battery in the shut down phone the more charge it looses. Typically the battery looses between 1% and 2% charge per hour when inside the shut down phone. After a few days without turning the fully charged phone on the battery is completely dead so that I cannot turn on the phone again without charging first. I don't know the previous owner and cannot ask them if they had the same problem. ### Things I tried so far: - Tried to spin the battery on a table to see if the battery is bloated. The battery did not spin at all. Its completely flat on both sides. - Removed the fully charged battery from the phone, let it sit outside for a few days, put it back in, turned on the phone to read the battery level: The battery was still full – so the battery shouldn't be the problem. - Bought a new battery from another vendor anyways and did the same tests as before. - Factory reset (multiple times) - Update to the latest available OS (Android 4.1.2) provided by the vendor. - Update to the latest available custom ROM (Lineage OS 14.1 / Android 7.1.2). - After the factory resets and after flashing I did not install or customize anything apart from the following things. Of course, they had no effect since the phone was turned off — I read about these things in some forums and tried them anyway. - Only on the vendor's OS: Stop the pre-installed Mail service (even though I didn't sign into anything). - Turn off everything (Wifi, GPS, ...) - Enable (extreme) power save mode Neither did I insert a SIM card nor an SD card into the phone, so these two can also be ruled out as the culprit. ### Question Why does the battery drain when the phone is shut down? Can I do anything about this?