Why is my battery charging and discharging very fast?

I have recently swapped a battery in my 5 year old P9 lite.

Before the swap the battery could not charge at all (it would drain and stay at 1 %). After the change to the new battery it is charging very fast (adds percent every few seconds), but also discharges fast over few hours even when phone is not used at all.

I wonder whether this could be a known P9 charging circuit fault or some kind of virus?