I live in Brazil and I have this same battery problem, after updating to android 9 Pie the consumption increased being that I keep the same applications and the same routine, in 8 Oreo I did not have that problem and Asus instead of a solution that is simple, do not care, a large and well-known company in the world claims that it does not support downgrade from android 9 to the oreo 8, regrettable.