Battery drains fast even if my phone power off

HI, im ready to help🫡

step 1: charge the phone to 90%, then use the phone until 10%, if that dosent work, go to step 2

step 2: update the android version of the phone, if that dosent work, go to step 3

step 3: remove the battery and try the same battery on another A5, if it drains faster when its off, bingo (50/50 chance), if it does drain faster while off, go to step 4. If it dosent, i dont know what else will fix this

step 4: replace the battery with this guide: Sostituzione batteria Samsung Galaxy A5 (2017)

hope it helps