I no longer have a old phone but I found that disabling bluetooth and mobile data when your not using it can help with battery drain.
You can also go into developer settings and set the maximum backround process limit to 3 and that does a good bit on its own.
sudoku@programming.dev 10 months ago
Normal stand-by drain is less than 1% per hour, for a new device or a phone from 2015. Something is very wrong for it to drain 35% overnight.