How do you prevent the battery from discharging when the phone is off?

[bcnaz]

I think sometimes it does not power down completely when turned off.
try bumping the power button quickly... after shutting it off.
Some of these OSes do not power down completely when you use the software to turn them off.

You need to hold the power button down for a few seconds when starting it from a cold start.
If it starts from a quick 'bump' it was not fully off.

Recently my Pinephone was off for over a week, it was 99% charged when I turned it on.