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

[vinnie]

a little offtopic.
I read that the piephone has an rtc, I remember the old phones that could turn on to work the alarm clock.
With android this seems to be no longer possible (maybe something to do with the rtc?) but once I happened a phone (very cheap, I don't remember if it was a mediatek) that could do it.
Maybe in the future it could be possible with the pinephone, planning phone turning off and on..