Modern lithium phone batteries do not suffer from the old Ni-Cad,"Memory" problems. They are very robust and don't mind being charged frequently or even left on a decent charger. There's no need to fully discharge them and it can be detrimental to them and in some cases make the phone unbootable(there are fixes).
For Android this is a neat little app that allows you to set charge level warning settings and much much more,
Totally free an no ads 🙂