I have found what I have waited for: A Limiter for charging Smartphones.
You find it at F-Droid. You may tell it:
Charge until 80% of Battery capacity,
use battery until it has reached 50%
and than charge again, as I did it in my example.
And it works!
To be exact: until now I have tested only the upper limit.
Edit: The app needs a rooted phone!