update:
measured under load the voltage of the battery break down under 1,5volt.
so the promotional description of moates "battery lasts as long as the car" seems to be a little bit too optimistic.
next step: unsolder the battery and install a battery holder. to use a changeable battery in future.
some coincells and a batteryholder is ordered now.
I 'll solder it parallel to the build-in battery first; that should allow to confirm my suspicion. or not.
to be continued...