Hello Pete
I’ve just rooted out the instructions for the electrobloc.
To charge the batteries the bloc must be switched on (“Batterie Ein/Aus”, on the unit itself) as the electrobloc is the battery charger.
The isolator switch near the door can be “off”. This allows charging but cuts power to services.
The advice I was given by Atlantic MHs was to charge periodically, but not leave it on constantly.
I hope this helps