Thanks to all for the above advices.
A bit more information. The yacht is in Greece and I am in Tasmania and cannot travel overseas. My concern is not so much for the batteries but the damage they might do to the yacht through a malfunction
of the controller. In retrospect I could have reduced the output of the panels
by partial covering and/or partial disconnection.
The chance of the controller malfunctioning and overcharging is small. But not impossible. Other than battery damage, what are the other possibilities? Explosion, fire acid damage?
I could arrange to have the solar panels
disconnected and I am sure the batteries would be OK for a few months. But I do not know when I will get back to Greece. April 2021?