I will get checked the batteries in a battery service
to be sure that they are dying.
The batteries are in front of one of the water tanks
and not much chance of expanding the case.
Before this discussion I did not have the concept
of deep cycle batteries I mostly did buy 3 car batteries
75-85 Ah. One for the starter and windlas and 2 for the board.
They usually lasted 3-4 years. Quite satisfactory
in my usage conditions. I checked and saw that
the last 3 batteries costed me 210$ in total in
May 2011. With all the advice I will be more choosy