Congratulations on your new boat
. Many years ago I owned a Sabre
30 and remember her so fondly.
A starting battery is - as the name suggests - meant only for engine
starting. It will be quickly ruined if repeatedly discharged to say 50% as you might when anchored out for the night.
What you want is a “deep cycle” battery. My old Sabre was set up so the you ran off one battery using the selector switch. Each day I would switch batteries (they were both deep cycle). Never leave the selector switch on “all” - someday you’ll forget and run down both batteries and not be able to start.
So I would do one if the following
1) get a deep cycle battery (not an AGM
which don’t last long either in deep cycle use) Always leave the selector set to that battery (except maybe one day a month to keep the other battery charged). If you ever can’t start your engine
because the deep cycle “house” is too discharged, switch to the start battery. If you go this route
consider getting a bigger than group 24 battery. A group 27 or even 31 will let you enjoy anchoring
out over night much more. Of course you need room and a suitable battery box
2) get rid of the start battery and buy two deep cycle. This was likely how it came from Sabre. Batteries only last about five years so your current
battery may be near the end of its life anyways.
3) with the help of an electrician or knowledgeable friend rewire the boat
to take out the selector switch. The start battery then only starts the engine. A new deep cycle house battery does everything else. This arrangement usually has a “parallel” relay to momentarily connect both batteries together if the start battery is dead to get the engine started.
And you are right that you should not be able to turn anything on when the battery switch is set to “off” (except the bilge
pump). Something is wired wrong.
Here’s a long but excellent discussion by MaineSail https://marinehowto.com/1-2-both-bat...onsiderations/