I have a cruiser with 3 - 12 volt batteries installed, the boat is twin screw
and has 2 battery switches (1,2,all,off) type, and finally a 30 amp battery
charger with terminals for all 3 batteries.

I always leave the charger on at the dock with shorepower but the past couple
of weeks I have 'cooked' 2 of the 3 batteries (maintenance free, probably 5
years old). Friends on the dock are guessing that the battery charger is
broken. Although it still charges fine, they suspect that it doesn't shut off
when the batteries are fully charged, causing them to boil over and eventually
dry up.

Here's my questions:

I just purchased 3 'dual purpose' Exide brand marine batteries that are not
maintenance free so I can pop the caps and see if there's water in the cells.
Do you think I can continue to use the original battery charger and just watch
to see that they don't boil dry?

A new charger which will charge 3 batteries is around $300. (ouch). I can get
a guest charger (2611) for $100 that is 10 amp on 2 circuits, for 2 batteries.
What will happen if I connect 2 batteries to one of the terminals in the
charger? Will that work or will I break something?

My other thought is to only hook up 2 of the 3 batteries to the new charger and
leave the 3rd one to charge when the engine is running, but not use that 3rd
battery for any accessories. This seems like a logical approach to me.

Any comments?

Thanks.