Thanks for the info. I'll look up Guy's earlier posting, his info is usually very informative.
I have two group 27 batteries, currently both deep cycle batteries, with a stock 60 amp alternator - which is absolutely not a "smart" one... I generally set the switch to "both" when running the engine, then to "house" when discharging the batteries at anchor, etc. - which may change now that I know what my battery combiner can do.
I use the batteries lightly - never intentionally running them down past 50%, as I know what that can do to them. They've been running down lately, which could be from a combination of factors. So, I'm starting with replacing the batteries, as they are over two years old, and I have no idea what the P.O. did as far as taking care of them. I am of the "buy em' at Costco" school, take good care of them, but don't cry if you have to replace them at three years. My last deep cycle battery on my O'Day 222 lasted five years, and was still in amazing condition.
I was thinking that the combination starting/marine battery would have more cranking amps, and therefore start the engine better. I would then use it for starting only, rarely (like when I forget to move the switch) for other duties.
I guess my concern has to do with the alternator charging them unevenly, which may be alleviated by the battery combiner, right?
While I am at it, I am also considering the purchase of a battery isolator, so that I can hook up a dual battery charger, without worrying about electrolysis. That way I can use the house battery "guilt free" - (well, under 50%) knowing it will be charged next time I go down to the boat.
There are a lot of reasons the batteries could be running down, but I'm starting with the batteries. Any further comments from the Oracles of "E"?