I noticed that a couple of my batteries are due to be replaced. I have been running three deep cycle and it has been working well.

Looking at my options today, I'm wondering if my single starting battery should be a starting battery and not a deep cycle? I have the other two on a circuit for my sound system.

I am guessing if it isn't broke don't fix it, but I'm curious at least for my education.

I would get one marine starting battery.

Starting or dual purpose batteries are designed to be maintained at a certain voltage and have more cranking amps.
Deep cycles are designed to be run up and down frequently. They like a slow charge rather than being charged with a 65amp alt.

I'm guessing I also take a pretty good amount of weight with the smaller battery too.

I agree with Griff. I want a marine starting battery to run the engine(s) and deep cycle for house batts.

Looked at Cost Co. They label their big batteries "starting/deep cycle." Is that a contradiction?

750 mca 115 amp hours for $85

No, they are dual purpose, but really are a deep cycle.

Here is an Interstate chart showing the differences http://www.interstatebatteries.com/c...o/marine_f.asp
Notice how the cranking batteries have a lot more CCA/MCA compared to the same size deep cycle.

Been using 2 group 27 deep cycles as house/start bank on 1 motor and a group 24 start battery on other motor for yrs no problem. Using 27's for the reserve capacity since the cruiser has a different demand than the Fountain
I don't get caught up in the big CCA numbers cause I'm not boating when it's zero deg outside where the CCA is more of a factor.

The Fountain has group 24 battery's and turn the 502's over just as good as the previous 27's did.

You can't beat a good AGM dual purpose battery IMO. The big CCA starting batterys have a big downside and you should only need them if you really have an engine that's especially difficult to spin and start. A modern fuel injected marine engine should not need a specialty starting battery.