Not to belabor the point, but this is where the agm's really shine. Totally discharging, and then letting it sit that way for a while, is usually the death blow to regular flooded cell batteries. Agm's seem to shrug this abuse off and keep on ticking. Turning the selector swith to "ALL" and letting the boat sit overnite, or connecting jumper cables between a charged and the dead battery and letting them sit for a day is a good way to bring an agm style back from the dead. Once you do this, just give it a regular slow trickle charge and you'll be good to go. This "equalizing" trick was passed on to me from the manufacturer of our agm's.

Update:

I purchased two Interstate Cranking batteries (same as what came with my Cobalt) Model 24m-xhd, 800 cca, 1000 mca, and reserve capacity of 135 minutes. I have them sitting in the garage floor right now. Interstate Batteries are also located in Dallas, so I thought I would keep it local:)

Should I hook them up to a battery tender before I place them in the boat? I have an onboard charger, but I guess it may not hurt to ensure they are fully charged before I put them in.

Question: If I take a reading with a voltmeter, what should it read to indicate it is fully charged?

Anyone???

I'd fully charge them before putting them in.
Conventional wisdom is to not store them on the concrete floor. Conventional wisdom is not always correct though, and this may be urban legend that storing on bare concrete will drain the batteries.
2.2 Volts per cell, so the total fully charged should read 13.2 volts.

I can assure you that storing them on bare concrete does NOT drain them. I have been storing batteries (a least three different brands too) that way for 7 years now and they hold their charge just fine.

According to Kevin Falvey of Boating Magazine "when tested at the terminals a fully charge 12 volt battery should read 12.6 volts nominally."

"A dead 12 volt will register 11.5 volts on your multimeter."

FWIW, I have been using a pair of West Marine deep cycle group 27s to crank my old carbed 4.3, since 2003 and they have never failed to start it. I used the deep cycles because this boat is left on a mooring, and if there was a period of really heavy rain and a leak in the cover, I want the bilge pump to be able to keep up, something that deep cycles are very good at. In fact we had a period of very heavy rain back in Oct 2005, 4 boats sank because of failed batteries in our harbor out back. But the old 4.3 cranked over, on the same batt that pumped all the water out, for that time peiord. But if you have a FI engine, you may have to go with what Volvo/Merc recommends and use a dual purpose.

First, good choice. I wouldn't use a 650cca in a V8.

Second, you can't accurately tell the state of charge of a battery with a voltmeter. If you have a trickle charger, just cook it overnight. A dedicated battery tender is meant to keep a charged battery charged.

I like to see at least 200 amps @10v on a 15 second carbon pile load test. If it is less than that, I use a fast charge rate and monitor amps and volts. It has taken years, but I can pick out a good, bad or marginal battery very easily.

Dave, a gear-reduction starter typically draws around 100 amps on a V8, more in colder weather.

Remember, alternators are not battery chargers. They will meet their demise in a premature fashion if they are repeatedly used as a charger, rather than a maintainer.

Oh, and the cement floor self-discharge is a wife's tale. Always keep your boat or car battery(s) clean, as dirty batteries will self-discharge.

Thanks Bob. I have them ready to go in the boat. Just need to get a little motivated to haul them over and put them in.

I should have done it today, but got caught up in honey-do lists.

I would like to add to this by asking a similar question.
My battery is past borrowed time and am looking for its replacement.

I have been using a Trojan Starting battery 24SM-4 with 700 CCA @ 0 degrees F and 860 MCA @ 32 degrees F with 125 min reserve @ 25 amps. I checked out an auto place and Wally World over lunch and most of the 24 series batteries have much lower CCA and I can't get the same ratings unless going to a 27 series battery but then the battery is too big. Any suggestions? I still have many other battery places to explore.

I have a Sea Ray 180 with a 3.0L.

If I would change batteries today I get a GSM type. For the sizes they offer more CCA or reserve. Check it out.

To crank a 3.0 assuming it's in good shape you shouldn't need that much in the way of CCA or MCA. I use 2 group 27s for my 88 FW, I had a pair of WM deep cycles but recently replaced them with a pair of Deka gp 27 dual purpose. I need the dual purpose because this is a moored boat and if there is a storm and water gets in the cover (some does no matter what you do with water proofing) the bilge pump will need to run a bit, probabay not enough to kill the batt, but I don't like to take chances with that, so I don't use a starting only batt.
Their dual purpose gp 24 gives you 550 cca (at 0*F) and 685 mca (at 32*F). I would think that would suffice for a 3.0. The gp 27 gives 650 cca and 810 mca, fine for my 4.3 V-6. It is not that much bigger, are you sure it will not fit? It's basically 1 1/2 in longer same width and height. Weight is 49 lbs for the 27 and 42 lbs for the 42. Trojans are known as the top of the heap in batteries, why not just get another one of those? To have those specs I bet it's an AGM battery which are great but usually about 2x the price of a conventional. A starting only batt will always have higher cranking amps than a dual purpose or pure deep cycle. The Dekas were about 79 each at a local marine shop, good deal for a gp 27. If you trailer the boat and you don't have a lot of accessories running the starting only batt will be fine. Deka's starting only gp 24 comes in various cca levels, ranging from 460 to 875, the higher cca ones weight more, all are the same size gp 24. So if a starting only batt is fine for you, that's a cheaper alternative to the Trojan.