12v AGM and 12v flooded are not to be put in the same category.
Trojan and USbattery make a group size GC12 'Floor Scrubber' battery. It is a Smidge longer by about 1/2 inch a bit wider by about 1/4 to 3/8" and about an inch taller than a group 31, and about 20 pounds heavier and ~20 more AH capaity 150AH capacity. Pretty sure East penn Deka make a GC12 size too.
These are true flooded deep cycle 12v batteries. Besides Roll$ $urrette, and a few other larger Trojan offerings, i know of no other 12v flooded batteries which Qualify as true deep cycle, but if you've come across in your extensive internet research in the last 6 months, and have a list of these 'quite a few' other 12v true deep cycle flooded lead acid batteries John, bust it out.
PIcking Nits, but the takeaaway is that a easily available flooded Marine/rv/trolling 12v battery, is commonly called/ stickered as a deep cycle battery by their purveyors, is marketing gone wrong and allowing people to misplace their confidence in an inferior product for the task of deep cycling. True deep cycle 12v exist, but not in any of the standard group 24/27/31 sizes
GC2 6v batteries are simply a 12v battery in two 60+ Lb pieces.
The Costco by me sells interstate GC-2 batteries for 84$ a piece. These are infact Trojan t-605 batteries relabelled by interstate, at least in thie region of the country( socal). They, at 58 Lbs are not as durable as t-105s(62 Lbs) which are pretty much the benchmark 6v GC-2 battery, Although USbattery would not be happy with that statement. What Sams club and COstco sell, well they could be made by different manufacturers in different regions of North America, so saying that COstco's lowest price GC-2s are not as good as Sams club GC-2s might or might not be true. Bring a bathroom scale, Heavier is better.
If one likes the Interstate brand, well they can get/order the GC12 size, likely made by USbattery, but most Interstate retailers likely will NOT stock it, and I would not want one if they did as it would likely be old and sulfated even if never used.
Crown makes a GC12 12v flooded battery too, which I would expect to be true deep cycle.
https://www.thesolarbiz.com/media/catalog/product/c/r/Crown-CR-GC150-Data-Sheet.pdf
Interesting that they say minimum recharge amperage is no less than 18 amps, Which is smak dab in the middle of trojans's 10 to 13% recommendation, 10 to 13 amps per 100AH, assuming a plug in charger and 12 hours minimum to recharge. Solar throws a wrench into that recommendation, but more solar wattage is mo Better.
If someone only requires a 150AH capacity battery, versus the ~220AH of a pair of GC-2s in series, then the GC12 size group will not be much more expensive than a group 31 flooded marine battery, and will likely last 2x as many cycles in the same usage patterns.
The 12v flooded marine batteries in the group 31 size would only be superior to the GC-12 in CCA for engine cranking, or powering high inverter loads. Trying to Stuff thick deepcycle plates in a group 31 or smaller size flooded battery casing yields very poor CCA and AH capacity. The easily available 12v flooded marine battery is closer to a starter battery in construction than it is to a deep cycle battery and it is a crime that they market them as 'deep cycle' . That is kind of like calling a campervan a track car, because one could indeed drive it around a track.
AGMS with their lower resistance can have the thicker plates and still yield nearly as many CCA as a flooded starter battery of the same size. Odyssey and Northstar AGM's can smoke other batteries of the same size group In CCA due to their very low resistance, but these are especially finicky as to their recharging regiment when deeply cycled. These thin plate pure lead AGMS prefer to be high amp recharged when deeply cycled.
Low and slow trickle charging AGMS time after time when deep cycling them tickles them to death. Those looking into AGMS would be wise to plan on somehow being able to deliver them at least 30 amps per 100AH of capacity when most depleted, at least once in a while. Odyssye states flat out 40 amps per 100Ah of capacity when deeply cycled. My 90Ah group 27 Northstar responds so well to high amperages. I occassionally intentionally drain it below 50% so that it can gobble 65 amps for longer, and it appears to love this treatement as much as it dislikes the low and slow solar only recharge regiment when deeply cycled.
At this moment, my battery monitor reads 12.0v under a 9.6 amp load, and 45Ah from full. Even at ~600 deep cycles this 90Ah battery is outperforming all the many previous batteries I have cycled to death in the last 16 years, and that includes when I had 230Ah of flooded house battery capacity. Two good quality group 27 Crown flooded marine batteries rarely lasted more than 2 years with my former charging abilities. This single Northstar AGM will turn 4 years old in November this year, but that means I can and do charge it fully, promptly, and often, and often at a very high rate, much to the chagrin of the 'trickle charge everything always' brigade that populate automotive forums, and parrot their unwise imprecision.
