6v batteries in AGM format, likely do not share the same significant advantage over their AGM 12v counterparts as they so obviously do when comparing 12v flooded marine batteries to flooded 6v golf cart batteries.
The plates of 6v flooded GC batteries are usually at least 2x as thick as flooded 12v, but might not be any thicker in AGM 6v vs 12v. Sadly this plate thickness seems to be some guarded secret as it would be nice to know for sure on any given battery.
IF one wants AGM, then 4 100AH 12v group 31 batteries wired in parallel Balanced, should give the same amount of total cycles as 400 AH of 6v batteries in series parallel.
400AH of 12v AGM batteries wired properly in parallel 'should' be able to maintin higher voltages during high inverter loads, compared to 400AH of 6v AGM batteries in series parallel.
400AH of 12v AGM might also be able to recharge slightly faster than an Equal capacity of 6v AGM, IF one has HUGE plug in charging sources at their disposal, Like 120 amps or more. but the time saved would not be extreme.
So Consider that and the weight of moving individual batteries, especially the 4 and 8d sizes.
While not really a likely occurrence, if a 6v battery shorts a cell, then one needs to remove 2 of the 4 batteries.
If one 12v battery shorts a cell, only one battery need be removed from the parallel string.
But it might not be obvious 1 of 4 parallel 12v batteries shorted a cell. One might not catch it unless they suddenly notice lower overnight voltages and know something is not right, and investigate. AGMS tend to not short cells nearly as often as their flooded counterparts, but it is not unheard of.
My personal Wish List Deep cycle AGM battery is the 12v Lifeline GPL-30HT at 150AH, with their group 31 GPL-31XT at 125AH a close second.
If the AGM were to be a dual purpose, like engine starting and house loads, I would get a group 31 Odyssey or Northstar AGM, even though they only have 100AH capacity compared to the 125AH of the Lifeline gpl-31XT.
The Lifeline will outcycle Northstar or Odyssey AGM given equal good treatment. BAd treatment, the best battery will not last much longer than the worst.
Odyssey and Northstar are TPPL AGMS. Thin plate pure lead. They have Extremely high CCA figures Which can crank engines faster in all temperatures, or maintain higher voltages during high inverter loads, perhaps keeping the inverter's low voltage alarm quiet when microwaving that baked potato.
Lifeline Odyssey and Northstar all can accept HUGE recharging amperages, just limit voltage to 14.4 or 14.7, and are better off for receiving at least 20 amps per 100AH capacity for lifeline and at least 40 amps per 100Ah capcity Odyssey.
All other AGMS basically say to limit amperage to no more than 30%, or 30 amps per 100AH of capacity.
All Agms when deeply cycled to the 50% range appreciate currents approaching these maximums, as long as they are not 90F when starting the higher amp recharge.
FullRiver are a good AGM, likely the best AGM coming out of China.
Good to hear reports of the Vmaxtanks still going strong, but this can easily be a testament to proper recharging and shallow discharges, rather than battery quality. Reports on performance or longevity are almost meaningless unless it is known how the battery is/was treated, and that is about how deeply discharged it gets and how promptly and fully it was then recharged.
AGMS are a bit more finicky in regards to how often theuy require the true full recharge, compared to flooded batteries. Both are not happy undercharged, but the AGMS are less so. Their greater price and other advantages, does not make them more resilient in this regard.
So the end user should consider the need to be able to occassionally recharge their deeply cycled AGMS at a higher amperage rate, from their most depleted state, and hold absorption voltage until amps taper to 0.5% of capacity. A low and slow solar only recharged battery cycled deeply daily, without ever being recharged occassionally at a higher rate, is not really a happy AGM battery.
How many cycles one can go before the true 100% recharge should really be sought? Well I try to not go more than 5 cycles to 50%. I have gone 20, but it took something like 14 hours for amps to taper to 0.5% when held @14.7v at an initial 46% charge rate.
I've now got about 500 deep cycles in 39 months on my group 27 Northstar AGM at 90AH capacity. I expect about 800 total deep cycles, with a few hundred more cumulative shallow cycles,1 to 15%, before its performance makes me decide it is time to replace. But ultimately, time will tell. So far it is behaving better than Expected, but I also recharge it properly, often, but I also deplete it well below 50% often too.
Once/IF more positive reports come out, my wishlist AGM battery is the Firefly Oasis, as these are 'supposed' to be able to be returned to full capacity after many partial state of charge cycles which would damage other AGMS. They are 480$ for a 115Ah group 31. That is the only size.
