Johnson controls acquisition of Optima AGM batteries had them go from many reports of ' gee i can't beleve this thing is still going', to many reports of 'it died in two years or less.'
Their rectangular AGM's.... who knows? You are my Guinea pig regarding them, unless my neighbors click 'place order'
If corrosion had formed on your ring terminals, it could easily have wicked up the stranding, imparting resistance between crimp and the copper stranding. I've seen one such wire, polished to perfection by a proud owner who declared it could not possibly be the problem, refuse to carry any current until I resquished the the barrell with a pair of channellocks, then it worked, but got really hot passing less than 20 amps.
It seems the Fiamms are viable. The JC AGMs resting full charge
voltage might be normal for that battery.
Separate them and check their resting voltages when fully charged or at same level of depletion occassionally, keep records you'll be able to see which of them has more capacity left with more observations and any which should be culled from the herd before they infect the others.
If anybody buys battery cables from the Autoparts stores, most of these use Stamped steel ring terminals. Steel tubing stamped over the copper stranding then a hole punched through both. These are horrible wire/cable terminations which WILL become highly resistive and heat up, and then become more so, and corrode faster.... The only question is how soon.
SAE wire gauge is aso 6 to 12% thinner than AWG wire with corresponding voltage drop, and autoparts stores packaged cables are SAE gauge.
Wirebrushing corroded terminals from green an white corrosion means the corossion has already wicked inside these stamped steel ring terminals. Cut the SOBs off and crimp on some real lugs.
Take a magnet to all conductor terminals. Those 'copper clamps' might attract a magnet.
Beware of marketers and their intentions, as they are not honest, only greedy.
