Some AGMS can indeed safely accept huge charging amps, upto about 80% state of charge, but that last 20% still takes 3.5 hours minimum. the whole 5x faster was never a believeable claim. When AGMS were first brought to mass market they were said to be immune to sulfation. Perhaps thats when the 5x faster claim came out too and the marketers were staining their shorts at the potential profits of their fables.
Many cheaper AGMS say to limit charging amps to 30% of capacity.
Only a few like Lifeline, Northstar, and Odyssey say there is No changing amperage limit, as long as voltage is not allowed past ~14.7v. Lifeline says no LEss than 20% and that more is better. Odyssey says no less than 40% when deeply cycled. My Northstar responds wonderfully to high amperage from a well depleted state, and it is now over 1000 deep cycles and 5 years of age.
When new I could get it fully charged in about 4.5 hours from well less than 50% charged with 65+ amps available, NOw it takes nearly 4.5 hours to get from 80% to 100% when held at 14.7v
AGMs on regular automotive forums, are now seemingly thought of as a far superior battery, and as usual low price sells, and manufacturers respond with the cheapest product they can make which is still AGM.
I doubt these starter specific AGMS last any longer underhood as starting batteries, but at least the terminals will not corrode as fast and the letters AGM give warm and fuzzies and perhaps bragging rights to those that do not need them
