I look at it this way. Chemistry, and specific gravity decide on the charge rate in amps at a set voltage point. The more sulfur on the plates, the greater the ionic potential to take it off the lead plates and put it back in ionic H2SO4 solution. It's all limited by chemistry and ionic potential. No sulfur or not much sulfur on the plates, not much potential to consume any amps to do so. Low SOC and lots of sulfur on the plate, 20 to 30% SOC, huge potential to take a lot of amps. Law of diminishing returns.
What I currently (pun intended) don't understand is how AGM batteries have so much more ionic potential to take huge amperage charging rates, and at an even lower voltage yet, at 14.4V? What is it that causes so much less resistance in the Chemistry? Denser acidic solution, less water? More chemically reactive?
