Phil, if they won't take any more amps at a higher voltage then AFAIK (not much!) that means they are truly full.
I don't know what the story is on charging at less than the rated charging spec with AGMs. Seems it is ok in your case--it just takes longer.
With Wets it is different, where you must get them to "gas" and 13.8 won't cut it--needs 14.x for gassing. My "learner set" of Wet batts never got up to proper SG with the 13.8 converter no matter how many days I left it on them. Only saw proper SG when I got a charger that could do 14.x.
My particular AGMs want around 14.8 for charging and around 13.7 to Float, so that is what I do. They also specify max of 27 amps per 100AH, so I also stick to that limit. I have no reason to go outside their specs. If I did have a reason, I would have to deal with that (get different batteries or take my chances with these) but I don't, so no problem. :)
