I dropped off my old and mostly defunct AGM's at AutoZone, who gives you a $10 gift card per dead car battery you bring in. Fantastic to know, especially with the stack of dead batteries back home in the garage I'd been keeping around to avoid the next "core charge" nonsense, if and when I ever got another lead acid battery.
So far, so good. They charged fast and even finally hit the float stage (the AGMs would just draw current, forever, probably just emitting a warm glow....) Strong and stable voltage (~13.2V). I'll run them through the paces tonight!