a year or so ago, i got one of those pulse battery chargers that supposedly work on any type of battery.
i had a sears AGM battery that only had a .07v reading, as it had sat for many years.
i hooked up a regular lead acid battery with jumper cables [4ga] and let them hooked up for a couple days, then hooked up the pulse charger to both of them for quite a while.
i let them both set for a couple of days, then hooked up the pulse charger again. after a while, both checked at 12.2v.
i then charged the lead acid battery up. that didn't take long, then hooked the charger to the AGM battery. it was on for quite a while, but didn't get over 12.2v.
i then let it sit for a couple of days, and it [the AGM battery] registered 10.3. after a couple of weeks, it checked at 9.7. the last time i checked it, it was 5.62. that was a few days ago.
a guy could get it back to 12.2 easy enough [i think], but i'm guessing it wouldn't load test fer $hit. might be a good battery for testing circuits not requiring much load.
this is just my trial and error experiment. i don't know if it relates in any way to the topic at hand, but i found it interesting i could charge an almost completely zero volt battery back to 12 volts, and after many months of sitting around, it still has 5.62 volts registering.