In a nutshell, here is the difference between an FLA (flooded lead-acid) battery, and an AGM (absorbed glass mat battery):
One has fiberglass matting between the lead plates and doesn't slosh.
That is all. Carry on.
In other words. What works for one, will work for the other.
So...yes, you can use your plain ol' automotive benchtop charger on an AGM battery. And...no, it won't get it fully charged. But...it won't get a flooded battery fully charged either. So...no difference there.
With any lead-acid battery (FLA, AGM or GEL), to get it fully charged, you have to let it soak up power until it can't soak up any more. At that point, the chemistry is said to be, "fully absorbed".
To get it really done right, you need to pump it up with "a bit of the old ultra-voltage" (yea, I liked A Clockwork Orange). For most FLAs and AGMs, that means taking it up to some higher than normal voltage and holding it there a while.
That's what a multi-stage charger does.
A 3-stage charger will do a "bulk stage" by raising the battery's voltage to (usually for most chargers) 14.4v, and then do an "absorb stage" by dropping the voltage to 14.2v and holding it there for a few hours, and then drop to "float stage" at something like 13.6v just to keep the battery from self-draining as it sits around.
That's pretty much the routine that all battery manufacturers recommend for both AGM and FLA batteries.
There are of course, a few exceptions to the rule. Interstate as I recall has one flooded battery that they recommend taking up to an unholy 15.8v (as if there is actually a charger on the market that can do that...).
But for the most part all AGMs and FLAs recommend the 14.4v/14.2v/13.6v 3-stage charge routine.
(GELs are different. They are like the females in the battery world. They are sensitive and must be treated with more care than AGMs and FLAs. They like a slightly lower voltage and don't you DARE go too high.)
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