The 12-volt outlet on a generator is for running 12-volt loads, not for charging 12-volt batteries. It'll work in a pinch for wet (and some AGM) batteries, but it's not the most efficient way to charge batteries. You can run a 40-50 amp charger off the AC outlet much more efficiently. The 8.3 amps won't even be 8.3 amps, it'll be more like 5.
The reason is says no to gel type batteries is that gel batteries require very specific voltages that are lower than that of wet and AGM batteries, and they are highly dependent on battery temperature, so a controlled, temperature-compensated charger should be used for those. More than a couple of tenths of a volt too low and it'll never, ever charge, and more than a couple too high and you'll fry the battery.
Wet cell and gel cell batteries at the same temperature, the wet cell might require 14.4 for the peak charging voltage, and the gel at the same temperature would require 13.8 volts. If the gel is fed 14.4 volts, then bubbles will be created in the electrolyte gel and the battery is toast.
Regular wet cell batteries, as a general rule of thumb (being temperature dependent, same as all batteries) might require between 13.8-14.1 for the bulk charging, and an AGM battery would require 14.4-14.8 volts (which is why you can't charge an AGM house bank and a wet cell or "maintenance-free" cranking battery at the same time with an alternator). The difference between AGM and wet cell, though, is the voltage and current relationship they require. For the most part, wet cells need constant current (amps going in) and the voltage can vary a great deal, while AGM batteries require constant voltage and the input amp current can vary all over the place.
With a 3-stage battery charger (Bulk, Absorption, Float) the Bulk stage handles about 80% of the charging where the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F.
The Absorption stage, (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged. This is because as the battery becomes more fully charge, its internal resistance rises and thus requires that much voltage (voltage is pressure, like water pressure, only amp pressure) to push the remaining amps into the battery. Because heat rises with resistance, the current is decreased to prevent excess heat.
The final Float stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.
But that's on a smart 3-stage charger, not a raw 12-volt outlet. On a 12-volt outlet you'll just get an uncontrolled voltage (could be anywhere from 13.1 to 14.1 volts depending on the load) and not nearly enough amps. With a wet cell battery, that's not a problem, because even if the charging voltage is a little low, as long as it's above 13.0 the battery will eventually get charged. May take a while, but it eventually will. With an AGM, if the internal voltage of the battery reaches 14.1 (which is about where it hits at the 80% mark) and the 12-volt outlet is cranking out 13.8 volts, you can readily see which voltage is pushing in which direction and all charging of the battery stops. Unlike a wet cell, when the charging voltage on a AGM battery ie less than the internal resistance of the battery, doesn't matter how long you leave it on the charger it'll never get fully charged.
So that's the problem with charging an AGM battery directly from a 12-volt outlet. It won't be damaged directly, like a gel battery would, but it'll be chronically undercharged, reducing the battery's lifespan.
"Must I run a charger specifically designed for AGM batteries off the 120 volt ac even when there is a good 12 volt output right there???"
Now you know there's
"good" and then there's
"good" when it comes to 12-volt outlets. A "good" 12 volt outlet isn't very good at all for charging an AGM battery. You can use it to throw a few amps into the battery in a pinch (although at about 5 amps it really won't be much), but you'll want to make sure it's properly and fully charged regularly (like every week).
"Isn't there some way to get around this?"
Yes, but my now I'm sure you realize you already answered your own question. I'd recommend most of the Samlex, QuickCharge or Iota chargers you can see
here. There are, of course, plenty of other options for chargers, but that'll give you an idea of the best way to go. Some of the Samplex chargers can be problematic when driven by a generator, so do make sure you call them (if you were to order one from that link) before you do.
