Unfortunately the ALT does not decide what amps the batts see the batts do. A 280 amp alt does not have 280 amps needing or having to go somewhere the batteries have to accept the current from the alt for the alt to put it out. Most all regulators are simply driving voltage or voltage / time with some external regs some can also limit amps. Unless you have a constant current alternator, not a voltage driven system, as most all are on boats, then the batts decide on what they will accept. I have a 100 amp bench charger and can charge even small lawnmower batteries and the batts generally take between 18-25% of the Ah rating if wets and depending upon age and condition. I can plug in my motorcycle battery and see 1-2 amps of charge off a 100 amp charger then hook up a 100 Ah group 31 and see about 18-25 amps. I can then hook up a 100Ah AGM and see over 80 amps, done it.. Same 100 amp charger three separate batteries all accepting different currents.

I think Balmar sums it up best:

"How much amperage is safe for my system?

Forget the rumor that an over-sized alternator will destroy your batteries ... the truth is that the acceptance rate of your batteries will dictate how much amperage the alternator will provide."

One thing not mentioned about AGM's is that if you have a stock alternator and a large AGM bank you may quickly fry your alt. I have seen this now on three separate occasions. I one of those instances a new replacement was installed (power boater) and it too failed in about two weeks time. By that time the old alt had been re-built, it was installed and then also cooked itself. After I got eh call I installed a Balmar reg and converted the alt to external regulation with temp sensing and dialed the output back in the settings and the alt is still running three years later. The dumb regulator in this instance will simply try to hit 14.2 volts on the bank and until it does it will allow the alt to put out max load. The previous wet cell bank was the same size but the acceptance was a max of about 75% of the alts rating. Max load into that AGM bank due to the very high acceptance was more than that alt could deliver so was not limited in any way and simply cooked itself.

If you got the money and want AGM's buy em but do make sure your system is up to snuff..