There is some misinformation above.
An AGM battery (like Optima and Odyssey) are
not gel batteries. The electrolyte in an AGM battery is
not a gel or a paste. The lead plates in an AGM battery are separated by a boron silicate glass mesh like thing. The AGM battery uses a regular sulphuric acid / water electrolyte
liquid. The
liquid is
absorbed into the fine spaces in the
glass mat. That's where the initials
AGM come from. Even if the case is cracked or cut the liquid acid will not run out of the separator mats.
An AGM battery is not truly sealed. There are valves that will release pressure if the battery is charged at too high a rate. When treated properly they should never off-gas. It is possible though. When being charged hydrogen and oxygen are released just as with a standard flooded battery. However the AGM battery has catalyst material that causes the gases to recombine into liquid form. An AGM battery should have its maximum amp charge rate listed someplace; you may need to download a spec sheet. There are also some brands of AGM's that can sustain very high charge rates without damage. This is because the internal resistance of an AGM is usually considerably less than a flooded battery. Low internal resistance means less heat is produced. There will also be a maximum voltage recommendation. An Optima (any color top) can be charged at up to 10 amps and up to 15 volts going by the mfg
specs. Higher amps can be used for rapid charging but then it is recommended to monitor the temperature. Keep the battery below 125 F.
It is true that a flooded battery is generally more forgiving to excessive charge amps and volts than an AGM or a GEL cell battery. As long as the plates are completely covered, that is. Excessive heat can damage a flooded battery so keeping the temperature below 125 F is important.
Both a flooded battery and an AGM battery can
generally be charged with the
same charger. Check the mfg specs though just to be certain. Look at it like this; Optima and Odyssey both sell AGM's for use a replacement batteries for all motor vehicles.There are other brands as well. None of those require any vehicle adjustments to use the AGM battery.
A GEL cell battery
does require a
different charge protocol. Usually the charge rate is critical; too high and the GEL battery dies early. However, once again check the specs. There is at least one GEL cell battery whose mfg claims a standard charger can be used, IIRC. I do not recall the name or details.
Hot Rod magazine
article on AGM's as used by auto enthusiasts.
Some AGM batteries used in the renewable energy industry have no current limits for charging. Once again, read the battery mfg specs.
Article from alt-energy dealer on batteries in general.
AGM's have a lower self discharge rate than flooded cell batteries. A trickle charger may be unnecessary, unless many many months pass between uses / chargers.. I leave an ATV with an AGM battery at our place in the mountains. In winter there can be as long as 4 months between uses. I have no problems and no trickle charger. Your experience may vary, but it is not nearly the worry as with a lead acid flooded battery.