What is an AGM Battery?
AGM batteries are a newer type of sealed battery using "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. The electrolyte (acid) is contained in the glass mats and will not spill, even if broken. This means that these batteries are non-hazardous, resulting in lower shipping costs. As there is no liquid to freeze and expand, they are practically immune from freezing damage as well.
AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.
Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.
Charging voltages are the same for AGM batteries as any standard battery. The internal resistance is extremely low and there is virtually no heating of the battery even under heavy charge and discharge currents.
AGM's have a very low self-discharge. 1% to 3% per month is typical. This allows the battery to sit in storage for much longer periods without charging than standard batteries.