AGM batteries are a type of sealed lead-acid battery using "Absorbed Glass Mats" between the plates. This is a very fine fibre boron-silicate glass matting. Electrolyte (acid) is fully contained in the glass matting and will not spill, even if broken. The result is a deep cycle battery which is considered non-hazardous for transport.
Traditional flooded lead-acid deep cycle batteries off-gas hydrogen during charging. This is avoided when using a sealed AGM battery. No off-gassing occurs under normal charging and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in an AGM battery are tightly packed and rigidly mounted and typically withstand shock and vibration better than traditional flooded lead-acid batteries.
Nearly all AGM batteries are "recombinant", meaning oxygen and hydrogen generated during charging recombines inside the battery case. The recombining process is typically 99+% efficient, so almost no water is lost.
Internal resistance is lower in sealed AGM batteries than traditional flooded models, allowing more efficient charging. AGM batteries have a very low self-discharge rate of 1% to 3% per month on average at 25°C (77°F).