The AGM hermetic battery (not to be confused with the GEL battery) is an accumulator that does not contain liquid acid but has absorbent felt inside it.
This material traps battery acid like a sponge while making it available for plates which, being very compact, make the battery very resistant to vibrations.
AGM accumulators are ideal both for uses in cyclic use (i.e. those that require many charge/discharge cycles) and for buffer use (i.e. as an energy reserve that remains available unused for a long time).