An AGM battery is a lead-acid eletric storage battery that:

• is sealed using special pressure valves and should never be opened.
  is completely maintenance-free.
• has all of its electrolyte absorbed in separators consisting of a sponge-like mass of matted glass fibers.
• uses a recombination reaction to prevent the escape of hydrogen and oxygen gases normally lost in a flooded lead-acid battery (particularly in deep cycle applications).
• is non-spillable, and therefore can be operated in virtually any position. However, upside-down installation is not recommended.