AGM is a newer type of sealed battery uses “Absorbed Glass Mats”, or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These
type of batteries have all the advantages of gelled, but can take much
batteries have several advantages over both gelled and wet (flooded)
batteries. Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.
Nearly all AGM batteries are “recombinant” — meaning 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.
The charging voltages are the same as for any standard battery — no
need for any special adjustments or problems with incompatible chargers
or charge controls. And, since the internal resistance is extremely low,
there is almost no heating of the battery even under heavy charge and
discharge currents. The Concorde (and most AGM) batteries have no charge
or discharge current limits.
AGMs have a very low self-discharge — from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.
AGMs 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.
Even with all the advantages listed above, there is still a place for the
standard flooded deep cycle battery. AGMs will cost 2 to 3 times as
much as flooded batteries of the same capacity. In many installations,
where the batteries are set in an area where you don’t have to worry
about fumes or leakage, a standard or industrial deep cycle is a better
economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes.