absorbed glass mat (AGM) battery
Absorbed glass mat (AGM) is a relatively new type of sealed battery construction that uses saturated absorbent glass (boron silicate) mats between the plates instead of a gelled or liquid electrolyte. AGM batteries are more expensive than the flooded (liquid) variety but offer a number of advantages.
Since all the electrolyte (acid) in AGM batteries is contained in the glass mats, it can't spill, even if the battery is broken. Being thus non-hazardous, AGM batteries are cheaper to ship. Additionally, since there's no liquid to freeze and expand, they're practically immune from freezing damage.
Nearly all AGM batteries are recombinant, which means that the oxygen and hydrogen recombine inside the battery. Recombinant batteries use gas phase transfer of oxygen to the negative plates to enable recombination back into water while charging, thereby preventing the loss of water through electrolysis. The recombining is typically more than 99% efficient.
The charging voltages are the same as for any standard battery, so there is no need for any special adjustments and no problems with incompatible chargers or charge controls. Since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.
AGMs have a very low self-discharge – from 1% to 3% per month is usual. As a result, they can sit in storage for much longer periods without charging than standard batteries.
Even under severe overcharge conditions, hydrogen emission of AGM batteries is far below the 4% maximum 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's still a place for the standard flooded deep cycle battery. AGMs 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.