Absorbent Glass Mat (AGM)

AGM technology became popular in the early 1980s as a sealed lead acid battery for military aircraft, vehicles and UPS to reduce weight and improve reliability. The acid is absorbed by a very fine fiberglass mat, making the battery spill-proof. This enables shipment without hazardous material restrictions. The plates can be made flat to resemble a standard flooded lead acid pack in a rectangular case; they can also be wound into a cylindrical cell.

AGM has very low internal resistance, is capable to deliver high currents on demand and offers a relatively long service life, even when deep-cycled. AGM is maintenance free, provides good electrical reliability and is lighter than the flooded lead acid type. It stands up well to low temperatures and has a low self-discharge. The leading advantages are a charge that is up to five times faster than the flooded version, and the ability to deep cycle. AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life.  The negatives are slightly lower specific energy and higher manufacturing costs that the flooded. AGM has a sweet spot in midsize packs from 30 to 100Ah and is less suitable for large UPS system.  

AGM batteries are commonly built to size and are found in high-end vehicles to run power-hungry accessories such as heated seats, steering wheels, mirrors and windshields. NASCAR and other auto racing leagues choose AGM products because they are vibration resistant. AGM is the preferred battery for upscale motorcycles. Being sealed, AGM reduces acid spilling in an accident, lowers the weight for the same performance and allows installation at odd angles. Because of good performance at cold temperatures, AGM batteries are also used for marine, motor home and robotic applications.

Ever since Cadillac introduced the electric starter motor in 1912, lead acid became the natural choice to crank the engine. The classic flooded type is, however, not robust enough for the start-stop function and most batteries in a micro-hybrid car are AGM. Repeated cycling of a regular flooded type causes a sharp capacity fade after two years of use. See Heat, Loading and Battery Life.

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. These batteries can be charged to 2.40V/cell (and higher) without problem; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive. See Charging Lead Acid.

AGM and other sealed batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49°C (120°F). While regular lead acid batteries need a topping charge every six months to prevent the buildup of sulfation, AGM batteries are less prone to this and can sit in storage for longer before a charge becomes necessary. Table 1 spells out the advantages and limitations of AGM.
 

Advantages	Spill-proof through acid encapsulation in matting technology High specific power, low internal resistance, responsive to load Up to 5 times faster charge than with flooded technology Better cycle life than with flooded systems Water retention (oxygen and hydrogen combine to produce water) Vibration resistance due to sandwich construction Stands up well to cold temperature
Limitations	Higher manufacturing cost than flooded (but cheaper than gel) Sensitive to overcharging (gel has tighter tolerances than AGM) Capacity has gradual decline (gel has a performance dome) Low specific energy Must be stored in charged condition (less critical than flooded) Not environmentally friendly (has less electrolyte, lead that flooded)

Table 4: Advantages and limitations AGM. The gel system shares many of the characteristics.