Absorbed Glass Mat
In the absorbed glass mat design, or AGM for short, the spacer between the cells is replaced by a
glass fibre mat soaked in electrolyte. There is only enough electrolyte in the mat to keep it wet, and if the battery is punctured the electrolyte will not flow out of the mats. Likewise, the mat greatly reduces evaporation, to the point that the batteries do not require periodic refilling of the water. This combination of features allows the battery to be completely sealed, which makes them useful in portable devices and similar roles.
To address the formation of hydrogen gas during discharge, calcium is added to the plates to absorb the gas. This only works during slow discharges, and gas buildup remains a problem when the battery is deeply or rapidly discharged. To deal with these events, AGMs often include a one-way blow-off valve, and are often known as "valve regulated lead-acid", or VRLA, designs.
Another advantage to the AGM design is that the electrolyte becomes the separator material, and mechanically strong. This allows the plate stack to be compressed together in the battery shell, slightly increasing energy density compared to liquid or gel versions. AGM batteries often show a characteristic "bulging" in their shells when built in common rectangular shapes.
The mat also prevents the vertical motion of the electrolyte within the battery. When a normal wet cell is stored in a discharged state, the heavier acid molecules tend to settle to the bottom of the battery, causing the electrolyte to stratify. When the battery is then used, the majority of the current flows only in this area, and the bottom of the plates tend to wear out rapidly. This is one of the reasons a conventional car battery can be ruined by leaving it stored for a long period and then used and recharged. The mat significantly prevents this stratification, eliminating the need to periodically shake the batteries, boil them, or run an "equalization charge" through them to mix the electrolyte. Stratification also causes the upper layers of the battery to become almost completely water, which can freeze in cold weather, AGMs are significantly less susceptible to damage due to low-temperature use.
