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Batteries are divided in two ways, by application (what they are used for) and construction (how they are built). The major applications are automotive, marine, and deep-cycle. Deep-cycle includes solar electric (PV), backup power, and RV and boat “house” batteries.
The major construction types are flooded (wet), gelled, and AGM (Absorbed Glass Mat). AGM batteries are also sometimes called “starved electrolyte” or “dry” because the fiberglass mat is only 95% saturated with sulfuric acid and there is no excess liquid. Flooded may be standard, with removable caps, or the so-called “maintenance free” (that means they are designed to die one week after the warranty runs out). All gelled are sealed and a few are “valve regulated” which means that a tiny valve keeps a slight positive pressure. Nearly all AGM batteries are sealed valve regulated (commonly referred to as “VRLA” – Valve Regulated Lead-Acid). Most valve regulated are under some pressure – 1 to 4 psi at sea level.