VRLA Batteries Explained
These types of batteries were designed to operate at wider temperature ranges than the flooded wet cell and to be installed in different orientations.
For example, a flooded wet cell needs to be installed in the upright position. The VRLA battery does not. It can be installed in various orientations.
The reason for the construction techniques employed is driven by the fact that oxygen is a byproduct of charging process where water in the battery gets broken down into its hydrogen and oxygen components. This is especially true if the battery is subjected to an overcharge.
In a serviceable flooded wet cell, this is not much of a problem. The battery is designed so that it can periodically be refilled with distilled water and thus replace the hydrogen and water that have escaped.
However, the VRLA is designed so that it can be tilted without leaking dangerous electrolyte. In other words, it is sealed. Therefore as part of the design, the VRLA not only incorporates a means of keeping the battery from spilling, but it also includes a means of allowing the oxygen and hydrogen to recombine.
Still, like any battery, there is a risk of overcharging and thus the creation of gas pressure within the battery. Thus the existence of the valve. The valves offers a means of allowing the pressure built up within the overcharged battery to escape.
Types of VRLA Batteries
There are two types of VRLA batteries being sold today. They are known as AGM and Gel Batteries.
AGM stands for Absorbed Glass Mat. Fundamentally this is a very porous mat that separates the negative and positive plates of the battery. This mat not only absorbs a great deal of electrolyte, but it allows oxygen to migrate and recombine with hydrogen.
As the name infers, the electrolyte in this battery is a gel and thus will not spill. Like the AGM, it offers a means for oxygen recombination.