Types of Sealed Batteries (VRLA’s)
Sealed Lead Acid batteries are often referred to as being "maintenance free" or valve regulated. In a sealed lead acid battery the unit is sealed, meaning that water loss is kept to a bare minimum and also that you needn't put any water into the battery. Most of the sealed VRLA batteries are recombinant which essentially means that the water lost through the venting of hydrogen and oxygen in a flooded battery is
The most common types of Sealed Lead Acid batteries that we carry are the AGM (Absorbed Glass Mat) batteries; another is the GEL type of battery.
AGM batteries have a very fine Boron-Silicate glass mat in between the plates inside the battery. The mats are about 95% filled up with electrolytes rather than completely saturated. The huge advantage of this type of battery is that they can be operated in almost any orientation and will even function underwater. They will also never leak, even if punctured.
The Gel cells have the electrolyte stored in a gel form, this also won't spill out even if the batteries are broken and also prevents the stratification that other battery types can have. Gel cells however do have some limitations; they take a slower charge than a comparable sized AGM battery and also if they are overcharged voids can form in the gel which permanently reduces the battery capacity.
For deep cycle applications however the Sealed Lead Acid type of battery can be a good deal more expensive than a comparable Flooded Lead Acid battery. However there are times when a sealed battery will better meet your needs than a flooded one. Particularly marine applications where stability isn't always a given, and ventilated spaces are hard to come by seem well tailored to a sealed lead acid battery. One other advantage that AGM and Gel batteries have over flooded types is the low self discharge rate. Where a Flooded battery will lose about 13% of its charge in a month a Gel or AGM will lose 1% -3%.