VRLA batteries are sealed, usually within polypropylene plastic. They were developed because they have the advantage of containing no sloshing acid that might leak or drip out when inverted or handled roughly.
The term valve-regulated refers to the method of gas release. If the gas pressure becomes too great inside the battery, the valve will vent when it reaches a certain pressure. During the charging of a lead-acid battery, hydrogen is normally liberated. In a vented battery, the hydrogen escapes into the atmosphere. In a VRLA battery, the hydrogen recombines with oxygen inside battery, so water loss is minimized. Under normal float conditions, virtually all the hydrogen and oxygen are recombined. Re-sealable valves vent non-recombined gases only when pressure exceeds a safety threshold.
A VRLA battery is distinguished from a flooded-cell battery by the rate at which oxygen is evolved from the positive plate and diffused to the negative plate, ultimately forming water. This rate is several orders of magnitude faster than a flooded-cell battery. Because water can’t be added, its recombination is critical to the life and health of the battery.
Any factor that increases the evaporation rate or water loss—such as ambient temperature and heat from the charging current—reduces the battery life.