VRLA batteries are sealed, usually within polypropylene plastic. They were developed because they have the advantage of containing no sloshing liquid that might leak or drip out when inverted or handled roughly. The term valve-regulated refers to the method of gas release.
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 a VRLA 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.