What is VRLA and what does “valve regulated” mean?

VRLA means the cell is sealed with a pressure relief valve and the electrolyte is immobilised, in a glass mat separator (AGM) or a thixotropic gel. Gas generated on charge recombines internally rather than escaping, so the battery needs no watering and can be installed in orientations and locations a flooded cell cannot. Recombination is not perfect: under over-charge or high temperature the valves vent hydrogen, which is why ventilation is still required.