 Both flooded and VRLA designs require suitable ventilation around the batteries; both to prevent highly flammable hydrogen gas concentrations from building up and ensure that the batteries receive adequate cooling.VRLA cells may be made of flat plates similar to a conventional flooded lead acid battery, or may be made in a spiral roll form to make cylindrical cells.VRLA batteries have a pressure relief valve which will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged.