US Patent 6949310 Leak proof pressure relief valve for secondary batteries

Secondary batteries are subjected to power cycling, and those which have a liquid electrolyte within the interior volume are equipped with a pressure relief valve. A void space over the liquid electrolyte may be subjected to gas pressure variations with respect to the ambient. The pressure relief valve structure comprises a vertically oriented gas flow chamber, and an outer relief vent adapted to open only when gas pressure within the battery and the gas flow chamber exceeds a predetermined pressure with respect to the ambient pressure. The gas flow chamber extends below the top surface of the battery, and has a dome-like cap which extends downwardly and has a plurality of openings so as to permit unimpeded gas flow between the interior of the battery and the gas flow chamber. There is a porous membrane covering the dome which permits gas flow between the interior of the battery and the gas flow chamber, but precludes flow of liquid electrolyte from the interior volume of the battery to the gas flow chamber.