NOVELTY - The valve (10) has a valve seal element (26) provided with an annular closed carrier body (30) and a sealing membrane (32). The carrier body is arranged on a housing wall (16b) that limits an exhaust opening (20). The membrane fits into the valve seal element, where an idle edge section (34) of the membrane is pre-tensioned at a seal surface (36) of a housing (16). The membrane is pressurized by a pressure medium (22) until a defined opening pressure is reached. The edge section is removable from an open-sided edge of the seal surface by increasing pressure progressively in a radial direction.    USE - Medium-operated relief valve for ventilating and degassing a battery cell of a container such as a storage battery (all claimed) e.g. valve regulated lead acidic (VRLA) battery, nickel cadmium battery and nickel metal hydride battery in a motor car. Can also be used for a storage or disposal tank and a gearbox.    ADVANTAGE - The sealing membrane is pressurized by the pressure medium until the defined opening pressure is reached, and the idle edge section is removable from the open-sided edge of the seal surface by increasing the pressure progressively in the radial direction, so that deviation of actual opening pressure values of the valve from a given opening pressure is reduced, thus enabling pressure control and ventilation/ degassing of the container in a reliable and reproducible manner even during long-term usage of the valve, while ensuring better sealing effect against contaminations in an improved and inexpensive manner. The valve safely prevents discharge of a battery acid.    DETAILED DESCRIPTION - The pressure medium is hydrogen gas. INDEPENDENT CLAIMS are also included for the following:    (1) a valve seal element for a relief valve    (2) a container.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a relief valve comprising an elastomeric valve seal element for control of an inner pressure of a container.    Medium-operated valve (10)    Housing (16)    Housing wall (16b)    Exhaust opening (20)    Pressure medium (22)    Valve seal element (26)    Annular closed carrier body (30)    Sealing membrane (32)    Idle edge section (34)    Seal surface (36)