Abstract: The invention relates to a pressure compensation system having a safety function for an electrolytic tank of flow batteries, in particular, vanadium redox flow batteries, and a head portion (5) of the electrolytic tank (3, 4) is connected to the surrounding area (2) of the flow battery via a pipeline (6), in which a primary bi-directional pressure compensation valve (7) is situated, and a bypass line (9, 20), in which a secondary bi-directional pressure compensation valve (10) having a second response pressure greater than the first response pressure is situated, branches off from the pipeline (6) having the primary pressure compensation valve (7) having a first response pressure, and the outlet (11) of said bypass line is situated within a housing (13) surrounding the electrolytic tanks (3, 4).