Redox flow battery and method of operation

The invention relates to a redox flow battery (1) comprising a cell assembly (2) and a tank device (3) for accommodating an electrolyte, the battery (1) comprising a measuring device (4) for determining an open circuit voltage and a circulation module, and the measuring device for determining the open circuit voltage comprises at least one measuring cell (4.1, 4.2, 4.3) and at least four interfaces, one connection is provided for conveying anolyte, one connection is provided for discharging anolyte, one connection is provided for conveying catholyte, and one connection is provided for discharging catholyte, and wherein the circulation module comprises at least one pump head (8.1, 8.2) and at least two pump impellers (9.1, 9.2), and wherein the pump impellers (9.1, 9.2) are connected to the pump head (8.1, 8.2). At least one measuring element is integrated in the pump head, and wherein a connection of the measuring device is connected to the pressure side of the pump impeller, and wherein the associated connec