Method for monitoring the state of a redox flow battery system

A method for monitoring a state of a redox flow battery system, the method comprising:

• providing a battery system with at least two battery modules, a bidirectional converter, and a control device, wherein the at least two battery modules are connected in a series circuit and are connected to the bidirectional converter, and wherein each of the at least two battery modules includes a cell arrangement having a multiplicity of redox flow cells and a tank device for storing electrolytes including a negatively-charged electrolyte and a positively-charged electrolyte and for supplying the electrolytes to the cell arrangement,
• the battery system further including, for each of the at least two battery modules, a first switch arranged in series with the respective battery module, and a second switch arranged in a bypass line around the respective battery module and the first switch, and
• the control device being connected to each of the first and second switches configured for determining a respective switch position in order to switch the at least two battery modules into or out of the series circuit;
• step S1:
• driving the first and second switches in such a way that at least one battery module is switched out of the series circuit;
• step S2:
• discharging at least a sub-volume of the electrolytes of the at least one battery module which has been switched out of the series circuit in step S1, repeatedly detecting by the control device a potential difference using a measuring device connected to the control device and the at least two battery modules, the potential difference being formed between a first potential of the negatively-charged electrolyte and a second potential of the positively-charged electrolyte of the at least one switched-out battery module;
• step S3:
• determining by the control device a State of Health, SoH, of the at least one battery module which has been switched out of the series circuit in step S1 from the potential difference detected by the control device in step S2;
• and
• implementing steps S2 and S3 at least partially in parallel with one another, and terminating by the control device step S2 when step S3 produces one of the following results:
• detection by the control device of a potential step having a step level that exceeds a predefined threshold value, the predefined threshold value lying within an open circuit voltage, OCV, equivalence range of from 0.7 to 1.2 volts;
• or
• detection by the control device of two potential steps.