Method for monitoring the state of a redox flow battery system

A method for monitoring a state of a redox flow battery system on the basis of vanadium, wherein the redox flow battery system comprises at least two battery modules, a bidirectional converter, and a control device,

• wherein the at least two battery modules are connected in series and are connected to the bidirectional converter,
• wherein each battery module of the at least two battery modules comprises a cell arrangement having a plurality of redox flow cells, a measurement device for detecting a potential difference of the cell arrangement, and a reservoir for storing negative and positive electrolyte and for supplying the cell arrangement with electrolyte,
• wherein the control device is configured to detect operating states of the bidirectional converter and to control pumps in the at least two battery modules,
• wherein the method comprises
• S1:
• identifying at least one battery module of the at least two battery modules with a suspicion of shifted electrolyte;
• S2:
• switching off the pumps of the at least one identified battery module at a time t1 while the redox flow battery system is in a discharge operating state;
• S3:
• repeatedly detecting potential difference values of the at least one identified battery module up to a later time t2, whereby the pumps of the at least one identified battery module remain switched off;
• and
• S4:
• determining an average oxidation state (AOS) of the at least one identified battery module from the potential difference values detected in step S3,
• wherein a duration of time interval Δt=t2−t1 is selected such that, at time t2, a potential difference of the at least one identified battery module is negative, but overcharging of the electrolyte located in the cell arrangement-(2) of the at least one identified battery module is avoided,
• wherein the redox flow battery system is discharged in the time interval [t1, t2], and
• wherein determining of the AOS in step S4 is redundantly performed from positive and negative potential difference values.