In situ bulk magnetization measurements reveal the state of charge of redox flow batteries.

We report two methods that use either NMR spectroscopy or direct magnetic susceptibility measurements for in situ (strictly online ) determination of the state of charge of redox flow batteries. These methods are demonstrated on the inorganic, redox-active potassium ferro/ferri cyanide catholyte cycled against 2,6-dihydroxyanthraquinone as the anolyte in a full cell, and should be applicable to a wide range of redox couples, provided that the magnetization of the electrolyte solution depends on its oxidation state.