Table : Definition of the total vanadium concentration C, mole fractions X i , and SOC for the posolyte and the negolyte under study as a function of the molar concentration of the four vanadium ions C i , i = {2, 3, 4, 5}. In the V IV /V V mixture C 45 denotes the concentration of the 1:1 stoichiometric mixed-valence complex V 2 O 3+ 3 at equilibrium with V IV and V V as described in AS7341 multi-spectral digital sensor from ams-OSRAM). The existence of such devices presents an opportunity to develop an affordable sensor for vanadium redox flow batteries, enhancing online diagnostic capabilities and improving the cost-benefit ratio, an essential factor for the widespread adoption of this technology.