Abstract
We describe methods for assessment of the state of charge (SOC) of vanadium redox flow batteries during operation using UV-vis spectrophotometry and full cell OCV measurements. The absorbance of the negative electrolyte solution at 433 nm, 600 nm and 750 nm is linear in SOC for SOC > 0.2. The absorbance of the positive electrolyte does not display a linear relationship with SOC, which we attribute to the formation of a strongly absorbing third species. An analysis of the excess absorbance of the positive electrolyte suggests that a complex with 1:1 stoichiometry is formed by VO2+ and VO2+. Full-cell OCV measurements were found to be an inaccurate gauge of the state of charge of the battery.