Zhijiang Tang

Monitoring the State of Charge of Operating Vanadium Redox Flow Batteries

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 VO 2+ and VO 2 + . Full-cell OCV measurements were found to be an inaccurate gauge of the state of charge of the battery.