Patent ID: 7855005

Claim:
A method for determining the state of charge of a redox flow cell battery, the redox flow cell battery comprising: at least one redox flow cell; an electrolyte composition comprising one or more metal ions, wherein the metal ions go between an oxidized form and a reduced form during charging/discharging of the redox flow cell battery; and at least one tank for holding the electrolyte composition, the tank coupled to the redox flow cell such that the tank supplies electrolyte composition to the redox flow cell during use, the method comprising: providing one or more test samples of the electrolyte composition while electrolyte composition is flowed between the tank to the redox flow cell; measuring an optical absorption of one or more of the test samples of the electrolyte composition; comparing the optical absorption of the test sample with reference optical absorption measurements of the electrolyte composition in various states of charge to provide a relative absorptivity of the test sample; and determining the state of charge of the redox flow cell battery from the relative absorptivity of the test sample.