NOVELTY - A rebalance system comprises electrochemical cell(s); and photochemical cell(s) containing source(s) of UV radiation. The electrochemical cell and the photochemical cell are in fluid communication with each other.    USE - Rebalance system for redox flow cells (100).    ADVANTAGE - The rebalance system effectively removes or recycles parasitic side products produced during the charge/discharge cycles of a flow cell battery to rebalance the charge states between the two electrolytic solutions.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a rebalance cell comprising source(s) of UV radiation; aqueous anolyte solution comprising acid(s) and metal salt(s); aqueous catholyte solution comprising acid(s) and metal salt(s); an anode compartment and a cathode compartment separated by a membrane; and an electrode disposed in each anode and cathode compartments;    (2) a control system for a rebalance cell of a flow cell battery comprising processor(s) operating the flow cell battery system in multiple states, where the multiple states comprises an initial drain mode, an initial fill mode, a delay state, and an on state transitioned to from the delay state, within which power is applied to the rebalance cell;    (3) a redox flow cell battery system comprising two electrodes, porous or ion-selective membrane separating the half-cell compartments, an electrolyte comprising chromium ions and iron ions, and sensor comprising heating element, temperature sensing element, and an enclosure; and    (4) a method for detecting a ratio of a first substance to that of a second substance in a mixture of substances comprising placing a sensor in a portion of a redox flow cell battery system comprising a mixture of hydrogen and chlorine; generating heat in the heating element; measuring a temperature proximate to the heating element with the temperature sensing element; and determining the ratio of hydrogen to chlorine from the measured temperature.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a redox flow cell filled with electrolyte.    Redox flow cell (100)    Electrodes (102, 104)    Half-cells (108, 110)    Catholyte (124)    Anolyte (126)