One of the biggest challenges related to the vanadium flow battery is to diagnose such an imbalance by accurately and independently measuring the State of Charge (SoC) of the negative and positive half-cells, that is, the relative concentration of charged species on the negative side (anolyte, V II /V III ) and the positive side (catholyte, V IV /V V ) . Following standard practice, the four oxidation states of vanadium V 2+ , V 3+ , VO 2+ , and VO + 2 are denoted here as V II , V III , V IV , and V V , or with subscripts i = {2, 3, 4, 5}. As shown in Table , the concentrations C i of these species determine both the SoC of each half-cell and the total concentration C of vanadium ions.