A method for the real-time monitoring of the state-of-charge of redox flow batteries is presented, which provides half-cell resolution. The well-known standard procedure for the state-of-charge monitoring, which utilizes the equilibrium potential of the monitored electrolyte versus a standard reference electrode in an open-circuit cell, is modified. It is demonstrated that the flow battery electrolyte itself provides a stable reference potential if used in the reference compartment of a symmetric open-circuit cell. This reference potential can be used to monitor the state-of-charge of a flow battery electrolyte during battery operation. The proposed method is validated with N,N,N-2,2,6,6-heptamethylpiperidinyloxy-4-ammonium chloride (TEMPTMA) catholytes and N,N′-dimethyl-4,4-bipyridinium dichloride (MV) anolytes in aqueous flow batteries using both a symmetric open-circuit cell and a standard open-circuit cell with a Ag/AgCl reference electrode. Reference potential drift rates are determined and drift mechanisms are investigated, demonstrating that the utilization of a symmetric open-circuit cell reduces the reference potential drift. It is validated that the exchange of the in-situ producible reference electrolyte can replace time-consuming re-calibration procedures.