A flow battery includes at least a cell that has a first electrode, a second electrode and an electrolyte separator layer arranged between the electrodes. A supply/storage system is external of the cell and includes a first vessel fluidly connected in a first loop with the first electrode and a second vessel fluidly connected in a second loop with the second electrode. The first loop and the second loop are isolated from each other. The supply/storage system is configured to fluidly connect the first loop and the second loop to move a second liquid electrolyte from the second vessel into a first liquid electrolyte in the first vessel responsive to a half-cell potential at the first electrode being less than a defined threshold half-cell potential.