The performance of an undivided flow battery based on the Pb(II)/Pb and PbO2/Pb(II) couples in aqueous methanesulfonic acid as a function of state of charge, current density, electrolyte flow rate and temperature is reported. In addition, it is demonstrated that the cell chemistry can be rebalanced after multiple charge/discharge by allowing the excess lead metal deposited at the negative electrode to react with oxygen on open circuit.