In this paper, we develop a mathematical model for soluble lead redox flow battery. The model accounts for simultaneous effect of forced convection and induced free convection in electrolyte domain of the battery. It predicts existence of dominant free convection over the forced convection in vicinity of the electrodes. The predictions suggests that both, electrode kinetics and ion transfer assisted by free convection alone, controls charge-discharge characteristics of the battery. The free convection augments ion transfer rate to the electrodes. By virtue of this, limiting current density at the electrodes increases to twice the theoretical limit under forced convection. The predictions coherently explains Collins et al. [1]'s observations of high charge efficiencies when charging currents are higher than the theoretical limit. Also, the model consistently explains Pletcher et al. [2]'s observation of insensitivity of the battery characteristics to the electrolyte flow rate. The model predicts satisfactory battery performance even at microscopic (mu L s(-1)) flow rates which opens up the possibility for significant reduction in electrolyte pumping cost.