The history of soluble lead flow batteries is concisely reviewed and recent developments are highlighted. The development of a practical, undivided cell is considered. An in-house, monopolar unit cell (geometrical electrode area 100 cm2) and an FM01-LC bipolar (2 × 64 cm2) flow cell are used. Porous, three-dimensional, reticulated vitreous carbon (RVC) and planar, carbon-HDPE composite electrodes have been used in laboratory flow cells. The performance of such cells under constant current density (10–160 mA cm−2) cycling is examined using a controlled flow rate (mean linear flow velocity <14 cm s-1) at a temperature of approximately 298 K. Voltage versus time and voltage versus current density relationships are considered. High charge (<90%), voltage (<80%) and energy (<70%) efficiencies are possible. Possible failure modes encountered during early scale-up from a small, laboratory flow cell to larger, pilot-scale cells are discussed.