The flow in a vanadium redox flow battery, which is determined by flow rate and geometry of flow channels, is an important factor in determining battery performance. Therefore, flow rate and flow channel must be carefully designed and controlled to provide smooth supplies of electrolyte to the areas where electrochemical reactions take place. Through a verified three-dimensional electrochemical steady-state model involving fluid motion, it is found that channel height is an important geometric parameter that determines battery performance. We investigate its impact on pressure drop and penetration of the electrolyte into the electrode, and then examine how it ultimately played a role in voltage response, energy efficiency, and battery efficiency. To identify the optimal channel shape, various operating parameters (including flow rate, current, and state of charge) are investigated at different channel heights. Overall, low-height cells are found to have better energy and battery efficiency, and can operate at relatively lower optimal flow rates. There is, however, a trade-off between an improved flow field and pumping loss, depending on geometry and loading conditions. A judicious balance between them would be useful in finding the optimal flow channel geometry and ideal loading conditions for the best performance of flow batteries. (C) 2017 The Electrochemical Society. All rights reserved.