Voltage loss in the bipolar plate (BP) is induced by in-plane current in the BP, which arises when the distributions of local current density over the surfaces of adjacent cells are different. We show that potential of BP satisfies Poisson equation with the right side proportional to the difference of local current densities on both sides of BP Solution to this equation is obtained for BP between two hydrogen cells with the single straight channels and ideally humidified membranes. The general relation for voltage loss in the BP is derived. (c) 2006 Elsevier B.V. All rights reserved.