This paper addresses the importance of the boundary conditions for the charge conservation equation in the modeling of fuel cells. In this context, we analyze the charge transport in an electric conductor, aiming to determine whether constant current and constant potential boundary conditions can be interchanged without disturbing the local current density distribution in the cell. Their interchangeability can be described with a dimensionless number, referred to as the "interchangeability number", which captures the relevant operating, geometrical and material parameters. The effect of the interchangeability number is further explored in a model for non-isothermal two-phase flow in a proton exchange membrane fuel cell, for which is it verified that the interchangeability number should be much less than 3, in order to ensure that the prediction for the local current density distribution at the catalyst layers remains the same regardless of galvanostatic or potentiostatic boundary conditions. (C) 2013 Elsevier Ltd. All rights reserved.