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Direct methanol fuel cells (DMFCs) constitute nowadays a promising alternative to lithium ion batteries for powering portable devices. The effective design of power-management units for interfacing DMFCs requires accurate models able to account for variable-load conditions and fuel consumption. A dynamic nonlinear circuit model for passive methanol fuel cells is presented in this paper. The model takes into account mass transport, current generation, electronic and protonic conduction, methanol adsorption, and electrochemical kinetics. Adsorption and oxidation rates, which mostly affect the cell dynamics, are modeled by a detailed two-step reaction mechanism. The fully coupled multiphysics equivalent circuit is solved by assembling first-order differential equations into a nonlinear state-variable system in order to simulate the electrical evolution of the fuel cell from its initial conditions. The fuel-cell discharge and methanol consumption are computed by combining mass-transport and conservation equations. As a result, the runtime of a DMFC can be predicted from the current load and the initial methanol concentration.