This paper examines the effect of the electrical double layer on the performance of a lithium ion battery electrochemical cell. We begin by introducing the Poisson Nernst–Planck equations of electrochemistry to describe ion transport within a representative liquid solvent and derive an expression for the current–voltage relationship in the electroneutral liquid within the separator pores. Different assumptions about the electrical double layer lead to variation of the lithium ion concentration profiles in the liquid electrolyte, which alter the cell voltage during discharge. The contribution of the electrical double layer to the cell overpotential is combined with the bulk liquid potential difference and a simplified treatment of the electrode solid phase to obtain an expression for the time-varying cell terminal voltage. We conclude by presenting experimental data for a cell using a graphite anode and lithium iron phosphate cathode to validate the new model, which includes electrical double layer effects, and construct modifications to the model to compensate for resistive effects that are associated with the cathode solid phase.