A 1D model based on physical and electrochemical processes of a lithium ion cell is used to describe constant current and hybrid pulse power characterization (HPPC) data from a 6Ah cell designed for hybrid electric vehicle (HEV) application. An approximate solution method for the diffusion of lithium ions within active material particles is formulated using the finite element method and implemented in the previously developed 1D electrochemical model as an explicit difference equation. Reaction current distribution and redistribution processes occurring during discharge and current interrupt, respectively, are driven by gradients in equilibrium potential that arise due to solid diffusion limitations. The model is extrapolated to predict voltage response at discharge rates up to 40C where end of discharge is caused by negative electrode active material surface concentrations near depletion. Simple expressions are derived from an analytical solution to describe solid-state diffusion limited current for short duration, high-rate pulses.