A mathematical model of a lithium-ion cell is used to analyze pulse and relaxation behavior in cells designed for hybrid-electric-vehicle propulsion. Predictions of cell voltage show good agreement with experimental results. Model results indicate the ohmic voltage loss in the positive electrode is the dominant contributor to cell overvoltage in the first instances of a pulse. The concentration overvoltage associated with the reduced lithium in the solid phase of the positive is of secondary importance through pulse duration, but dominates after current interruption. Effects of anisotropy in the particle diffusion coefficient are also studied. Heaviside mollification functions are utilized to describe the thermodynamic open-circuit voltage of lithiated graphite, and the “pleated-layer model” is extended to realize the phase behavior of primary-particle aggregates during cell operation. The negative electrode contributes little to the cell overvoltage, and two-phase behavior results in a reaction front within the electrode. No voltage relaxation is associated with the negative electrode, and after full relaxation, a stable composition gradient of lithium exists throughout the solid phase. Internal galvanic coupling removes the composition gradients in the positive electrode during relaxation.