A mathematical model has been developed in order to simulate the three-dimensional, transient behavior during discharge of a spirally wound lead-acid battery cell designed for use in hybrid-electric vehicles. Several aspects of battery behavior are predicted, including potential and current distributions, heat generation rates, concentration changes, and temperature. The influence of the lead grids which support both electrodes has been described and found to have a significant impact on both the power available from the cell and heat generation rates in the cell. The model predictions compared well with available experimental data (current-voltage-time). Models such as the one presented in this paper represent a valuable tool for understanding battery behavior, solving problems with existing battery designs, and generating and optimizing new designs.