Battery packs used in hybrid applications often operate at high state of charge, especially during charging. To model battery performance during charge/overcharge, side reactions that affect the performance and cause degradation should be included. Control-oriented models must be accurate enough to predict side reactions but simple enough to be used for control algorithm analysis, design, and implementation. In this work, a lumped, nonlinear state space model for lead-acid cells is developed including electrolysis (gassing or water loss), the main side reaction for this chemistry during overcharge. The model is validated against voltage and pressure test data from a VRLA battery during charge and overcharge. The model predicts the rapid rise in voltage and pressure during overcharge as the input current is diverted from the main reaction to the electrolysis side reaction.