An empirical sulfation model for valve-regulated lead-acid (VRLA) batteries is presented. The model is based on measurements of the internal resistance during cycling in different state of charge (SOC) ranges. Further influence factors like voltage, current and depth of discharge (DOD) are varied and their effect on the gradient of the internal resistance is investigated. The behavior of the internal resistance correlates to the charge acceptance which is related to the sulfation of the battery. The model is tested by separate measurements.