VRLA batteries in hybrid electric vehicles are operated at a partial state of charge with high current draws for acceleration and regenerative braking. Adding larger amounts of carbon particles to the negative plate material extends battery life. Water loss and increasing internal resistance are a cause of a subsequent failure mode that is related to the carbon and other organic additives in the negative plate. Previous studies of the composition and volume of gases vented from valve-regulated lead-acid (VRLA) batteries and acid-limited batteries at various temperatures and current levels are reviewed and used to develop an understanding of carbon reactions and their effects on battery state of health.