Publisher Summary Lead–acid batteries are employed in a wide variety of different tasks, each with its own distinctive duty cycle. In internal-combustion engine vehicles, the battery provides a quick pulse of high-current for starting and a lower, sustained current for other purposes; the battery remains at a high state-of-charge for most of the time. However, the drive toward increased convenience through eliminating the need for water maintenance and avoiding the release of acid-carrying gases has led, however, to the widespread adoption of the valve-regulated form of the lead–acid battery. This paradigm shift in battery design has brought about a new set of scientific and technological challenges. The valve-regulated lead–acid (VRLA) battery is designed to operate by means of an internal oxygen cycle (or oxygen-recombination cycle), where oxygen is evolved during the latter stages of charging and during overcharging of the positive electrode. The function of the oxygen cycle is subtly linked to the microstructure of the separator material and to the nature of the charge algorithm applied, especially near top-of-charge. However, significant modifications in VRLA design will be required for new-generation transportation applications.