Abstract Valve-regulated lead–acid batteries (VRLAB) with operating closed oxygen cycle. McClelland and Devitt invented the valve-regulated lead–acid cell with electrolyte immobilized in microporous absorptive glass mat (AGM) and a pressure relief valve. The AGM separator absorbs electrolyte, provides gas transport channels between the two electrodes and minimizes the pulsation of plate volumes during cycling. AGM separators have to be under compression in the cells. The pore system of the AGN separator depends on the proportion between fine and coarse glass fibers in its structure. The electrical characteristics of VRLA cells depend strongly on the AGM saturation with electrolyte. Depending on the state of charge of the cell, oxygen reduction and hydrogen evolution proceed at the negative plates during charge of VRLA batteries. The reactions of the closed oxygen cycle generate heat. The cell temperature increases also due to the Joule effect. A linear dependence is established between cell current and temperature. Self-accelerating interrelations are established between the processes on the two electrodes, which result in uncontrolled increase of cell temperature and current. This phenomenon is known as “thermal runaway.” Reduction of oxygen at the negative plates may proceed via several mechanisms: electrochemical mechanism or reduction of oxygen with formation of PbO, H2O2, PbSO4 as intermediate products. Some of these mechanisms are discussed in this chapter.