Sulfuric acid is one of the three active materials that take part in the reactions in lead-acid batteries. As a result of the reactions of current generation and accumulation in the battery the concentration of H2SO4 in the electrolyte changes. The battery has the highest performance parameters when the variations in H2SO4 concentration on cycling are within the range from 1.10 to 1.28 specific gravity (SG). In this acid concentration range, lead dioxide has the highest electrochemical activity, lead sulfate is highly soluble, and the H2SO4 solution is most electroconductive. Acid concentration determines open circuit battery voltage. This should be taken into account when selecting the upper charging voltage limit so as to guarantee fully charged battery. Battery performance depends on the utilization coefficients of the three active materials: Pb, PbO2 and H2SO4. When the utilization of H2SO4 is higher than that of Pb and PbO2, the battery has lower initial capacity, but longer cycle life. When Pb and PbO2 are the capacity-limiting active materials, the initial capacity is high, but its life is shorter. The sulfuric acid purity is very important for normal operation of the battery and is regulated by a H2SO4 purity standard. Tests with various additives to the electrolyte (phosphoric acid, boric acid, polymer emulsions etc.) have proved that they have but limited applications. During battery cycling, vertical stratification of the H2SO4 electrolyte occurs. This creates local potential differences in height and leads to self-discharge and capacity loss of the cell. Gas evolution in the cell homogenizes the electrolyte concentration throughout the cell volume.