If, with a fully charged battery, the density of the electrolyte in one of the cells is 0.040 or more lower or higher than in the others, then the reason for this may be the leakage of electrolyte from this cell. Add a solution of 1 part sulfuric acid and 2.5 parts distilled water to the element. If the electrolyte level in it is already at the upper mark, suck the electrolyte with a hydrometer. When sulfuric acid is mixed with water it is not allowed to pour water into acid; always pour the acid in a thin stream into a glass container of water. When pouring water into sulfuric acid, an explosion will occur. Fill the cell with fresh electrolyte, recharge the battery and check the density of the electrolyte.