Inside a lead storage battery is a series of plates. Half of the plates are made of lead dioxide and the other half are made of a spongy form of lead. The plates are bathed in a solution of sulfuric acid which serves as an electrolyte (a chemical solution that conducts electricity). Two posts extend to the outside of the battery through sealed openings in the battery wall. One of the posts---the negative post---is connected to the lead plates, and the other---the positive post---to the lead dioxide plates.
Sulfuric acid, chemically, is composed of two hydrogen atoms, a sulfur atom and four oxygen atoms. Inside the battery, the sulfuric acid molecules are in solution with water and so are dissociated. This means that the sulfur atom with the four oxygen atoms attached to it are in the water, separated from the hydrogen atoms. The sulfur with the four oxygen atoms is called a sulfate ion and has a double negative charge. The free hydrogen atom is called a hydrogen ion and has a positive charge. (An ion is simply an atom or a molecule with a positive or negative charge.)
The battery performs its function through a series of chemical reactions involving these ions. In the condition described in the introductory paragraph, the battery is charged and has the capacity to supply an electric current through cables connected to the two posts. As the battery supplies electric current, the sulfuric acid reacts by giving up its sulfate ions to the lead and lead dioxide plates. This forms lead sulfate that deposits on the plates. While this process occurs, the sulfuric acid concentration is decreasing, and the battery is discharging.
Supplying an electric current to the battery rather than drawing it out---such as what happens in an automobile when the engine is running---reverses the chemical reaction and the battery recharges. When recharging, the lead and lead dioxide plates give up sulfate ions to the electrolyte solution. This restores the amount of sulfuric acid in the electrolyte solution and restores the lead and lead dioxide plates to their charged condition.
