The storage batteries of commerce (Fig. 76) are built up with electrodes composed principally of lead peroxide (PbO2) as the positive electrode, and sponge lead as the negative electrode. The positive plate is hard, like soapstone, while the spongy lead is so soft that it may be cut by the finger nail. Both plates are immersed in a dilute solution of sulphuric acid. On discharging the battery, the metallic lead, peroxide, and sulphuric acid react forming lead sulphate and water. On charging, the reverse takes place; the lead sulphate forms metallic lead, lead peroxide, and sulphuric acid.

When the battery is fully charged and in good condition, the positive plate is a dark reddish brown or chocolate color, while the negative plate is slate-colored. On discharging the battery, the SO3 isobtained from sulphuric acid, which combines with water and forms lead sulphate with lead. When the battery is recharged the current releases the SO3, restoring the plates to their previous condition. Storage batteries are measured in ampere-hours. Thus a 100 ampere-hour battery will give a continuous discharge of 12 1/ 2 amperes for 8 hrs. Theoretically, it should give a discharge of 25 amperes for 4 hrs., or of 50 amperes for 2 hrs.

The capacity of a cell is proportional to the exposed area of the plates, the number of plates, and the active material present.