The following paragraphs describe the general operation and construction of lead-acid batteries.

Lead-Acid Battery Active Materials

The active materials in a battery are those that participate in the electrochemical charge/discharge reaction. These materials include the electrolyte and the positive and negative electrodes. As mentioned earlier, the electrolyte in a lead-acid battery is a dilute solution of sulfuric acid (H_ZSO,). The negative electrode of a fully charged battery is composed of sponge lead (Pb) and the positive electrode is composed of lead dioxide (PbO,).

Electrochemistry of the Lead-Acid Cell

All lead-acid batteries operate on the same fundamental reactions. As the battery discharges, the active materials in the electrodes (lead dioxide in the positive electrode and sponge lead in the negative electrode) react with sulfuric acid in the electrolyte to form lead sulfate and water. On recharge, the lead sulfate on both electrodes converts back to lead dioxide (positive) and sponge lead (negative), and the sulfate ions (SO,'^-)are driven back into the electrolyte solution to form sulfuric acid. The reactions involved in the cell follow.

At the positive electrode

At the negative electrode

For the overall cell

Therefore the maximum open-circuit voltage that can be developed by a single lead-acid cell is 2.041 V.

Negative and Positive Plate Construction Methods

The simplest method for the construction of lead-acid battery electrodes is the plante plate, named after the inventor of the lead-acid battery. A plante plate is merely a flat plate composed of pure lead. Since the capacity of a lead-acid battery is proportional to the surface area of the electrodes that is exposed to the electrolyte, various schemes are employed to increase the surface area of the electrodes per unit volume or weight. Pate plates are grooved or perforated to increase their surface area. A typical plante plate is shown in Figure 4.

The most commonly used method to increase surface area is to make the active material into a paste that acts like a sponge where the electrolyte fills all the pores. The paste, or active material, is mounted into a frame or grid structure that mechanically supports it and serves as the electrical conductor carrying the current during both the charge and discharge cycle. The most commonly used plate today is the pasted plate, also known as the flat plate. This grid structure is a lattice-work that resembles the cross section of a honeycomb, with the paste filling all of the rectangular windows on the structure. Figure 5 shows a typical construction of a pasted plate grid. The flat plate construction is used as the negative electrode plate in almost all cases, and serves as the positive plate in most standby applications.

Figure 4. Typical plante plate.

Figure 5. Typical construction of a pasted plate grid.

Positive electrodes are usually of pasted plate or tubular construction. Tubular electrodes are popular positive plates for heavy cycling applications. This construction uses a frame structure consisting of a series of vertical spines connected to a common bus. The paste is held in micro-porous, non-conductive tubes which are placed over the individual spines. A simplified view of tubular plate construction is shown in Figure 6. Regardless of the plate type used, the capacity of any battery is increased by adding multiple plates in parallel.