Lead-acid batteries contain three separate 2-volt compartments, known as cells. Inside each cell is a series of thick, parallel lead plates (Figure 7.1). The cells are connected internally (wired in series) so that they produce 6-volt electricity. The cells are filled with sulfuric acid (hence the term “flooded”). A partition wall separates each cell so that fluid cannot flow from one cell to the next. The cells are encased in a heavy-duty plastic case.
As illustrated in Figure 7.1, lead-acid batteries contain two types of plates: positive and negative. The positive plates connect to a positivemetal post or terminal; the negative plates connect to a negative post. The posts allow electricity to flow into and out of batteries.
The positive plates of lead-acid batteries are made from lead dioxide (PbO2). The negative plates are made from pure lead. The
Fig. 7.1: Anatomy of a Flooded Lead-Acid Battery.
sulfuric acid that fills the spaces between the plates is referred to as the electrolyte.
How Lead-Acid Batteries Work
Like all other types of batteries, lead-acid batteries convert electrical energy into chemical energy when they are charged. When discharging, that is, giving off electricity, chemical energy is converted back into electricity. Electricity, of course, consists of electrons, tiny negatively charged particles, that flow through conductors. The electrons flow out of the battery at the positive post, creating an electrical current. During the chemical reactions that take place during discharge, lead on the surface of the negative plates reacts with sulfuric acid in the battery, creating tiny lead sulfate
crystals on the surface of the plates.
Although the chemistry of lead-acid batteries is a bit complicated, it is important to remember that this system works because electrons can be stored in the chemicals within the battery when a battery is charged. The stored electrons can be drawn out by reversing the chemical reactions. Through this reversible chemical reaction, the battery is acting as a “charge pump,” moving electrical charges through a circuit on demand.
