110 Electricity & Electronics Lead-Acid Cell Lead-acid batteries are also referred to as a storage battery. A storage battery does not store electricity. Rather, it stores chemical energy, which in turn produces electrical energy. Th e active ingredients in a fully charged battery are lead peroxide (PbO2), which acts as the positive plate, and pure spongy lead (Pb) for the negative plate. Th e liquid electrolyte is sulfuric acid (H2SO4) and water (H2O). Th e positive plates are a reddish-brown color. Negative plates are gray. See Figure 7-9. Th e chemical reaction is rather involved. However, study the information given in Figure 7-10. Notice that during discharge, both the spongy lead and the lead peroxide (also called lead dioxide) plates are being changed to lead sulfate, and the electrolyte is being changed to water. When the cell is recharged, the reverse action occurs. Th e lead sulfate changes back to spongy lead and lead peroxide the electrolyte to sulfuric acid. Th e electrolyte of a fully charged battery is a solution of sulfuric acid and water. Th e weight of pure sulfuric acid is 1.835 times heavier than water. Th is is called its specifi c gravity. Specifi c gravity is the weight of a liquid as it compares to water. Th e specifi c gravity of water is 1.000. Th e acid and water mixture in a fully charged battery has a specifi c gravity of approximately 1.300 or less. As the electrolyte changes to water when the cell discharges, the specifi c gravity becomes approximately 1.100 to 1.150. Th erefore, the specifi c gravity of the electrolyte can be used to determine the state of charge of a cell. Th e instrument used to measure the specifi c gravity is a hydrometer. Th e principle of the hydrometer is based on Archimedes’ principle in physics. Th is principle states that a fl oating body will displace an amount of liquid equal to its own weight. If the cell is in a fully charged state, the electrolyte liquid is heavier, so the fl oat in the hydrometer will not sink as far. Th e distance that the fl oat does Voltages in Series In this experiment you will demonstrate and observe how voltages add in series. MATERIALS ■ 4 fl ashlight cells (D-type) ■ 4 cell holders ■ 1 multimeter PROCEDURE 1. Connect the four cells in series. See the schematic in Exhibit 7-1A. 2. Set your multimeter to read voltages in the 0 V to 10 V range. 3. Measure and write down the voltage between points A and B. 4. Measure and write down the voltage between points A and C. 5. Measure and write down the voltage between points A and D. 6. Measure and write down the voltage between points A and E. Do your results follow the formula for voltage sources in series? What conclusion can you draw from this experiment? E x p e r i m e n t 7 - 1 A B C D E − + + − + − + − Goodheart-Willcox Publisher Exhibit 7-1A Cake78/Shutterstock.com Figure 7-9. A lead-acid battery consists of many thin lead plates suspended in a bath of sulfuric acid called the electrolyte. Copyright Goodheart-Willcox Co., Inc.