Chapter 7 Sources of Electricity 111 sink is calibrated in specifi c gravity on the scale. Th is can be read as the state of charge of the cell. In the 12-volt automotive battery, six lead-acid cells are placed in a molded hard rubber case. Each cell has its own compartment. At the bottom of each compartment a space, or sediment chamber, is provided. Th is is where particles of chemicals broken from the plates due to chemical action or vibration can collect. Otherwise, these particles would short out the plates and make a dead cell. Th e individual cells are connected in series by lead alloy connectors. The chemical formula for a storage cell PbO2 + Pb + 2H2SO4 Charged 2PbSO4 + 2H2O Discharged Discharged Electrolyte Electrolyte (Sulfuric acid and water) Maximum sulfuric acid Minimum water Negative plate Positive plate Sponge lead Lead dioxide Decreasing sulfuric acid Increasing water Decreasing sponge lead Increasing lead sulfate Decreasing lead dioxide Increasing lead sulfate Electrolyte Increasing sulfuric acid Decreasing water Increasing sponge lead Decreasing lead sulfate Increasing lead dioxide Decreasing lead sulfate Electrolyte Minimum sulfuric acid Maximum water Minimum sponge lead Maximum lead sulfate Minimum lead dioxide Maximum lead sulfate Diagram of Chemical Action Charged Discharging Charging ESB Brands, Inc. Figure 7-10. How a lead-acid cell works. C A U T I O N Expensive storage batteries may be destroyed by excessive vibration and rough handling. Chemicals may break off from the plates and cause internal short circuits and dead cells. Handle a battery gently and be sure it is securely clamped and bolted in your car. Copyright Goodheart-Willcox Co., Inc.