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Chapter 8: Batteries 181simple lead-acid battery cell that will be examined in the next few sections as itundergoes the four stages: fully charged, discharging, fully discharged, and charging. Itconsists of an electrode made of sponge lead (Pb), another electrode made of leadperoxide (PbO 2), and an electrolyte made of a mixture of sulfuric acid (H 2SO 4) dilutedwith water (H 2O).Overall Chemical ReactionCombining active material elements into compounds that further combine with theaction of the electrolyte significantly alters their native properties. The true operation ofany battery is best described by the chemical equation that defines its operation. In thecase of the lead-acid battery, this equation is given asPb 1 PbO 21 2H 2SO 41 2PbSO 41 2H 2OThe left side of the equation represents the cell in the charged condition, and theright side represents the discharged cell. In a charged lead-acid battery, its positiveanode plate is nearly all lead peroxide (PbO 2), its negative cathode plate is nearly allsponge lead (Pb), and its electrolyte is mostly sulfuric acid (H 2SO 4) (see the top of Figure8-1). In a discharged condition, both plates are mostly lead sulfate (PbSO 4), and the acidelectrolyte solution used in forming the lead sulfate becomes mostly water (H 2O) (seethe bottom of Figure 8-1).Discharging Chemical ReactionThe general equation gives a more accurate view when separately analyzed at eachelectrode. The discharging process is described at the anode asPbO 21 4H – 1 SO 4 –– 1 2e – g PbSO 41 2H 2OThe discharging process is described at the cathode as:Pb 1 SO 4 –– – 2e – g 4 PbSO 4When discharging, the cathode acquires the sulfate (SO 4) radical from the electrolytesolution and releases two electrons in the process. These electrons are acquired by theelectron-deficient anode. The electron flow from negative cathode to positive anodeinside the battery is the source of the battery’s power and external current flow frompositive anode to negative cathode through the load. In the process of discharging (rightof Figure 8-1), both electrodes become coated with lead sulfate (PbSO 4)—a goodinsulator that does not conduct current—and the sulfate (SO 4) radicals are consumedfrom the electrolyte. At the same time the physical area of the sponge-like platesavailable for further reaction decreases as it becomes coated with lead sulfate; thisincreases the internal resistance of the cell, and results in a decrease of its output voltage.At some point before all the sulfate (SO 4) radicals are consumed from the electrolyte,there is no more area available for chemical reaction and the battery is said to be fullydischarged.Charging Chemical ReactionThe charging process is described at the anode asPbSO 41 2H 20 – 2e – g PbO 21 4H – 1 SO 4 ––