A lead acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in a electrolytic solution of sulfuric acid and water.
The Nernst equation for the lead-acid cell can be written by adding the two half-cell reactions given in equations 1 and 2. Note: The affect of sulfuric acid concentration on the electrode potential, is clearly seen in equation 10, which is a simpler form of equation 9. Using equation 8, the Nernst equation for the lead acid cell is,
In general, this H2SO4 electrolyte solution can have a strong effect on the energy output of lead-acid batteries. In most batteries, the electrolyte is an ionic conductive liquid located between the positive and negative electrodes. Its primary function is to provide a
Voltage of lead acid battery upon charging. The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved.
Note: The maximum concentration of acid, 3.0M used here, is lower than the nominal concentrations, 4.5 – 6.0 M reported for auto batteries. The 3.0 M acid cell produces a potential above 2.0 volts, and is adequate for demonstrating our objectives.
The formation of this lead sulfate uses sulfate from the sulfuric acid electrolyte surrounding the battery. As a result, the electrolyte becomes less concentrated. Full discharge would result in both electrodes being covered with lead sulfate and water rather than sulfuric acid surrounding the electrodes.