The Lead-Acid Battery: Its Voltage in Theory and in Practice

The common 12-volt lead-acid battery used in automobiles consists of six electrochemical cells connected in series. The voltage produced by each cell while discharging or required for its recharging is a matter of practical importance. The Nernst equation can be used to calculate the cell voltage as a function of the electrolyte concentration. Two theoretical models are developed for this purpose using thermodynamic data from the literature. The voltages thus calculated are open-circuit voltages for a cell at thermodynamic equilibrium. They differ from the voltages of an actual cell through which a current is flowing. A flowing current produces overvoltages and ohmic polarization, which decrease the voltage the cell delivers when discharging and increase the voltage required for its recharging.