Why does the emf of a lead-acid cell decrease after prolonged use?

The decrease in emf (electromotive force) of a lead-acid cell after prolonged use is primarily due to the consumption of key reagents involved in the electrochemical reactions. In a lead-acid battery, the chemical reactions during discharge convert lead dioxide (PbO2), sponge lead (Pb), and sulfuric acid (H2SO4) into lead sulfate (PbSO4) and water. Over time, as the cell undergoes repeated cycles of discharge and recharge, the sulfuric acid and the lead compounds are gradually depleted or converted into lead sulfate.

When the reagents, particularly the sulfuric acid and the active materials (PbO2 and Pb), are used up during these reactions, there are fewer reactants available to drive the electrochemical reaction efficiently. As a result, the cell can no longer generate a strong enough emf to maintain its output voltage, leading to a decrease in emf over time.

Consequently, maintaining an adequate concentration of sulfuric acid and the active materials is essential in prolonging the battery's performance and voltage output. This depletion of essential reagents directly correlates with the observed decline in emf during prolonged use of the battery.