Question

Lead acid battery can be charged where as primary cell cannot be charged. Why ?

Answer 1

The key difference between a lead-acid battery and a primary cell lies in their construction and chemical reactions.

Lead-Acid Battery (Secondary Cell):

A lead-acid battery is rechargeable because it undergoes reversible chemical reactions. In a lead-acid battery, the reaction that occurs during discharging and charging is reversible. When you discharge the battery, lead dioxide (PbO2) on the positive plate and sponge lead (Pb) on the negative plate react with sulfuric acid (H2SO4) to produce lead sulfate (PbSO4) and water. When charging, this reaction is reversed, and the lead sulfate converts back to lead and lead dioxide.

1. Discharging: Pb + PbO2 + H2SO4 → PbSO4 + H2O
   

1. Charging: PbSO4 + H2O → Pb + PbO2 + H2SO4
   

This ability to reverse the chemical reactions is what allows the lead-acid battery to be recharged and used multiple times.

Primary Cell:

A primary cell, on the other hand, is non-rechargeable. The chemical reactions that take place in a primary cell, such as in a standard alkaline battery (zinc-carbon or similar), are irreversible. During discharge, chemicals inside the battery undergo a reaction that creates electrical energy. However, once these reactions occur, the chemicals are used up and cannot be restored to their original state by applying an external current.

For example, in an alkaline battery:

1. Discharging: Zinc reacts with manganese dioxide and produces energy, but the zinc and manganese compounds get chemically altered in a way that can’t be undone by recharging.
   

Since the chemical process is irreversible, a primary cell cannot be recharged and used again once the chemicals are exhausted.

Summary:

1. Lead-acid battery: The chemical reactions are reversible, so the battery can be recharged.
   

1. Primary cell: The chemical reactions are irreversible, so the cell cannot be recharged.