Lead Accumulator: Working Principle and Reactions

Lead Accumulator (Lead-Acid Battery)

What is a Lead Accumulator?

A lead accumulator, commonly known as a lead-acid battery, is a type of secondary cell. Unlike primary cells, it can be recharged multiple times by applying an external voltage greater than the cell's own electromotive force (EMF), which reverses the chemical reactions.

It acts as an accumulator, storing electrical energy in the form of chemical energy. The voltage of the cell depends primarily on the concentration of the sulphuric acid electrolyte, rather than the physical size of the electrodes.

Construction

The battery consists of:

• Negative Electrode: Lead plates.
• Positive Electrode: Lead plates coated with Lead Oxide (PbO₂).
• Electrolyte: 38% sulphuric acid solution housed in a non-conducting vessel (glass, plastic, or ebonite).

Construction of a standard Lead Accumulator.

Chemical Reactions

Discharging (Battery in use)

During discharge, the battery converts chemical energy into electrical energy:

• Anode: $Pb(s) + SO_4^{2-}(aq) \to PbSO_4(s) + 2e^-$
• Cathode: $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \to PbSO_4(s) + 2H_2O(l)$
• Overall: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \to 2PbSO_4(s) + 2H_2O(l)$

Charging (Reversing the reaction)

When an external EMF is applied, the process is reversed, regenerating Pb and PbO₂.

Uses of Lead Accumulators

1. Automobiles: Commonly used as car batteries for starting and ignition.
2. Telecommunications: Powering telegraph and telephone exchanges.
3. Backup Power: Used in radio sets, electric clocks, and alarm systems.
4. Laboratories: Utilized as a reliable constant DC voltage source.