Lead-acid car batteries are a type of rechargeable battery widely used in vehicles to start the engine and power electrical systems when the engine is not running.

Structure of a Lead-Acid Car Battery

Cells:

A typical lead-acid car battery consists of six cells connected in series, each producing approximately 2.1 volts, totaling around 12.6 volts when fully charged.

Plates:

Positive Plates: Made of lead dioxide (PbO₂).
Negative Plates: Made of sponge lead (Pb).
These plates are alternately stacked in each cell with separators in between to prevent short circuits.

Electrolyte:

The electrolyte is a mixture of sulfuric acid (H₂SO₄) and water. It reacts with the plates to produce electrical energy.

Case:

The plates and electrolyte are housed in a plastic or rubber case, which is designed to be durable and resist the corrosive effects of the acid.

How Lead-Acid Batteries Work

1. Chemical Reactions During Discharge:

• When the battery is in use (discharging), a chemical reaction occurs between the lead dioxide on the positive plates, the sponge lead on the negative plates, and the sulfuric acid in the electrolyte.
• At the Positive Plate:
  • Lead dioxide reacts with sulfuric acid, producing lead sulfate (PbSO₄), water (H₂O), and releasing electrons.
• At the Negative Plate:
  • Sponge lead reacts with sulfuric acid to form lead sulfate and releases electrons.
• Flow of Electrons:
  • The flow of electrons from the negative plate to the positive plate through an external circuit provides electrical energy to power the vehicle.

2. Chemical Reactions During Charging:

• When the engine is running, the alternator recharges the battery by reversing the chemical reactions that occurred during discharge.
• At the Positive Plate:
  • Lead sulfate converts back into lead dioxide and sulfuric acid.
• At the Negative Plate:
  • Lead sulfate converts back into sponge lead and sulfuric acid.
• Restoration of Electrolyte:
  • As the battery charges, the sulfuric acid concentration in the electrolyte increases, restoring the battery's ability to deliver power.

Applications

Starting the Engine: The primary role of a lead-acid car battery is to provide the large burst of power needed to start the engine.
Powering Electrical Systems: When the engine is off, the battery powers the vehicle’s electrical components, such as lights, radio, and other accessories.
Stabilizing Voltage: The battery helps stabilize the voltage in the vehicle’s electrical system, protecting components from damage due to voltage spikes.

Lead-acid car batteries work through a reversible chemical reaction between lead dioxide, sponge lead, and sulfuric acid, which generates and stores electrical energy. They are essential for starting the engine and powering electrical systems in vehicles, with a design that balances cost, reliability, and performance. Proper maintenance can extend the life of these batteries, making them a reliable choice for automotive applications.