Question

When charging a lead acid battery it is noticed that there are bubbles coming to the surface in the electrolyte like it is boiling. What is causing the bubbles? What are the gasses in the bubbles? Are the gasses in the bubbles potentially explosive?

Answer 1

When charging a lead acid battery causing the bubbles. Bubbling gasses are hydrogen and oxygen.

At anode

Pb + HSO₄^- → PbSO₄ + H⁺ + 2 e^-

The metal lead in the anode reacts with the ionized sulphuric acid to produce lead sulphate, hydrogen ions in solution and two excess electrons

At cathode

PbO₂ + HSO₄^- + 3H⁺ +2 e^- → PbSO₄ + 2H₂O

This says that the lead dioxide reacts with the ionized sulphuric acid and the available hydrogen ions, plus some donated excess electrons from the anode via the connecting wire, to produce lead sulphate and water

The balance of the current decomposes the water of the electrolyte into hydrogen and oxygen, causing gassing. Hydrogen and oxygen gases evolved during charging make a very explosive mixture. An open flame brought near a battery on charge or freshly charged, will probably produce an explosion