The Chemistry Behind Lead Acid Batteries: Charging vs. Discharging

The chemical reactions that occur during charging and discharging of a lead acid battery.

Dr. Sasanka Deka

9/9/20261 min read

Lead acid batteries are among the most widely used rechargeable energy storage devices. From cars to UPS systems, they quietly power our lives. But what really happens inside when they charge and discharge? Let’s break it down in a simple, blog‑style explanation.

🔋 Discharging – Delivering Power

When you connect a lead acid battery to a load (say, starting your car), the battery begins to discharge.

At the negative plate (lead, Pb):

Pb+SO42−→PbSO4+2e−

Lead reacts with sulfate ions from the sulfuric acid electrolyte, forming lead sulfate and releasing electrons.

At the positive plate (lead dioxide, PbO₂):

PbO2+4H++SO42−+2e−→PbSO4+2H2O

Lead dioxide reacts with hydrogen ions and sulfate ions, also forming lead sulfate and water.

👉 The electrons flow through the external circuit, providing usable electricity. Meanwhile, the electrolyte’s sulfuric acid concentration drops as water content increases.

⚡ Charging – Restoring Energy

When you plug the battery into a charger, the process reverses.

At the negative plate:

PbSO4+2e−→Pb+SO42−

Lead sulfate is converted back into pure lead.

At the positive plate:

PbSO4+2H2O→PbO2+4H++SO42−+2e−

Lead sulfate is converted back into lead dioxide, regenerating the active material.

👉 The electrolyte regains its sulfuric acid strength as sulfate ions return, preparing the battery for another cycle.

🌍 Why It Matters

Efficiency: The reversibility of these reactions makes lead acid batteries reliable for repeated use.
Challenges: Over time, incomplete conversion can cause sulfation (hard PbSO₄ crystals), reducing capacity.
Applications: Their robustness keeps them popular in automotive, backup power, and renewable energy storage.

A lead acid battery is essentially a reversible chemical factory. Discharge turns active materials into lead sulfate while producing electricity, and charging restores them back to their original forms.