Lead accumulator

1.0Introduction

The lead-acid battery is a type of rechargeable battery invented in 1859 by French physicist Gaston Planté. It is the first rechargeable battery ever developed. While it has a low energy density compared to modern batteries, it offers a high power-to-weight ratio due to its ability to deliver high surge currents.

These characteristics, along with low cost, make lead-acid batteries ideal for motor vehicles, where they provide the high current needed for engine starters. Despite newer battery technologies offering higher energy densities, lead-acid batteries remain widely used due to their affordability and reliability.

Lead-acid batteries are also used in:

• Backup power systems (e.g., cell phone towers, hospitals, stand-alone power systems)
• Uninterruptible power supplies (UPS)
• Industrial machinery and renewable energy storage

Modified versions of standard lead-acid batteries improve storage capacity and reduce maintenance requirements. VRLA (Valve-Regulated Lead-Acid) batteries, including gel-cell and absorbed glass-mat (AGM) batteries, are commonly used in these roles.

2.0Components of a Lead-Acid Battery

1. Electrodes: Alloyed metallic lead (Pb) with antimony (Sb) and/or calcium (Ca) for durability
2. Electrolyte: Diluted sulfuric acid (H₂SO₄)
3. Plastic separators & casing: To prevent short circuits and provide structural support

Sulfation & Battery Maintenance

Prolonged discharge leads to lead sulfate buildup, which can become difficult to remove. To prevent this, lead-acid batteries should be charged promptly. If the sulfuric acid or other components degrade, charging may become inefficient, reducing battery life. 

3.0Electrode Composition & Reactions

The electrodes, or grids, are composed of a lead alloy frame to enhance strength and durability. The active mass is a mix of:

• Ultrafine Pb powder
• Lead oxide (PbO or Pb₃O₄)
• Diluted H₂SO₄

This mass primarily consists of lead sulfate (PbSO₄) and unreacted lead oxides upon drying. The final electrodes are formed by spreading this mixture on the grids and charging them in H₂SO₄ electrolysis.

During discharge, lead sulfate (PbSO₄) forms at both electrodes while the sulfuric acid concentration decreases.

The chemical energy in a lead-acid battery is stored in the potential difference between:

• Pure lead (Pb) at the negative electrode
• Lead dioxide (PbO₂) at the positive electrode Aqueous sulfuric acid (H₂SO₄) as the electrolyte

4.0The lead-acid accumulator

The lead-acid accumulator remains one of the most widely used rechargeable batteries due to its cost-effectiveness, reliability, and high surge current capability. Although newer battery technologies offer higher energy densities, lead-acid batteries dominate applications where affordability and durability are key factors.

The lead-acid accumulator, commonly used in car batteries, is also known as a storage cell because it requires charging by passing an electric current through it.

The anode in a lead storage battery consists of metallic lead, while the cathode is made of lead(IV) oxide. The electrolyte is a dilute solution of tetraoxosulfate(VI) acid. When both electrodes are connected to an external circuit, the battery discharges, generating electricity.

Lead Accumulator Discharging:

The lead-acid accumulator remains one of the most widely used rechargeable batteries due to its cost-effectiveness, reliability, and high surge current capability. Although newer battery technologies offer higher energy densities, lead-acid batteries dominate applications where affordability and durability are key factors.

At the Anode (During Discharge):

When the accumulator is used as a direct current source, lead atoms at the anode release two electrons each, oxidising into lead(II) ions (Pb²⁺). These ions then react with tetraoxosulfate (VI) ions (SO₄²⁻) present in the electrolyte, forming lead(II) tetraoxosulfate(IV) (PbSO₄), which gets deposited on the anode.

Pb(s) → Pb(aq)²⁺ + 2e⁻

The released electrons travel through an external circuit, creating an electric current that can be utilised for purposes such as:

• Starting a vehicle engine
• Powering car headlights before reaching the cathode

At the Cathode (During Discharge):

At the cathode, electrons are accepted, leading to a redox reaction between lead(IV) oxide (PbO₂) and hydrogen ions (H⁺) from the electrolyte, as shown below:

PbO₂(s)+4H⁺(aq)+2e⁻→Pb(aq)²⁺+2H₂O(l)

The lead(II) ions (Pb²⁺) formed then react with tetraoxosulfate(VI) ions (SO₄²⁻) from the electrolyte, resulting in the deposition of lead(II) tetraoxosulfate(IV) (PbSO₄) at the cathode:

Pb²⁺ (aq) + SO₄²⁻ (aq) → PbSO₄

Charging Process

When charging, an external current source reverses the reaction, restoring PbO₂ at the positive electrode and pure lead at the negative electrode while increasing the sulfuric acid concentration.

5.0Rechargeable Nature of Lead-Acid Batteries

Lead-acid batteries are secondary cells, meaning they can be recharged multiple times. The electrode reactions are reversible, allowing electrical energy to be stored and later discharged.

Formation Process

1. Electrodes are inserted into dilute H₂SO₄
2. A forming current is passed through the cell
3. This converts PbO on the negative plate into finely divided sponge lead (Pb)
4. PbO on the positive plate is converted into lead dioxide (PbO₂)

Storage Cell & Voltage Regulation

• The voltage of a lead-acid battery depends on the concentration of sulfuric acid, not the size of the electrodes or cell.
• Charging reverses the cell reaction by applying an external electromotive force (emf)