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
- Electrodes: Alloyed metallic lead (Pb) with antimony (Sb) and/or calcium (Ca) for durability
- Electrolyte: Diluted sulfuric acid (H₂SO₄)
- 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
- Electrodes are inserted into dilute H₂SO₄
- A forming current is passed through the cell
- This converts PbO on the negative plate into finely divided sponge lead (Pb)
- 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)