A lead acid battery is a widely used type of rechargeable battery. It converts chemical energy into electrical energy through reversible chemical reactions involving lead and sulfuric acid. It is commonly found in vehicles, backup power systems, and various other applications due to its reliability and cost-effectiveness.
Components of a Lead Acid Battery
Battery Case
A durable, acid-resistant container that houses all internal components and protects them from damage.
Positive Plates
Made of lead dioxide (PbO₂), these plates act as the cathode during discharge.
Negative Plates
Composed of spongy or porous lead (Pb), these serve as the anode during discharge.
Electrolyte
A mixture of sulfuric acid (H₂SO₄) and water (H₂O) that facilitates the chemical reactions by allowing ion movement.
Separator
A porous insulating material placed between positive and negative plates to prevent short circuits while permitting ion flow.
Terminal Posts
Metal connectors that link the battery to the external electrical circuit.
These components are arranged in cells, with each cell providing about 2 volts. A typical 12V battery contains 6 such cells connected in series.
How the Valve Regulated Lead Acid Battery Works
Discharging Process
When the battery powers an external device, the following occurs:
- The sulfuric acid electrolyte dissociates into positive hydrogen ions (H⁺) and negative sulfate ions (SO₄²⁻).
- Sulfate ions move toward the negative plates (anode), where they react with lead (Pb) to form lead sulfate (PbSO₄), releasing electrons.
- These electrons travel through the external circuit, providing electrical energy to the load.
- At the positive plates (cathode), lead dioxide (PbO₂) reacts with hydrogen ions and sulfate ions to also form lead sulfate and water (H₂O) as a byproduct.
- As a result, the concentration of sulfuric acid in the electrolyte decreases, and water content increases, reducing the battery’s voltage.
The overall chemical reaction during discharge is:
PbO₂ + Pb + 2H₂SO₄ → 2PbSO₄ + 2H₂O
This reaction releases electrons, creating electric current that powers connected devices.
Charging Process
When the battery is connected to a charger:
- The external current forces the lead sulfate deposits on both electrodes to convert back into lead (negative plate) and lead dioxide (positive plate).
- Water in the electrolyte breaks down into hydrogen ions and oxide ions.
- Oxide ions react with the positive plates to reform lead dioxide, releasing sulfate ions.
- Electrons flow back to the negative plates, converting lead sulfate back to lead and releasing sulfate ions.
- The sulfate ions recombine with hydrogen ions to regenerate sulfuric acid, restoring the electrolyte’s original concentration.
- This reverses the chemical changes caused during discharge, replenishing the battery’s charge.
2PbSO₄ + 2H₂O → PbO₂ + Pb + 2H₂SO₄
This process restores the battery to its original state.
Additional Considerations
Sulfation
If a lead acid battery is not fully recharged after discharge, lead sulfate crystals can harden on the plates, reducing capacity and lifespan. Preventing sulfation by timely full charging is crucial since reversing it is difficult and unverified by independent research
Stratification
Over time, the electrolyte can separate into layers of varying acid concentration, impairing performance. Periodic overcharging or mechanical agitation helps mix the electrolyte and maintain uniform concentration
Construction Details
Modern lead-acid batteries use lead-calcium alloy grids for strength and durability, and separators made of glass fiber cloth for heat resistance and ion conductivity. Safety features include relief valves to release excess gas pressure.
How do Lead-acid Batteries Handle Deep Discharge
Lead-acid batteries handle deep discharge with notable limitations and risks, as they are not inherently designed for frequent or very deep discharges without damage.
What Happens During Deep Discharge
Formation of Lead Sulfate and Sulfation
During discharge, lead sulfate (PbSO₄) forms on both the positive and negative plates. In a deep discharge, excessive lead sulfate accumulates and can crystallize into large, hard sulfate deposits-a process called sulfation. These sulfate crystals are difficult to convert back into active material during charging, leading to permanent capacity loss and reduced battery life.
Plate Shedding and Physical Damage
Deep discharge can cause mechanical stress on the battery plates, leading to shedding of active material. This shedding reduces the effective surface area for chemical reactions and can cause internal short circuits, resulting in battery failure.
Increased Internal Resistance
Deeply discharged lead-acid batteries exhibit higher internal resistance, making recharging less efficient and potentially causing overheating during charging cycles.
Voltage Drop and Cut-Off Limits
The battery voltage drops significantly during deep discharge. For lead-acid batteries, discharging below about 10.5 volts (for a 12V battery) is considered deep discharge and can harm the battery if sustained.
Lead-Acid Battery Deep Discharge Design
Deep-Cycle Lead-Acid Batteries
Unlike standard automotive lead-acid batteries designed for short, high-current bursts, deep-cycle lead-acid batteries have thicker plates and more robust construction. This design allows them to withstand deeper discharges (often 80% or more of capacity) and more frequent cycling without immediate damage.
Absorbent Glass Mat (AGM) and Gel Variants
These sealed lead-acid types tolerate deep discharges better than flooded batteries. AGM batteries, for example, reduce maintenance and are less prone to stratification and sulfation, making them more suitable for deep discharge applications.
Battery Management Systems (BMS) and Cut-Off Devices
To protect lead-acid batteries from harmful deep discharge, battery management systems or low-voltage cut-off devices are used. These systems monitor voltage and state of charge to prevent discharging beyond safe limits, thus extending battery life.
Related Sealed Lead Acid Battery
Practical Considerations and Maintenance
Avoid Prolonged Deep Discharge
Leaving a lead-acid battery in a deeply discharged state for extended periods accelerates sulfation and permanent damage. Prompt recharging after use is critical.
Regular Charging and Maintenance
Maintaining proper electrolyte levels (for flooded types), cleaning terminals, and ensuring a healthy charging system (alternator and regulator) help prevent unintended deep discharges and prolong battery life.
Reconditioning
Some deep-cycle lead-acid batteries can be reconditioned after deep discharge to restore partial capacity, but this is not always effective and depends on the extent of sulfation and damage.