Understanding Lead-Acid Batteries: Construction, Operation and Maintenance

Lead-acid batteries are among the oldest and most widely used rechargeable energy storage devices. They are employed in diverse applications including automotive, industrial, and backup power systems. This article explores their construction, operation, care, and the factors affecting their performance and lifespan. If your a battery manufacture, MAC Engineering & Equipment can help you make quality batteries for your customers.

1. Construction of Lead-Acid Batteries

A lead-acid battery consists of multiple cells connected in series. Each cell contains a set of positive and negative plates immersed in an electrolyte composed of sulfuric acid and distilled water. The typical cell voltage is 2 volts, and a 12-volt battery comprises six such cells.

• Positive Plates: Made of lead dioxide (PbO₂) supported on a lead alloy grid.
• Negative Plates: Composed of spongy lead (Pb) on a similar grid structure.
• Electrolyte: A mixture of sulfuric acid and water with a specific gravity of around 1.270 at 60°F (15.6°C).
• Separators: Insulating materials placed between the plates to prevent short circuits.

To increase current output, multiple plates are used within a cell. Positive plates are always sandwiched between negative ones, leading to one more negative plate than positive per cell.

2. Electrochemical Reactions and Battery Operation

During discharge, the following chemical reactions occur:

• At the negative plate: Pb + HSO₄⁻ → PbSO₄ + H⁺ + 2e⁻
• At the positive plate: PbO₂ + H⁺ + HSO₄⁻ + 2e⁻ → PbSO₄ + H₂O

These reactions convert active materials on the plates into lead sulfate (PbSO₄) and generate water, producing a voltage. When recharging, these reactions reverse, restoring the active materials.

3. Charging Characteristics

Lead-acid batteries are charged in three main phases:

• Bulk Charge (I): Constant current phase where voltage rises.
• Absorption (U): Constant voltage; current gradually decreases.
• Equalization (I): A controlled overcharge to balance the charge among all cells.

Proper charging is critical. Overcharging causes gassing, water loss, and plate damage, while undercharging leads to sulfation.

4. Discharge and Capacity

Battery capacity (Ah) depends on discharge rate. Faster discharge reduces available capacity due to internal resistance and chemical limitations.

• A 100Ah battery rated at a 20-hour rate provides 5A for 20 hours.
• At higher discharge rates, capacity declines significantly (e.g., 59Ah over 1 hour).

Avoid discharging beyond 75% depth of discharge (DoD) to preserve battery life.

5. State of Charge (SOC) Assessment

Voltage alone is a poor SOC indicator due to temperature and aging effects. The most accurate method is measuring the specific gravity of the electrolyte with a hydrometer:

Low specific gravity readings indicate diminished charge and increased risk of sulfation.

6. Sulfation and Equalization Charging

Sulfation is the buildup of lead sulfate crystals that cannot be converted back during charging. It occurs due to:

• Deep discharges
• Undercharging
• Long storage without recharging

Equalization charging can reverse moderate sulfation by applying a controlled overcharge that restores plate chemistry.

7. Maintenance Practices

• Watering: Use only distilled or demineralized water. Plates must remain covered.
• Storage: Store on non-conductive surfaces; recharge every two months.
• Ventilation: Ensure proper venting to avoid gas buildup.

Avoid tap water, which contains minerals that corrode internal components. Automatic watering and catalytic caps can reduce manual maintenance but must be used with care.

8. Temperature Effects

• Cold temperatures slow charging and increase absorption.
• High temperatures accelerate discharge and reduce lifespan.
• Ideal operating temperature: 59–77°F (15–25°C) for charging, 68–77°F (20–30°C) for discharging.

Charging hot batteries can cause thermal runaway; they must cool before charging resumes.

Conclusion

Lead-acid batteries require careful handling to ensure longevity and performance. Regular maintenance, proper charging, and adherence to usage guidelines can prevent premature failure and optimize battery capacity. While more modern chemistries exist, lead-acid remains a dependable choice for many applications due to its cost-effectiveness and mature technology.

State-of-the-Art Battery Manufacturing Equipment can be purchased from MAC Engineering & Equipment and members of the Energy Storage Equipment Manufacturing Alliance.