Sealed lead-acid batteries work by converting chemical energy into electrical energy, similar to flooded lead-acid batteries, but with a key difference: they are designed to be maintenance-free, meaning they don't require the addition of water. This is achieved through a clever recombination process that prevents the loss of water and allows the battery to be sealed.
The Basic Chemical Process
Like all lead-acid batteries, sealed lead-acid batteries rely on a chemical reaction between lead and lead dioxide in the presence of sulfuric acid electrolyte. This reaction generates electricity when the battery is discharged.
- Discharge: During discharge, lead (Pb) at the negative plate reacts with sulfuric acid (H₂SO₄) to form lead sulfate (PbSO₄) and releases electrons, which then flow through an external circuit. Simultaneously, at the positive plate, lead dioxide (PbO₂) also reacts with sulfuric acid and accepts these electrons, also forming lead sulfate.
- Charge: The charging process reverses this reaction, converting the lead sulfate back to lead and lead dioxide, and replenishing the sulfuric acid.
Key Differences: The 'Acid-Starved' Design and Oxygen Recombination
The primary feature that separates a sealed lead-acid battery from a traditional flooded one is its design, which prevents water loss. Here's how:
- Acid-Starved Condition: As the reference states, sealed lead-acid batteries contain less electrolyte than flooded batteries, making them "acid-starved". This means there is not enough electrolyte to completely immerse the plates.
- Oxygen Recombination Cycle: When the battery is charged, both oxygen and hydrogen gas are produced. However, instead of escaping, the oxygen travels through separators to the negative plate.
- Recombination: At the negative plate, this oxygen reacts with the lead and sulfuric acid to form lead sulfate. Critically, this lead sulfate then reacts with hydrogen released from the negative plate and also sulfuric acid. The result of this second reaction is that water is produced, completing the "oxygen recombination cycle", and preventing water loss. This is also why the batteries are often called valve-regulated lead-acid (VRLA) batteries, as there is a one way valve to release excessive pressure buildup.
Advantages of Sealed Lead-Acid Batteries
This oxygen recombination process offers significant advantages:
- Maintenance-Free: The batteries do not require water refilling, making them more convenient and suitable for various applications.
- Reduced risk of acid spills: The sealed design prevents spills and leaks.
- Flexible Orientation: Sealed batteries can be operated in various orientations.
Types of Sealed Lead-Acid Batteries
There are two main types of sealed lead-acid batteries:
- Absorbent Glass Mat (AGM): The electrolyte is absorbed into a fiberglass mat, allowing for improved performance in vibration and high-current applications.
- Gel: The electrolyte is in the form of a gel, providing good performance at higher temperatures and preventing leakage when damaged.
Summary
In summary, sealed lead-acid batteries leverage a unique oxygen recombination cycle to prevent water loss, making them maintenance-free and versatile. Their 'acid-starved' design, in combination with the recombination process, is central to their functionality.
