The lead-acid battery is a standard for automotive and backup power due to its ability to deliver high current efficiently. Central to this function is the battery acid, which serves as the electrolyte and actively participates in the chemical reactions that store and release electrical energy. This electrolyte is the crucial medium that enables the controlled movement of ions between the battery’s plates, completing the internal electrical circuit. Understanding how the battery acid works involves examining its composition, the structure it operates within, and its role during discharge and recharge cycles.
The Chemical Composition of Battery Acid
Battery acid is a solution of sulfuric acid (\(\text{H}_2\text{SO}_4\)) diluted with deionized water. In a fully charged state, the sulfuric acid concentration is typically between 29% and 32% by weight, corresponding to a specific gravity of around 1.25 to 1.28. This concentration is carefully optimized to ensure high conductivity and limit corrosion of the internal components.
The primary function of this dilute acid is to act as an electrolyte, a substance that allows the movement of electrically charged particles, or ions. When dissolved in water, sulfuric acid dissociates into positively charged hydrogen ions (\(\text{H}^+\)) and negatively charged sulfate ions (\(\text{SO}_4^{2-}\)). These mobile ions are responsible for carrying the charge between the positive and negative plates, enabling the electrochemical reaction.
Anatomy of a Lead-Acid Battery
The battery acid interacts directly with the active materials contained within the battery’s structure, which is divided into multiple individual cells. Each cell contains two distinct types of plates submerged in the electrolyte. The positive plate is coated with lead dioxide (\(\text{PbO}_2\)).
The negative plate is composed of spongy lead (\(\text{Pb}\)), a porous form designed to maximize the surface area for chemical reactions. A thin layer of insulating material called a separator is placed between the plates. The separator prevents the positive and negative plates from touching and causing a short circuit while allowing the electrolyte and ions to pass freely.
The Discharge Cycle: Generating Power
When an external circuit is closed, such as turning on a car’s headlights, the battery begins its discharge cycle, converting stored chemical energy into electrical energy. This process is driven by a double sulfate reaction involving the battery acid and the plates. The sulfuric acid reacts simultaneously at both the positive and negative plates.
At the negative plate, the spongy lead reacts with the sulfate ions from the acid to form lead sulfate (\(\text{PbSO}_4\)) and releases two electrons. These released electrons flow out of the negative terminal and through the external circuit, providing the electrical current. At the positive plate, lead dioxide, hydrogen ions, and sulfate ions react to form lead sulfate and water.
The electrons flowing from the negative plate are consumed at the positive plate, completing the circuit. As discharge continues, the active materials on both plates are converted into lead sulfate, which is deposited on the surfaces. Crucially, the sulfuric acid is consumed and water is produced, causing the electrolyte to become less concentrated and its specific gravity to drop. A fully discharged battery will have a significantly lower acid concentration, consisting primarily of water.
Recharging and System Restoration
The lead-acid battery is rechargeable because the chemical reaction that occurs during discharge is fully reversible. When an external power source, like an alternator or a battery charger, is applied, it forces an electrical current back through the battery. This external current reverses the flow of electrons and the chemical processes that occurred during discharge.
The energy supplied by the charger breaks down the lead sulfate that coated the plates. At the negative plate, the lead sulfate is converted back into spongy lead, and at the positive plate, it is converted back into lead dioxide. Simultaneously, the sulfate ions are driven back into the water, regenerating the sulfuric acid.
This restoration process increases the electrolyte’s concentration, returning it to its fully charged, highly conductive state. The concentration of the acid is directly related to the battery’s state of charge, which is why a measurement of the electrolyte’s specific gravity is a reliable method for determining how much charge remains.