The common term “battery acid” specifically refers to sulfuric acid (\(\text{H}_2\text{SO}_4\)) used as the electrolyte in lead-acid batteries. The electrolyte is the medium that allows the flow of ions between the battery’s internal components to generate an electrical current, facilitating the reversible chemical reactions necessary to store and release energy.
Sulfuric Acid and Lead-Acid Batteries
Sulfuric acid (\(\text{H}_2\text{SO}_4\)) is the definitive chemical identity of battery acid. This acidic liquid serves as the electrolyte in traditional lead-acid batteries, which are frequently found in vehicles and uninterruptible power supplies. The acid interacts with electrodes made of lead (\(\text{Pb}\)) and lead dioxide (\(\text{PbO}_2\)).
During discharge, the sulfuric acid reacts with the lead plates to produce lead sulfate (\(\text{PbSO}_4\)) and water. This chemical transformation releases electrons, creating electrical power. Recharging the battery reverses this process, converting the lead sulfate and water back into lead, lead dioxide, and the sulfuric acid electrolyte.
The Role of Dilution and Concentration
Battery acid is not pure, highly concentrated sulfuric acid; it is an aqueous solution mixed with water. This dilution is necessary for the electrochemical reaction to occur efficiently and safely. The typical concentration of the acid in a fully charged lead-acid battery ranges from approximately 29% to 37% sulfuric acid by weight.
The remainder of the solution is primarily distilled water, which allows for ion mobility. The concentration is often monitored by measuring the electrolyte’s specific gravity, which indicates the battery’s state of charge. As the battery discharges, the acid is consumed to form water and lead sulfate, causing the concentration and specific gravity to decrease.
Other Common Battery Chemistries
Not all batteries contain sulfuric acid; many common types use completely different electrolytes. Standard household alkaline batteries rely on an alkaline electrolyte, most often potassium hydroxide (\(\text{KOH}\)), which is chemically distinct from sulfuric acid.
Nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries also use potassium hydroxide. Lithium-ion batteries, which power modern consumer electronics and electric vehicles, use a non-aqueous electrolyte. This system typically involves lithium salts, such as lithium hexafluorophosphate (\(\text{LiPF}_6\)), dissolved in organic solvents.
Essential Safety Precautions
Because battery acid is diluted sulfuric acid, it remains highly corrosive and requires careful handling. Direct contact with the liquid can cause severe chemical burns to the skin and permanent damage if splashed into the eyes. Proper personal protective equipment, including splash-proof goggles, acid-resistant gloves, and an apron, should always be worn when handling or servicing lead-acid batteries.
Working in a well-ventilated area is important because charging a lead-acid battery can release hydrogen gas and acid mist. Hydrogen gas is highly flammable and explosive. In the event of a spill, the acid can be neutralized by applying a weak base like baking soda before rinsing it with water. If contact occurs, the affected area, especially the eyes, must be flushed immediately and continuously with clean water for at least 15 minutes before seeking medical attention.