What Acid is in Car Batteries?

If you are wondering what acid is in car batteries, the answer is a specific dilution of sulfuric acid. Commonly known as battery acid or car battery electrolyte, this solution is critical for energy storage. In standard lead-acid systems, the battery fluid is exactly 37% sulfuric acid (H2SO4) and 63% deionized water. Even a slight misspelling like "battery acud" in search logs points to the same vital chemical.

This 37% concentration (CAS 7664-93-9) provides the optimal balance of ionic conductivity and freezing point depression required for automotive and industrial batteries. Sulfuric Acid 37% - Battery Acid is a clear, colorless liquid that is fully miscible with water.

The Role of Car Battery Electrolyte

Lead-acid batteries convert chemical energy into electrical energy through reversible reactions between lead plates and the electrolyte. The 37% sulfuric acid solution provides the sulfate ions (SO4 2-) necessary for this electrochemical process.

During discharge, the sulfuric acid reacts with the lead and lead dioxide plates to form lead sulfate and water. This consumes the acid and dilutes the electrolyte. During charging, the process reverses: the lead sulfate breaks down, regenerating the sulfuric acid and restoring the electrolyte's concentration.

Physical Properties of Battery Fluid Components

A healthy battery relies on the precise physical properties of its two electrolyte components: 37% sulfuric acid and deionized water. The 37% sulfuric acid has a molecular weight of 98.08 g/mol, a boiling point of 337°C, and a melting point of 10°C. It is non-flammable. Deionized Water (CAS 7732-18-5) has a molecular weight of 18.015 g/mol, boils at 100°C (212°F), and melts at 0°C (32°F).

Battery Fluid Maintenance: When to Add Deionized Water

Over time, the water in car battery electrolyte naturally depletes through evaporation and electrolysis (gassing) during the charging cycle. The sulfuric acid, however, does not evaporate. When battery fluid levels drop, the remaining acid becomes overly concentrated, which can accelerate plate corrosion and sulfation.

To restore the fluid level, you must only add pure Deionized Water. Never use tap water, spring water, or filtered drinking water. Tap water contains dissolved minerals (like calcium, magnesium, and iron) that will react with the sulfuric acid, short out the battery cells, and permanently destroy the battery's capacity.

Measuring Specific Gravity

Because the concentration of sulfuric acid changes as the battery charges and discharges, measuring the specific gravity (density) of the battery fluid is the most accurate way to determine the state of charge. This is done using a battery hydrometer.

A fully charged lead-acid battery typically exhibits a specific gravity between 1.265 and 1.280. As the battery discharges and acid turns into water, the specific gravity drops closer to 1.000 (the specific gravity of pure water).

Safety and Handling of 37% Sulfuric Acid

Sulfuric acid is a highly corrosive chemical. Even at a 37% dilution, it can cause severe skin burns and eye damage. Always wear appropriate personal protective equipment (PPE), including acid-resistant gloves, chemical splash goggles, and a protective apron when handling battery acid or servicing flooded lead-acid batteries.

For specific hazard classes, UN numbers, and packing group information regarding transportation and storage, consult the linked Safety Data Sheet (SDS) on the product page.

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