What Is Battery Acid? Sulfuric Acid Facts

What Is Battery Acid
Car battery acid is around 35% sulfuric acid in water.

Battery acid is a solution of sulfuric acid (H2SO4) in water that serves as the conductive medium within batteries. It facilitates the exchange of ions between the battery’s anode and cathode, allowing for energy storage and discharge.

Sulfuric acid (or sulphuric acid) is the type of acid found in lead-acid batteries, a type of rechargeable battery commonly found in vehicles, emergency lighting systems, and backup power supplies.

Properties of Battery Acid

In a standard car battery, the electrolyte is a mixture of around 35% sulfuric acid and 65% water by weight. This leads to an approximate molarity of about 4.2 M and a density of 1.28 g/cm³. The mole fraction for sulfuric acid in this solution is approximately 0.39. But, battery acid strength ranges anywhere from 15% to 50% acid in water.

Sulfuric acid is a strong acid with a very low pH value. A 35% w/w solution has a pH of approximately 0.8.

Sulfuric acid is colorless and odorless in its pure form, but has a slight yellow hue when impurities are present. It’s highly corrosive and causes severe burns on contact with skin.

How Lead-Acid Batteries Work

A lead-acid battery has two types of electrodes: a lead dioxide (PbO2) positive electrode (or cathode) and a lead (Pb) negative electrode (or anode). The battery acid is the electrolyte that allow for ion movement between the electrodes. This type of battery is rechargeable.

When the battery discharges, a redox reaction occurs that involves both electrodes. Lead dioxide is reduced at the cathode and combines with the hydrogen ions (H+) from the sulfuric acid and forms lead sulfate (PbSO4) and water:

PbO2(s) + HSO4 + 3H+(aq) + 2 e → PbSO4(s) + 2 H2O(l)

At the anode, lead reacts with the sulfate ions (SO42-) from the sulfuric acid and also forms lead sulfate:

Pb(s) + HSO4(aq) → PbSO4(s) + H+(aq) + 2 e

The net reaction when a lead-acid battery discharges is:

PbO2(s) + Pb(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)

Charging and Discharging

When the battery is charging, these reactions reverse, where lead oxide forms lead, lead dioxide, and sulfuric acid. An applied electrical current drives the chemical reactions. The positive lead sulfate electrode (cathode) (PbSO4) oxidizes to lead dioxide (PbO2). The negative electrode (anode), also lead sulfate, is reduced to form elemental lead (Pb). The overall effect of these reactions regenerates the sulfuric acid (H2SO4) in the electrolyte:

2PbSO4 + 2H2O → PbO2 + Pb + 2H2SO4

The battery is considered fully charged when the sulfuric acid has been regenerated and lead sulfate is no longer present on the electrodes. At this point, the specific gravity of the electrolyte is its maximum, reflecting the high sulfuric acid concentration.

Dead Batteries

When a battery is fully discharged, the lead and lead dioxide electrodes have both converted to lead sulfate, and the sulfuric acid has been mostly transformed into water:

PbO2 + Pb + 2H2SO4 → 2PbSO4 + 2H2O

At this stage, the electrolyte is primarily water, and the specific gravity is at a minimum. If left in this state for extended periods, the lead sulfate crystallizes and won’t easily reverse back into lead and lead dioxide. This phenomenon is “sulfation” and it can produce a permanently dead battery.

However, if you promptly recharge a discharged battery, the lead sulfate can convert back into lead, lead dioxide, and sulfuric acid and preserve the battery’s ability to produce electrical current. Regular charging and discharging cycles help prevent sulfation and extend the battery’s lifespan.


It’s also worth noting that overcharging damages a battery as well. When a battery is overcharged, it produces excess heat that breaks down the electrolyte, releasing oxygen and hydrogen gas. This leads to a dangerous situation where the battery could explode if exposed to a spark or flame.

Other Concentrations of Sulfuric Acid

Different concentrations of sulfuric acid carry various names:

  • Concentration less than 29% or 4.2 mol/L: The common name is dilute sulfuric acid.
  • 29-32% or 4.2-5.0 mol/L: This is the concentration of battery acid found in lead-acid batteries.
  • 62%-70% or 9.2-11.5 mol/L: This is chamber acid or fertilizer acid. The lead chamber process yields sulfuric acid with this concentration.
  • 78%-80% or 13.5-14.0 mol/L: This is tower acid or Glover acid. It is the acid recovered from the bottom of the Glover tower.
  • 93.2% or 17.4 mol/L: The common name for this concentration of sulfuric acid is 66 °Bé (“66-degree Baumé”) acid. The name describes the density of the acid as measured using a hydrometer.
  • 98.3% or 18.4 mol/L: This is concentrated or fuming sulfuric acid. While making nearly 100% sulfuric acid is theoretically possible, the chemical loses SO3 near its boiling point and subsequently becomes 98.3%.

Handling and Safety

Battery acid is corrosive and can cause severe chemical burns. In the event of a spill or contact with the skin, immediately flush the affected area with copious amounts of water. If the acid makes contact with the eyes, flush with water and seek immediate medical attention.

In terms of battery safety, proper handling and maintenance are key. Keep batteries upright to prevent leakage and store them in a well-ventilated area away from any flammable materials. When dealing with battery acid, wear appropriate protective equipment, including gloves and safety glasses.

Indications of potential acid exposure risk include corrosion around the battery terminals, a strong sulfur smell indicating a leak, or visible damage to the battery casing. If you notice any of these, seek professional help to handle the situation and avoid potential harm.


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