How it is Made: Car Battery Acid

Each cell provides 2.1 volts for a total of 12.6 volts at full charge. Each cell of a lead storage battery consists of alternate plates of lead (cathode) and lead coated with lead dioxide (anode) immersed in an electrolyte of sulfuric acid solution.

Table of Content:
  • What is battery acid?
  • Percentage of H2SO4
  • Lead-Acid Battery Construction and Chemical Reaction
  • Battery pH

What is battery acid?

Battery acid could refer to any acid used in a chemical cell or battery, but usually this term s applied to describe the acid used in a lead-acid battery, such as is found in a motor vehicle.

Percentage of H2SO4

Car or automotive battery acid is 30-50% sulfuric acid(H2SO4) in water. Usually the acid has a mole fraction of 29%-32% sulfuric acid, density of 1.25–1.28 kg/L and concentration of 4.2–5 moll/L. Battery acid has a pH of approximately 0.8.

Lead-Acid Battery Construction and Chemical Reaction

A lead-acid battery consists of two lead plats separated by a liquid or gel containing sulfuric acid in water. The battery is rechargeable, with charging and discharge chemical reactions. When the battery is being used (discharged), electrons move from the negatively-charged lead plate to the positively-charged plate. The negative plate reaction is:

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

The positive plate reaction is:

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

Which may be combined to write the overall chemical reaction:

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

When the battery is fully charged, the negative plate is lead, the electrolyte is concentrated sulfuric acid, and the positive plate is lead dioxide. If the battery is overcharged, electrolysis of water produces hydrogen gas and oxygen gas, which are lost. Some types of batteries allow water to be added to make up for the loss.

When the battery is discharged, the reverse reaction forms lead sulfate on both plates. If the battery is fully discharged, the result is two identical lead sulfate plates, separated by water. At this point, the battery is considered completely dead and can’t recover to be charged again.

Battery pH

The pH scale is used to show how acidic or alkaline a solution is. If something is pH 7, it is neutral. If something is pH 0, it is strongly acidic. If something is pH 14, it is strongly alkaline.

Battery acid is sulfuric acid with a concentration typically between 4M and 5M, and so, the pH is actually lower than 1.5. Battery acid has a pH between -0.6 and -0.7.

And yes, you can have negative pH values. Remember, pH is the “negative” logarithm of the hydrogen ion concentration (pH = -log[H+]). Logs of numbers less than 1 are negative, the (-) in the equation makes most pH’s positive.

Measuring specific gravity

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