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What is Battery Acid? Its composition and Roles

The battery acid is the acid used in lead- acid batteries. A battery can not work without battery acid or electrolyte.The battery converts electrical energy into chemical energy during charging process. The battery converts the chemical energy into electrical energy during discharging. We use lead-acid battery in  motor vehicles, DG sets and 110 volts DC source for electrical operated breakers. The electrolyte plays very vital role in battery operation. In this article, we will learn about composition of battery acid, its role in battery charging and discharge process.

Battery Acid Composition

The battery acid is made of sulfuric acid (H2So4) diluted with purified water to get  overall concentration around 29-32, a density of 1.25-1.28 kg/L and concentration of 4.2 mol/L.
pH value of electrolyte is about 0.8, and therefore we need to take utmost care while handling battery acid.

Why do batteries contain Acid?

The battery converts

  • Electrical energy into chemical energy during  charging process
  • Chemical energy into electrical energy during  discharging process

The electrolyte is the medium which helps in converting electrical energy into chemical during charging and chemical energy into electrical during discharging.Thus, without battery acid the functioning of the battery is impossible.

Construction of Battery 

A lead-acid battery consists of two lead plates separated by electrolyte. The positive plate has lead peroxide (PbO2), and negative plate has lead (Pb). The diluted sulfuric acid as a electrolyte remains in between the plates. The other part of the battery is the separator. The separators are the  insulating material which keep the battery plates apart to avoid touching of plates. The battery is rechargeable, through charging and discharging chemical reactions.

Role of Battery Acid  in battery charging & discharging

Electrons move from the negatively-charged lead plate to the positively-charged plate during discharging of the battery.
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)
The combined overall chemical reaction is ;
Pb(s) + PbO2(s) + 2 H2SO4(aq) → 2 PbSO4(s) + 2 H2O(l)
The battery charging and discharge equation are as given by
From above equations, it is clear that without electrolyte the electrochemical  reactions are not possible.

What happens during  Battery Charging and Discharge process?

When the battery is fully charged, the negative plate is lead(Pb) , the electrolyte is concentrated sulfuric acid , and the positive plate is lead dioxide(Pb02). Under overcharging condition, electrolysis of water produces hydrogen gas and oxygen gas. Some types of batteries allow water addition for making up the loss.
After battery discharging, the reverse reaction forms lead sulfate on both the  plates.  The fully discharged battery cause  formation of lead sulfate on both the plates separated by water.  At this stage, the battery is completely dead and can’t recover or charged again. Thus deep discharge of battery cause permanent damage to battery.
Never overcharge and deep discharge the batter