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 the charging process. The battery converts the chemical energy into electrical energy during discharging. We use the lead-acid batteries in motor vehicles, DG sets, and 110 volts DC source for electrical operated breakers. The electrolyte plays a very vital role in battery operation. In this article, we will learn about the composition of battery acid and its role in the battery charging and discharge process. 

lead-acid battery internals

Battery Acid Composition

The battery acid is made of sulfuric acid (H2So4) diluted with purified water to get an overall concentration of around 29-32, a density of 1.25-1.28 kg/L, and a concentration of 4.2 mol/L.

battery acid composition

The 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 the charging process
  • Chemical energy into electrical energy during the discharging process

The electrolyte is the medium that 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 an electrolyte. The positive plate has lead peroxide (PbO2), and the negative plate has lead (Pb). The diluted sulfuric acid as an electrolyte remains in between the plates. The other part of the battery is the separator. The separators are the insulating material that keeps the battery plates apart to avoid touching 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

battery charging and discharge equations

From the above equations, it is clear that without electrolytes 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 conditions, the electrolysis of water produces hydrogen gas and oxygen gas. Some types of batteries allow water addition for making up for the loss.

After battery discharging, the reverse reaction forms lead sulfate on both 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 be charged again. Thus deep discharge of the battery causes permanent damage to the battery.

Never overcharge and deep discharge the battery. 

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