What Is the Function of Sulfuric Acid in Car Batteries?

by Phil Borges // in Car

In automobile batteries, sulfuric acid is combined with water to form an electrolyte. This electrolyte facilitates the chemical process within the battery and enables ions to move freely from one terminal to another. However, sulfuric acid can cause problems, such as damaging vehicles or injuries. This blog post will provide in-depth information about the function of sulfuric acid in car batteries and the potential issues that may affect you and your car.

Why Is Sulfuric Acid Used in Car Batteries?

Sulfuric acid in batteries aids the chemical processes and reactions inside a car’s lead-acid battery. Once combined with water, sulfuric acid forms an electrolyte in the car battery.

Inside this battery, a process that converts chemical energy into electrical energy occurs between the negative and positive electrodes of the battery.

The direction of this movement is determined by the process being performed, including charging or discharging through usage. During the charging process, sulfuric acid gets discharged into the electrolyte but gets used up during the discharging process.

The chemical reaction that takes place inside the battery with the assistance of sulfuric acid can be summed up through the following steps:

  • Once the electrons from the negative terminal reach the positive terminal, the sulfate formed from the sulfuric acid makes its way to the negative terminal and forms lead sulfate by combining with lead.
  • Lead sulfate gets formed in the positive terminal, while the electrolyte supports water formation due to oxygen that arrives through the positive terminal. This reaction leads to the battery’s usage in functioning the car.
  • After a point, the battery needs to be charged, which will reverse the process and bring back the sulfuric acid to the electrolyte.

How Much Sulfuric Acid Is Present in Car Batteries?

Usually, 30%-50% of sulfuric acid is contained in the electrolyte in car batteries, while the rest is water. The density of this sulfuric acid tends to be around 1.25 kilograms per liter with varying levels of concentration (usually about 30%-35%). The level of this sulfuric acid tends to fluctuate in the electrolyte throughout the discharging and charging processes.

Should You Add More Sulfuric Acid?

Throughout the process, it’s essential to maintain a certain level of the electrolyte to function the battery. However, you shouldn’t add additional acid to the electrolyte during this time but should top it up with distilled water.

It is usual for the content of sulfuric acid to keep fluctuating inside the electrolyte. While the discharging process takes the acid away from the electrolyte, the charging process brings it back to life.

However, this would reduce the overall sulfuric acid content in the electrolyte due to the small dissipation that takes place each time. Thus, reducing the battery’s efficiency and the number of charging cycles remaining in the battery.

Potential Issues

  • In case of charging does not take place, or if the voltage level during the charging process is insufficient, this can lead to an occurrence known as layering that separates the water from the acid in the electrolyte, leading to a small amount of it being used during the discharging and charging processes. If this happens too frequently, the overall lifespan of the car battery reduces significantly.
  • Also, Sulfation occurs when a low voltage is presented while charging. This results in the crystallization of the lead sulfate that forms at the negative and positive terminals.
  • If crystallization occurs, it becomes difficult for the reaction to reverse itself, preventing the sulfuric acid from returning to the electrolyte. This reduces the battery’s lifespan while also preventing it from recharging appropriately.
  • Furthermore, leakages or spills may also happen if overheating or cracks occur, causing the sulfuric acid to release out into the open. This leads to several damages to the battery and the engine while also causing potential health hazards.

Frequently Asked Questions

Why does sulfuric acid in a car produce electricity?

Sulfuric acid converts chemical energy into electrical energy, allowing it to produce electricity to start the battery and use the car engine efficiently. The electrical energy needs to turn back into chemical energy so that the sulfuric acid continues to produce electricity through its ions at the battery’s terminals.

Can any other acid be used in a car battery?

Sulfuric acid works efficiently in a car battery since car batteries have a lead-acid composition. Therefore, the acid must be compatible with the lead, so it can combine with it and decompose effectively. Not being able to react well to the element leads to several issues with the car battery, ranging from inefficiency, leakages, or even explosions.

What form is the electrolyte available in?

Most commonly, electrolyte comes in its usual wet form that involves a combination of water with sulfuric acid. However, with innovations and technologies, several car batteries have electrolytes in the form of a gel or a stable mat that provides improved efficiency throughout the charging and discharging processes.

Does sulfuric acid require certain conditions to function well?

It is crucial for the sulfuric acid and the car battery to be kept at cool temperatures. Extreme heat or cold temperatures could reduce the efficiency of the battery.

Is it possible to maintain the condition of the electrolyte?

Yes, it’s possible to maintain the electrolyte condition through proper routine cleaning. In addition, several additives such as chemicals are added during the production process to prevent significant issues with the electrolyte.


If there’s no sulfuric acid present in automobile batteries, the lead-acid battery that powers the vehicle will not undergo any chemical processes or reactions. As a result, no electrolytes will be produced. The electrolyte is brought back to life during the charging process after the acid has been removed during the discharging process. However, if the charging process is insufficient, it will separate the water and the acid in the electrolyte. If the battery is overheated, you should get it checked by a professional right away since there’s a risk of leaks or spills, which harm the battery and pose health risks.

About the author, Phil Borges

Phil Borges is a battery aficionado. He's written extensively about batteries, and he loves nothing more than discussing the latest innovations in the industry. He has a deep understanding of how batteries work, and he's always on the lookout for new ways to improve their performance.