What Kind Of Acid Is In A Car Battery?

Car battery acid is a sulfuric acid.

It is a gloomy Sunday morning when someone came on our front porch, tap my shoulders, and asked me to check his car’s battery. He told me that his car is not starting and he believes it’s the battery’s fault.

I asked him, what is the type of your car’s battery? He said, “I have a lead-acid battery. I always use tap water to top off the water that has been lost; do you think it’s one of the factors?”

Have you ever wondered what kind of acid is in a car battery when you heard “lead-acid” battery, a deep-cycle battery, wet-cell battery? Have you ever think that these batteries have acid inside all of them?

It is not a myth that there’s a kind of acid that is inside a car battery. However, the question is, what type of acid is inside those cells?

The answer – is sulfuric acid. And this sulfuric acid has a great deal on the car battery’s performance. Do you want to know more about it? Well, let’s get it on!

What Is Sulfuric Acid?

Each cell in a lead-acid battery composed of other plates which are made of a lead metal full of cathode lead or anode. Each of this cell is full of sulfuric acid which is commonly called as the electrolyte.

But what exactly is a sulfuric acid?

Sulfuric acid is commonly called as vitriol. It is a mineral kind of acid that is composed of sulfur, hydrogen, and oxygen. Its molecular formula is H2SO4. Sulfuric acid has no smell, no color, and looks like syrup that is water-soluble. It is also highly exothermic.

Because it has a strong acidic quality, it could be a cause of corrosion. Sulfuric acid can cause chemical burns and thermal burns; that’s why it is really important to avoid car battery explosion which can happen due to overcharging.

Sulfuric acid is a common chemical that is actually needed in the industry. If your country is rich in sulfuric acid, this is a good indication that the nation is leading in mechanical aspects. Sulfuric acid can be obtained through different processes: wet sulfuric acid process, contact process, and lead chamber process to name a few.

It is an essential element in the chemical industry because it is usually used in the fertilizer section and also vital in the mineral process, oil industry, and chemical synthesis. Aside from lead-acid batteries, sulfuric acid is used as a cleaning agent for drains and other markets.

When Does A Car Battery Need Sulfuric Acid?

It doesn’t usually happen; however, there are some who are an exception to the rule. Most of the batteries are mailed dry in which state you have to add additional sulfuric acid to maintain the life of the car battery.

If the sulfuric acid spilled out for no apparent reason, that is the time that you need to add sulfuric acid onto your car’s battery. You can test the strength of the sulfuric acid/electrolyte using a hydrometer or a refractometer.

What Is The Ratio Of Acid To Water In A Car Battery?

Some might ask, what is the ratio of the sulfuric acid to the water inside the car battery? Which has more percentage, the water or the sulfuric acid? Can we replace the sulfuric acid with a different type of acid or we just needed it to be the same?

There should be 80 percent of water and 20 percent of sulfuric acid only inside the battery cells. It is the correct ratio of water to sulfuric acid in the battery, and it cannot be interchanged. You can actually see that there are some changes with the percentage, but it is always the water that has a higher rate between the two components.

Is it okay to add sulfuric acid onto a car battery when you see the level of acid going down?

No, it is not advisable to put additional acid onto the car’s battery unless it was tipped all over and all of the acids slipped over. In the event that it happened, you have to check the battery with a mechanic. If there’s only a normal change with the battery’s acid going down, you can top it up with distilled water and not additional acid.

Why Distilled Water Instead Of Tap Water?

A lot of drivers, old and new ones have been asking why you need a distilled water to add into the battery instead of regular tap water just like every simple driver is doing.

The reason why is because distilled water is one of the agents that weaken lead acid in the battery and it triggers the flow of the electricity because of it. You cannot use ordinary tap water because this type of water contains metal ions that respond with the acid and other electrodes causing changes on the charging function of the car battery.

Check the post on how to make distilled water if you are interested in.

Why Not Use Tap Water To Fill The Car Battery?

It is indeed a debate when it comes to which type of water to use to top off the decreasing acid or electrolyte in your car’s battery. There are some, actually a lot of people that have been using tap water to replace the electrolytes in the car battery but it is not really recommended.

Car battery manufacturers recommend only distilled water or deionized water to top off the needed level of electrolyte on the cells. The main reason why it not advisable is because tap water is composed of disintegrated masses that can change the role of the battery especially in charging it. Some people might not notice it right away, but it is working inside the cells; that is why you will only realize when it is too late.

You can only use tap water in the event that you have a filter that will actually remove the dissolved masses on the water – but if you don’t have I tell you not to do so because it will be harmful and we don’t want corrosion build up inside the cells that may lead to even serious problems.

Can We Use Rainwater In Case No Distilled Water Is Available?

When it is not really a solution, rainwater can still be used as a replacement when there is no available distilled water to refill your car battery’s water. Since the best and only option to use distilled water because it has about 5 ppm melted solids.

While rainwater has around 20 ppm, it is better to use it instead of using tap water that has approximately 300-2000 which is higher than the two and it has contained minerals that are not good for the components of a car battery.


We found out what kind of acid is in a car battery through the above paragraphs. It has been mentioned that sulfuric acid has no smell, no color, and looks like syrup but is water soluble. The correct ratio of the water and acid has been discussed as 80% for the water and 20% for the sulfuric acid. It can be changed, but the water is always higher than the acid percentage.