How Much Acid Should Be In A Battery?

Although solid-state battery technology is becoming more advanced with each passing month, lead-acid batteries are still the most popular form. They’re inexpensive and can last for years without requiring much maintenance. According to recent statistics, the global market for all types of acid-based batteries shall grow to USD 59.7 billion by 2026. As such, it helps to get the most out of their life span until more affordable solid-state batteries are available at your nearest Walmart store.

One of the critical shortcomings of lead-acid batteries is that they have a water-soluble electrolyte. As such, it’s prone to evaporate with extensive usage or when subjected to extreme heat. That disturbs the balance of the acidic solution, making it less capable of storing an electrical charge. That’s why you should ensure the electrolyte level remains adequate at all times.

Throughout this guide, you’ll learn how much acid should be in a battery and how you can retain the balance should it lose it to reactions or the atmosphere.

How Much Acid Should Be in a Battery?

A lead acid battery works similarly to a LecLanche cell, which was first patented by a French scientist in 1866. The only difference is that the lead and lead oxide electrodes allow you to retain the ions when the battery is recharged. The sulfate ions from the sulfuric acid react with lead to form lead sulfate and release a current within the connected circuit. The process is reversed during recharging.

Still, the reaction of ions causes some thermal runoff that evaporates the water within the solution. That translates to fewer H+ ions being available within the electrolyte, reducing its effectiveness in storing a charge. It’s when you should add distilled water to the battery to retain its effectiveness. In optimal conditions, a lead acid battery should have anywhere between 4.8 M to 5.3 M sulfuric acid concentration for every liter of water.

What Is the Ratio of Acid to Water in a Battery?

In a functional lead-acid battery, the ratio of acid to water should remain close to 35:65. You can use a hydrometer to analyze the precise ratio. Alternatively, you can dip a pH strip within the battery cell and observe its color. Since the solution has around 1 pH, it should give the strip a dark pink or crimson coloration.

What if We Add Battery Acid Instead of Distilled Water to a Battery?

Battery acid is highly corrosive and accelerates the reactions within the battery. It might be more effective to increase the battery’s potential for the short term, but if left unchecked, it can quickly erode the electrodes and destabilize the solution. Moreover, adding pure sulfuric acid increases the risk of spillage and may even cause an electrical fire. Therefore, you must always add acid through a distilled water solution, even when reviving a dead battery.

When Should I Add Extra Sulfuric Acid to My Battery?

You should only add extra sulfuric acid to your battery once it’s been depleted to less than 30% of its original capacity. The solution becomes highly diluted at that point, and you can observe sulfate deposits on the electrodes. The acid added alongside distilled not only makes the electrolyte functional again but also neutralizes the basic sulfate solution off the electrodes that you may have used to clean them.

Note: Determining the precise electrolyte levels may be challenging, depending on your battery type. In the case of flooded lead-acid (FLA) batteries, the electrolyte is shared among all the cells. So, you can easily observe the electrolyte level from one glance. However, Absorbed Glass Mat (AGM) batteries have electrolytes filled in individual cells. Thus, you have to check the level in each one to ensure that your battery has enough acid to hold the rated charge.

Frequently Asked Questions

Can You Put Too Much Acid in a Battery?

Yes. It’s easy to put too much acid in a battery, as you won’t immediately notice any reactions. The degradation of electrodes happens over a while so the excess acid can cause damage weeks or even months in advance.

Can I Add Battery Acid to a Battery if It Is Low?

It depends upon the battery’s usage. If it’s only been a few months, you should add only distilled water to retain the electrolyte to adequate levels. For batteries that have been used for a few years, it helps to drain the remaining electrolyte and add a new 5 M sulfuric acid solution with distilled water.

How Much Acid Is in a 12V Battery?

A 12 V battery typically has around 2.7 Kg of electrolyte, of which 1 Kg is sulfuric acid, and the rest is distilled water. Those figures often vary with the manufacturer and battery application since a few might require higher CCAs or increased resilience to extreme weather conditions.

What Happens if You Don’t Put Distilled Water in a Battery?

Distilled water is often the safest way to retain appropriate electrolyte levels in a battery. It increases the concentration of hydronium ions without affecting any of the other components of the battery.

What Is the Percentage of Sulfuric Acid in Battery Water?

The electrolyte solution used in lead-acid batteries should have a 35% sulfuric acid concentration to remain effective. That concentration is retained even after months of use, as it’s mostly the water that evaporates, not the acid.


Adding distilled water and acid to your lead-acid battery properly can grant you an extended lifespan well past its warranty date. We hope this guide on how much acid should be in a battery helps you gain valuable insights. Remember, always use protective measures and place your battery in a safe area when working on it. That minimizes the risks to yourself and any pets or kids who might be nearby.


  1. When Should I Add Extra Sulfuric Acid to my Battery?

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