Battery electrolyte is a liquid substance found in most car batteries. It is sometimes called battery acid because it is highly acidic. In fact, the battery electrolyte is made from a mixture of water and sulfuric acid.

When the electrolyte level in a lead-acid car battery gets low, you may wonder if you can use a common electrolyte alternative, something like sea water or baking soda. Do not do that. Never put electrolyte in a lead-acid car battery.

If your battery is low on electrolyte, you should only add clean water. In some cases sulfuric acid can be added for example, if the battery has tipped over and leaked, but do not add anything else.

What does it mean when battery electrolyte is low?

When your mechanic tells you that the electrolyte level in your battery is low, it means that the liquid level in one or more of the battery cells has dropped below the top of the lead plates. What does it mean? Car batteries are made up of row of lead plates immersed in a bath of water and sulfuric acid. This creates a chemical reaction that accumulates electrons, which are eventually discharged as an electrical current.

If the electrolyte in a battery falls below the top of the plates and is exposed to air, a chemical process called sulfation begins. Sulfation can shorten battery life because it interferes with the normal operation of the cells. Under normal conditions, the sulfuric acid in the electrolyte solution is absorbed by the lead plates as the battery discharges. This is then released back into the electrolyte solution as the battery is being charged.

Adding the Right Kind of Electrolyte to the Battery

The only electrolyte that can be used in lead-acid batteries is sulfuric acid. Adding anything other than water to a battery can instantly damage it, but some substances are worse than others.

For example, baking soda can neutralize the sulfuric acid present in a battery’s electrolyte solution. Despite poor internal battery performance, a mixture of baking soda and water is a good way to remove corrosion from battery terminals and cables.

Under certain circumstances, you can add water to the battery to keep the fluid level above the lead plates, but water should only be added when the battery is fully charged. If it is not fully charged, the battery will overflow when turned on and cause damage.

How can water be an electrolyte?

Water by itself is not an electrolyte. It can only be an electrolyte when mixed with sulfuric acid, so it is understandable that you will have to top up the battery with a mixture of sulfuric acid and water, not straight distilled water.

The reason you can, in some cases, add water to the battery is because when a lead acid battery loses water, it doesn’t lose sulfuric acid either. Water is naturally lost during the electrolysis process and can also be lost due to evaporation, especially during hot weather. Meanwhile, the volume of sulfuric acid does not fundamentally change at these pressures or is lost much more slowly.

An easy way to understand how this works is to think about boiling a pot of sea water. The water evaporates, but the salt is left behind. If you add plain water to the pot, the salt will mix again and you will have salt water again. The same thing happens when you add distilled water to a lead acid battery.

The only exception is if the fluid level is low due to the battery tipping over. When this happens, all the sulfuric acid and water solution is lost. In this case, you need to fill the empty cells with a dilute mixture of water and sulfuric acid.

Extending the life of a car battery by filling it with electrolyte

While you can extend the life of a lead-acid battery by leaving it fully charged, leaving it empty, or leaving the charge too low, you can cause irreparable damage.

Once the battery reaches a certain tipping point, there is nowhere to go back. So if the battery dies and it won’t accept or hold a charge when you try charge the battery you are probably dealing with irreversible sulfation.

The best way to prevent this from happening is to keep the electrolyte fully charged as part of a regular battery maintenance schedule.

This knowledge is not very useful when dead battery has already put you in a difficult position in not really ideal conditions, but if you keep it in your hands, it will help you avoid the same fate in the future.

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