What Can Replace Battery Electrolyte?

When and when not to top off your battery's electrolyte solution

Man removing a car battery in the dead of winter

Jan Stromme / Getty

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

When the electrolyte level in your lead-acid car battery gets low, you may find yourself wondering if you can use a common electrolyte alternative—something like saltwater or baking soda. Do not do this. Never put any kind of electrolyte in a lead-acid car battery.

If your battery electrolyte is low, the only thing you should ever add is straight water. There are some specific circumstances where sulfuric acid may be added, such as if the battery has tipped over and leaked, but never add anything else.

What Does it Mean When Battery Electrolyte is Low?

When your mechanic tells you your battery's electrolyte level is low, it means the fluid level in one or more of the battery cells has dropped below the top of the lead plates. What does that mean? Car batteries are composed of a series of lead plates submerged in a bath of water and sulfuric acid. This creates a chemical reaction that builds up electrons, which eventually discharge in the form of electrical current.

If the electrolyte in a battery drops below the top of the plates and is exposed to air, a chemical process called sulfation starts to take place. Sulfation can shorten the life of a battery because it interferes with the normal operation of the cells. Under normal conditions, sulfuric acid in the electrolyte solution is absorbed into the lead plates as the battery discharges power. It is then released back into the electrolyte solution as the battery charges.

Adding the Right Kind of Electrolyte to a Battery

The only electrolyte that can be used in a lead-acid battery is sulfuric acid. Adding anything but 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. While bad for the internal operation of the battery, a mixture of baking soda and water is a good way to clean corrosion from battery terminals and cables.

Under certain circumstances, you can add water to a battery to keep the fluid level above the lead plates—but water must only be added when the battery is fully charged. If it is not fully charged, the battery will overflow as it powers up and cause damage.

How Can Water Be an Electrolyte?

Water, on its own, is not an electrolyte. It can only be an electrolyte when mixed with sulfuric acid, so it stands to reason that you would have to top off a battery with a mixture of sulfuric acid and water, rather than straight distilled water.

The reason why you may, in some cases, be able to add straight water to a battery is that when a lead-acid battery loses water it does not also lose sulfuric acid. Water is naturally lost during the process of electrolysis and can also be lost due to evaporation, especially in hot weather. The volume of sulfuric acid, meanwhile, does not fundamentally change under these pressures, or it is lost at a much slower rate.

An easy way to understand how this works is to think about boiling a pot of saltwater. The water evaporates, but the salt remains behind. If you add plain water to the pot, the salt mixes back in, and you have saltwater again. The same thing happens when you add distilled water to a lead-acid battery.

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

Prolonging Car Battery Life by Filling Electrolyte

Although you can prolong the life of a lead acid battery by keeping it topped off, leaving it empty, or allowing the charge to drain too low, can cause irreparable harm.

Once a battery reaches a certain tipping point, there's no coming back. So if a battery dies, and it won't accept or hold a charge when you try to charge the battery, you're likely dealing with irreversible sulfation.

The best way to prevent this type of situation from happening is to keep the electrolyte topped off as part of a regular battery maintenance schedule.

That knowledge isn't a whole lot of use when a dead battery has already left you stranded in less than ideal conditions, but keeping on top of it can help you avoid the same fate in the future.