When Does a Battery Need Electrolyte Instead of Water?

Adding electrolyte to a car battery is a complicated subject, so it's important to understand what battery electrolyte is, what it does, and why it gets low before you try to service your own battery.

When you hear about electrolyte in reference to car batteries, what people are talking about is a solution of water and sulfuric acid. This solution fills the cells in traditional lead acid car batteries, and the interaction between the electrolyte and the lead plates allows the battery to store and release energy.

That's why you may have seen people add water to a battery when the liquid inside seemed low. The water itself isn't the electrolyte, but the liquid solution of sulfuric acid and water inside the battery is.

The Chemical Composition of Lead-Acid Battery Electrolyte

When a lead acid battery is fully charged, the electrolyte is composed of a solution that consists of up to 40 percent sulfuric acid, with the remainder consisting of regular water.

As the battery discharges, the positive and negative plates gradually turn into lead sulfate. The electrolyte loses much of its sulfuric acid content during this process, and it eventually becomes a very weak solution of sulfuric acid and water.

Since this is a reversible chemical process, charging a car battery causes the positive plates to turn back into lead oxide, while the negative plates turn back into pure, spongy lead, and the electrolyte becomes a stronger solution of sulfuric acid and water.

This process can occur many thousands of times over the life of a car battery, although the life of a battery can be significantly shortened by draining it below a certain threshold.

Adding Water to Battery Electrolyte

Under normal circumstances, the sulfuric acid content in battery electrolyte never changes. It's either present in the water solution as an electrolyte, or absorbed into the lead plates.

In batteries that aren't sealed, it is necessary to add water from time to time. Some water is lost during normal use as a consequence of the electrolysis process, and the water content in the electrolyte also tends to evaporate naturally, especially during hot weather. When that happens, it has to be replaced.

The sulfuric acid, on the other hand, doesn’t go anywhere. In fact, evaporation is actually one way to obtain sulfuric acid from battery electrolyte. If you take a solution of sulfuric acid and water, and allow it to evaporate, you'll be left with sulfuric acid.

If you add water to the electrolyte in a battery before damage occurs, the existing sulfuric acid, either in solution or present as lead sulfate, will ensure that the electrolyte will still consist of about 25 to 40 percent sulfuric acid.

Adding Acid to Battery Electrolyte

There usually isn’t any reason to add additional sulfuric acid to a battery, but there are some exceptions. For instance, batteries are sometimes shipped dry, in which case sulfuric acid must be added to the cells before the battery is used.

If a battery ever tips over, or electrolyte spills out for any other reason, then sulfuric acid will have to be added back into the system to make up for what was lost. When this happens, you can use a hydrometer or refractometer to test the strength of the electrolyte.

Using Tap Water to Fill Battery Electrolyte

The last piece of the puzzle, and possibly the most important, is the type of water used to top off the electrolyte in a battery. While using tap water is fine in some situations, most battery manufacturers recommend distilled or deionized water instead. The reason is that tap water typically contains dissolved solids that can affect the function of a battery, especially when dealing with hard water.

If the available tap water has an especially high level of dissolved solids, or the water is hard, then it may be necessary to use distilled water. However, processing the available tap water with an appropriate filter will often be enough to render the water suitable for use in battery electrolyte.