When Does a Battery Need Electrolyte Instead of Water?

Man working on car battery

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When you hear about “battery electrolyte,” what people are talking about is a solution of water and sulfuric acid, and it’s the interaction between this electrolyte and the lead plates in a car battery that allows it to store and release energy. So it's right to add water to a battery if the electrolyte was low, and it's also true that the liquid in the battery is an electrolyte.

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 and 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.

Adding Water to Battery Electrolyte

Under normal circumstances, the sulfuric acid content in battery electrolyte never has to be added to, but the water does have to be topped off from time to time. The reason is that the water is lost during the electrolysis process. The water content in the electrolyte also tends to evaporate, especially during hot weather, and it is lost when that happens. 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 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. A hydrometer or refractometer can be used 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.