Adding electrolyte to a car battery is tricky, so it’s important to understand what battery electrolyte is, what it does, and why it drains before attempting to service your own battery.
When you hear about electrolyte in relation to car batteries, people talk about a solution of water and sulfuric acid. This solution fills elements 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 adding water to a battery when the fluid inside seemed low. Water itself is not an electrolyte, but a liquid solution of sulfuric acid and water inside the battery.
Chemical composition of lead-acid battery electrolyte
When lead acid battery fully charged, the electrolyte consists of a solution containing up to 40 percent sulfuric acid, and the remainder consists of ordinary water.
As the battery discharges, the positive and negative plates gradually turn into lead sulfate. During this process, the electrolyte loses much of its sulfuric acid content and it eventually becomes a very weak solution of sulfuric acid and water.
Since this is a reversible chemical process, car battery charging causes the positive plates to turn into lead oxide and the negative plates turn into pure spongy lead, and the electrolyte becomes a stronger solution of sulfuric acid and water.
This process can occur many thousands of times during the life of a car battery, although battery life can be significantly shortened by discharging it below a certain threshold.
Adding water to battery electrolyte
Under normal conditions, the sulfuric acid content of the battery electrolyte never changes. It is either present in an aqueous solution as an electrolyte or is absorbed into the lead plates.
Batteries that are not sealed need to be refilled with water from time to time. Some water is lost during normal use as a result of the electrolysis process, and the water content of the electrolyte also tends to evaporate naturally, especially in hot weather. When this happens, it needs to be replaced.
Sulfuric acid, on the other hand, isn’t going anywhere. In fact, evaporation is actually one way to get sulfuric acid from battery electrolyte. If you take a solution of sulfuric acid and water and let it evaporate, you are left with sulfuric acid.
If you add water to the electrolyte in the battery before damage occurs, the existing sulfuric acid, either in solution or as lead sulfate, will ensure that the electrolyte is still about 25-40 percent sulfuric acid.
Adding acid to battery electrolyte
There is usually no reason to add extra sulfuric acid to a battery, but there are some exceptions. For example, batteries are sometimes shipped dry, in which case sulfuric acid must be added to the cells before the battery can be used.
If the battery ever capsizes or the electrolyte is spilled for any other reason, then sulfuric acid must be added back into the system to make up for what was lost. When this happens, you can use a hydrometer or refractometer, to check the strength of the electrolyte.