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.
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.