A chemical solution, either a liquid or a gas, that conducts electrical current by releasing ions (unbalanced atoms) that unite with oppositely charged ions on the electrodes. Negative ions, atoms having an excess of electrons, move through the electrolyte to the positive electrode. Positive ions, atoms having a deficiency of electrons, move to the negative electrode.

A lead-acid battery uses an electrolyte of sulfuric acid and water. When the battery is being charged, negative sulfate ions from the plates join positive hydrogen ions in the water and form sulfuric acid. When the battery discharges, negative sulfate ions leave the electrolyte and go to both plates. Positive hydrogen ions left from the sulfuric acid join negative oxygen ions from the positive plate and form water.

The condition of charge of a lead-acid battery can be determined by measuring the specific gravity of the electrolyte with a hydrometer. The specific gravity shows the amount of acid in the electrolyte.