When an automotive battery is fully charged, the sulfuric acid and water mixture will have a specific gravity of about 1.3. Specific gravity is actually the difference in the weight of water in comparison to a specific fluid. It is measured by a hydrometer. The amount of charge in the battery is normally measured by the specific gravity of the battery. The specific gravity of water is 1 and anything less than one is considered less dense while anything that has a specific gravity of more than 1 is considered more dense than water.