Specific Gravity of electrolyte refers to the its relative density. Specific gravity is the ratio of the density of a liquid to the density of water. The specific gravity is measured by means of a hydrometer. The specific gravity gives an indication of the amount of charge in a battery.
When a lead acid battery is charged, the sulphuric acid which is the electrolyte is transformed into water. The specific gravity of the electrolyte varies between 1.1 and 1.3.
The specific gravity should be periodically checked. If the specific gravity becomes more than 1.3, the electrolyte may be overly acidic and can damage the plates. If the specific gravity is less than 1.1, the plates can become hydrated.
The specific gravity is directly linked with the open circuit voltage (OCV) of the battery. The open circuit voltage rises and falls with the specific gravity of the electrolyte.
Individual manufacturers give a graph or a table describe the exact relationship between the open circuit voltage and specific gravity.
Specific gravity of the electrolyte also varies in accordance with temperature, it decreases with increase in temperature and increases in colder conditions.