Specific Gravity

In Flooded batteries this relates the density of electrolyte compared to water
Battery charge can be determined in a flooded battery by using a hydrometer to measure the specific gravity of a battery.

Volume of electrolyte expands as temperature rises and contracts as temperature drops,
therefore affecting the density or specific gravity reading. As the volume of electrolyte expands, the readings are lowered and, conversely,
specific gravity increases with colder temperatures, therefore temperature must be calculated also when determining charge state.