The state of discharge for a lead acid cell or battery is generally checked by measuring the specific gravity of the electrolyte. Specific gravity is a ratio comparing the weight of a substance with the weight of water. For instance, concentrated sulfuric acid is 1.835 times as heavy as water for the same volume. Therefore, its specific gravity equals 1.835 times. The specific gravity of water is 1, since it is the reference.
In a fully charged automotive cell, the mixture of sulfuric acid and water results in a specific gravity of 1.280 at room temperatures of 70 to 80°F. as the cell discharges, more water is formed , lowering the specific gravity. When it is down to about 1.150, the cell is completely discharged.
Specific gravity readings are taken with a battery hydrometer, such as the one in below figures. Note that the calibrated float with the specific gravity marks will rest higher in an electrolyte of higher specific gravity. The decimal point is often omitted for convenience. For example, the value of 1.220 in figure. In figure (c), is simply read twelve twenty. A hydrometer reading of 1260 to 1280 indicates full charge, approximately 1250 is half charge, and 1150 to 1200 indicates complete discharge.
The importance of the specific gravity can be seen from the fact that the open circuit voltage of the lead acid cell is approximately equal to
V= specific gravity +0.84