Hydrometer (Specific-Gravity) Test

The hydrometer tests battery state-of-charge by measuring the specific gravity of the electrolyte. This test cannot be performed on maintenance-free batteries with non-re movable cell caps. Specific gravity is a comparison of the density of the electrolyte to the density of pure water. Pure water has a specific gravity of 1.000, and sulfuric (battery) acid makes up approximately 35% of the electrolyte by weight, or 24% of its volume. In a fully charged battery the electrolyte will have a temperature-corrected specific gravity of 1.260 to 1.290. However, a specific gravity of 1.235 or above, is satisfactory for battery load testing and/or return to service.

Measure specific gravity with a hydrometer. Refer to the instructions supplied with the hydrometer for recommendations on the correct use of the hydrometer.