A battery hydrometer tests lead-acid batteries by drawing fluid into a glass tube containing a calibrated float. The depth to which the float sinks shows the specific gravity of the sulfuric acid solution. A fully charged battery cell will show a specific gravity around 1.265, while a dead cell drops below 1.120. If one cell reads significantly lower than the others, that cell is shorted and the battery must be replaced. Make sure to wear rubber gloves and safety glasses. Battery acid will burn your skin and eat holes in your clothes.
Mechanic Glossary
Battery Hydrometer
A test instrument that measures the specific gravity of battery electrolyte fluid to determine the state of charge and overall cell health.
Frequently Asked Questions
No. Hydrometers only work on flooded lead-acid batteries where you can unscrew the cell caps to access the liquid electrolyte.
Cold fluid makes the electrolyte denser, raising the reading. You must add or subtract points to the reading if the temperature is not 80F.
Try charging the battery slowly. If the specific gravity does not rise after a full charge cycle, the battery is worn out.