A battery hydrometer is a diagnostic instrument used to measure the specific gravity of the electrolyte solution in lead-acid batteries. It helps determine the battery’s state of charge and provides an indication of its overall condition and performance. The tool typically consists of a transparent tube, a rubber bulb or syringe for drawing electrolyte, and a calibrated float or scale that displays the reading. It is commonly used by automotive technicians, electricians, maintenance personnel, and battery service professionals during routine inspections and troubleshooting. Regular use of a battery hydrometer helps identify weak or failing battery cells, supports preventive maintenance, improves battery reliability, and extends service life when used according to recommended safety procedures.