A battery hydrometer is a tool used to measure the specific gravity of the electrolyte solution in lead-acid batteries. This measurement helps determine the state of charge and health of the battery. By floating a calibrated scale in the electrolyte, the hydrometer provides readings that indicate how much charge the battery holds. Using a battery hydrometer is straightforward. The user simply draws a sample of the electrolyte into the device and observes the level on the scale. Regular checks with a hydrometer can help maintain the battery's performance and longevity, making it an essential tool for anyone using lead-acid batteries.