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The state of charge of a lead-acid battery is most accurately determined by using a hydrometer. This device measures the specific gravity of the electrolyte, which changes depending on the battery's state of charge. When a lead-acid battery is fully charged, the specific gravity of the electrolyte is at its highest due to the concentration of sulfuric acid. As the battery discharges, the specific gravity decreases, indicating a lower state of charge.

While measuring voltage can provide a general indication of the battery's state of charge, it does not offer the same level of precision as a hydrometer, as voltage readings can fluctuate based on several factors, including temperature and load conditions. Additionally, the use of a voltmeter is essentially a method of measuring voltage, which also does not account for the electrolyte's density or composition. Checking the temperature, while useful for maintenance and performance considerations, does not directly reflect the state of charge in lead-acid batteries. Thus, using a hydrometer is the most reliable method for accurately determining the state of charge in this type of battery.