Which instrument is used to determine the state of charge by measuring the electrolyte's density in a lead-acid battery?

• A. Hydrometer
• B. Voltmeter
• C. Ammeter
• D. Thermometer

Answer: (A)

In a lead-acid battery, the state of charge is reflected by the electrolyte’s density because charging and discharging change the sulfuric acid concentration in the electrolyte. A hydrometer is the instrument designed to measure this density, usually expressed as specific gravity. When the battery is fully charged, the electrolyte is more concentrated with sulfuric acid, giving a higher specific gravity. As the battery discharges, the acid is converted to water and lead sulfate, lowering the density. So by reading the electrolyte’s density with a hydrometer, you get a direct indication of how charged the battery is. Remember to account for temperature, since electrolyte density changes with temperature and readings should be temperature-corrected.

The other instruments don’t measure density: a voltmeter gauges electrical potential, which can loosely indicate charge but not via electrolyte density; an ammeter measures current; and a thermometer measures temperature.