What instrument is used to measure the specific gravity of the electrolyte in a lead-acid battery?

• A. Hydrometer
• B. Multimeter
• C. Thermometer
• D. Refractometer

Answer: (A)

Measuring the electrolyte’s specific gravity is a way to gauge a lead-acid battery’s state of charge. The sulfuric acid solution in the cells becomes denser as the concentration of acid increases, which happens in a charged cell, and lighter as the charge depletes. A hydrometer works by floating in the electrolyte; the float sinks more in a lighter (lower SG) solution and rises higher in a denser (higher SG) one, giving a direct reading of specific gravity on its scale. Reading should be done at eye level, and you adjust the reading for temperature because electrolyte density varies with temperature. The other tools aren’t used for this purpose: a multimeter checks electrical properties like voltage or resistance, not density; a thermometer only measures temperature; a refractometer measures refractive index, which can relate to concentration in some liquids but isn’t the standard tool for determining electrolyte SG in aircraft battery maintenance.