Which instrument is used to measure the specific gravity of a battery cell?

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

Answer: (D)

Measuring how dense the battery’s electrolyte is, which tells you its state of charge, is the idea here. The instrument used for that purpose is a hydrometer. It works by buoyancy: you place the hydrometer in the liquid, and the float rises higher in denser liquids and sinks lower in lighter ones. Since the lead‑acid electrolyte becomes more concentrated (and thus denser) as the battery charges, reading the scale on the hydrometer gives you the specific gravity. A higher reading means a more charged cell, while a lower reading indicates a discharge. Temperature can affect density, so readings are often corrected to a standard temperature.

Other common tools don’t measure density. A voltmeter checks electrical potential, an ammeter measures current, and a multimeter can measure voltage, current, or resistance, but none of these directly assess the electrolyte’s density or specific gravity. In practice, you’d use the hydrometer on each cell to assess overall battery health and charge state.