Which statement about the hydrometer reading of a lead-acid battery electrolyte is true?

Which statement about the hydrometer reading of a lead-acid battery electrolyte is true?

• A. The hydrometer reading does not require a temperature correction if the electrolyte temperature is 80 deg.
• B. A temperature correction is required for all electrolyte temperatures.
• C. The hydrometer will give a true indication of the capacity of the battery regardless of the electrolyte temperature.
• D. Temperature corrections are required only for temperatures above 100 deg.

Answer: (A)

The key idea is that a hydrometer measures the electrolyte’s specific gravity, which reflects the battery’s state of charge, but that reading is affected by temperature. The hydrometer is calibrated at a standard temperature (usually 80°F), so at that temperature the reading directly corresponds to the true specific gravity. If the electrolyte is exactly 80°F, no temperature correction is needed—the reading can be used as is. When the temperature is different, you must apply a temperature correction to normalize the reading to the standard reference so you can accurately interpret the charge level. Temperature corrections aren’t needed for all temperatures, and the hydrometer doesn’t give a true indication of battery capacity regardless of temperature; it measures density, which, after correcting for temperature, informs charge state. Corrections aren’t limited to temperatures above 100°F; they apply whenever the electrolyte temperature differs from the calibration temperature.

The key idea is that a hydrometer measures the electrolyte’s specific gravity, which reflects the battery’s state of charge, but that reading is affected by temperature. The hydrometer is calibrated at a standard temperature (usually 80°F), so at that temperature the reading directly corresponds to the true specific gravity.

If the electrolyte is exactly 80°F, no temperature correction is needed—the reading can be used as is. When the temperature is different, you must apply a temperature correction to normalize the reading to the standard reference so you can accurately interpret the charge level. Temperature corrections aren’t needed for all temperatures, and the hydrometer doesn’t give a true indication of battery capacity regardless of temperature; it measures density, which, after correcting for temperature, informs charge state. Corrections aren’t limited to temperatures above 100°F; they apply whenever the electrolyte temperature differs from the calibration temperature.