In a lead-acid aircraft battery, what does the electrolyte specific gravity indicate?

• A. The battery temperature
• B. The amount of acid relative to water in the electrolyte and the state of charge.
• C. The battery casing type
• D. The electrolyte viscosity

Answer: (B)

In a lead-acid aircraft battery, the electrolyte specific gravity measures how dense the electrolyte is, which reflects the amount of sulfuric acid relative to water. The acid concentration changes with charge state: when the battery is charged, more sulfuric acid is present and the solution becomes denser; when it’s discharged, the acid is consumed and the solution becomes thinner as water content increases. So the specific gravity effectively indicates the state of charge.

Temperature matters, so readings are corrected for ambient temperature for accurate interpretation. Typical fully charged values are around 1.265 to 1.300 at 25°C, while discharged cells run lower, roughly in the 1.110 to 1.190 range. Use a hydrometer to measure each cell’s gravity; uniform readings across cells suggest a healthy battery, while a single low cell can indicate a failing cell.

This parameter doesn’t tell you battery temperature, casing type, or electrolyte viscosity.