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Specific gravity spread between cells

Aviation Mechanic Airframe (AMA) practice question 10 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.K16 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA hydrometer check of a vented lead-acid battery shows the highest cell at 1.285 and the lowest cell at 1.225. What does this difference indicate?

  1. AThe battery is nearing the end of its useful life
  2. BThe battery is fully charged and ready for service
  3. CThe low cell needs electrolyte added to it
Show the answer and explanation

Correct answer: A. The battery is nearing the end of its useful life

  • A — correct The spread is 0.060, and a difference of 0.050 or more between cells means the battery is approaching the end of its useful life and replacement should be considered.
  • B — incorrect A spread this large is a defect indication regardless of how high the best cell reads.
  • C — incorrect Electrolyte level is adjusted only with distilled water; electrolyte is not added to raise a cell.

BACKGROUND

A hydrometer is used to determine the specific gravity of lead-acid battery electrolyte, which is the weight of the electrolyte compared with the weight of pure water. Care must be taken to return the electrolyte to the cell it was drawn from. When a specific gravity difference of 0.050 or more exists between cells of a battery, the battery is approaching the end of its useful life and replacement should be considered. Electrolyte level may be adjusted by adding distilled water, and electrolyte itself must not be added. Frequent refilling with no evidence of external leakage may point to a poor battery, a charging system problem, or an overcharge condition.

STUDY TIP

Subtract the cell readings first and compare the spread with 0.050; the absolute value of the best cell is not what the rule is about.

KEY POINTS

  • A hydrometer measures electrolyte specific gravity.
  • A spread of 0.050 or more between cells signals end of useful life.
  • Top up level with distilled water only, never with electrolyte.

SOURCES

  • sources/ama-airframe/ch09.txt "Battery Maintenance"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.