FAA Aviation Mechanic General (AMG)Ground Operations and ServicingMedium
A mechanic checks a lead-acid aircraft battery and finds the electrolyte level below the plates. What should be added to correct the level?
- AA 50/50 mixture of sulfuric acid and distilled water
- BDistilled water only
- CConcentrated sulfuric acid to restore full strength
- DTap water, since it is readily available in the shop
Show answer & explanationAnswer & explanation
Correct answer: B. Distilled water only
Low electrolyte level in a lead-acid battery results from water loss during charging, not acid loss; only distilled water should be added to restore the level, never acid or tap water, which contains impurities that damage the plates.
Why the other options are wrong
- A. A fresh acid mixture would overcorrect the specific gravity and damage the battery.
- C. Adding acid would make the electrolyte overly concentrated and damage the plates.
- D. Tap water contains minerals that contaminate the electrolyte and accelerate plate corrosion.
Lead-Acid Battery Servicing
Maintenance practice for correcting electrolyte level and monitoring charge state of lead-acid aircraft batteries.
- Add distilled water only, never acid
- Specific gravity checked with hydrometer
- PPE required due to corrosive electrolyte
Memory trick: Water leaves, acid stays — refill with distilled water only.