FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician performs a hydrometer check on a fully serviced, fully charged lead-acid battery and finds the electrolyte specific gravity reading is only 1.150, corrected to standard temperature. What does this reading most likely indicate?
- AThe battery electrolyte is overfilled with distilled water
- BThe battery is significantly discharged and needs charging
- CThe battery has been overcharged and is gassing excessively
- DThe battery is fully charged and within normal limits
Show answer & explanationAnswer & explanation
Correct answer: B. The battery is significantly discharged and needs charging
A fully charged lead-acid battery normally reads a specific gravity of approximately 1.275 to 1.300. A reading of 1.150 is well below this range, indicating the battery is significantly discharged, since specific gravity drops as sulfuric acid is consumed during discharge.
Why the other options are wrong
- A. Overfilling with water dilutes gravity slightly but not to this degree, and doesn't indicate charge state directly.
- C. Overcharging does not lower specific gravity; it can cause excessive gassing and water loss instead.
- D. A fully charged battery reads much higher, around 1.275–1.300, not 1.150.
Lead-Acid Specific Gravity
A hydrometer measurement of electrolyte density used to determine the state of charge of a lead-acid battery.
- Fully charged: approx. 1.275–1.300
- Discharged: drops toward 1.150–1.200
- Readings must be corrected for electrolyte temperature
Memory trick: Heavy acid = healthy battery; watery acid = weak battery.