FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
During postflight inspection, a technician finds a nickel-cadmium battery case warped and discolored with a strong odor. Which condition most likely caused this damage?
- ALow electrolyte specific gravity
- BReverse polarity connection during installation
- CNormal sulfation from age
- DThermal runaway from excessive charging current
Show answer & explanationAnswer & explanation
Correct answer: D. Thermal runaway from excessive charging current
Thermal runaway in Ni-Cd batteries occurs when internal resistance drops and charging current increases uncontrollably, generating excessive heat that boils electrolyte, warps the case, and produces a strong odor. Sulfation and low specific gravity are lead-acid battery issues, and reverse polarity would cause different symptoms such as reversed cell voltage, not case warping from heat.
Why the other options are wrong
- A. Low specific gravity doesn't apply to Ni-Cd cells the way it does lead-acid.
- B. Reverse polarity would not typically produce case warping from overheating.
- C. Sulfation is a lead-acid battery problem, not Ni-Cd.
Thermal Runaway (Ni-Cd Battery)
A destructive condition where decreasing internal resistance causes charging current and temperature to increase uncontrollably, potentially destroying the battery.
- Signs: excessive heat, case swelling/warping, strong odor, high charge current
- Caused by internal short or voltage regulator malfunction
- Requires immediate removal of battery from charging circuit
Memory trick: Runaway battery runs hot and never stops until you pull the plug