FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician troubleshoots a battery that will not connect to the bus even though the battery master switch is selected ON and battery voltage is present at the switch. The technician suspects the battery contactor (solenoid) itself. What is the correct explanation of how this component normally functions?
- AIt converts battery DC power into regulated AC for instrument lighting
- BIt steps down battery voltage before it reaches the main bus
- CIt limits inrush current to the battery during initial charging only
- DIt uses a small control current to close heavy contacts that carry the main battery current
Show answer & explanationAnswer & explanation
Correct answer: D. It uses a small control current to close heavy contacts that carry the main battery current
A battery contactor (master solenoid) is an electromagnetic relay: a small control current from the battery switch energizes a coil that closes heavy-duty contacts, allowing the full battery current to flow to the main bus without routing that heavy current through the cockpit switch itself.
Why the other options are wrong
- A. AC conversion is the function of an inverter, not a battery contactor.
- B. Voltage step-down is performed by transformers or regulators, not contactors.
- C. Current limiting during charging is unrelated to the contactor's switching function.
Battery Contactor (Master Solenoid)
An electromagnetic relay that uses a low-current control circuit to open or close the high-current path between the battery and the main electrical bus.
- Energized by the battery master switch
- Closes heavy contacts to connect battery to bus
- Protects cockpit switch from carrying high battery current
Memory trick: Small switch, big muscle — the solenoid does the heavy lifting.