Glossary
alternator
The alternator is the belt-driven generator that makes electricity while the engine runs, using it to charge the battery and run the lights, ignition and electronics.
The alternator is the belt-driven generator that makes electricity while the engine is running. It produces alternating current, which a rectifier built into the unit converts into direct current for the vehicle's electrical system.
It is spun by a belt from the crankshaft pulley, so its output rises with engine speed. That is why the battery warning light can glow at idle and go out as you rev the engine — at low speed the alternator is not producing enough to keep up.
Battery, starter and alternator are the three names questions mix. The battery stores charge and supplies the current for starting; the starter is the motor that turns the engine over; the alternator recharges the battery and powers the car once the engine is running.
Taught in
Related terms
- crankshaft — The crankshaft turns the pistons' up-and-down motion into rotation. Each piston's connecting rod rides a journal on the crank, and the spinning shaft is the engine's output.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)