Alternator

Once the engine is running, almost all electrical power comes from the alternator, not the battery. It is driven by the auxiliary (serpentine) belt and produces alternating current that internal diodes convert to direct current for the 12-volt system. Output is controlled by a voltage regulator, typically keeping system voltage around 13.5 to 14.8 volts. Many modern cars use smart charging, where the engine control unit varies the output to save fuel, so the voltage you measure can change from moment to moment.

Alternator output is measured in amperes; a modern car with many electrical consumers may use an alternator rated well above 100 A. Some vehicles with start-stop systems use a reinforced alternator or a combined starter-generator.

A rotating magnetic field inside the stator windings induces current. A rectifier bridge turns it into DC, and the regulator adjusts the field current so the output matches demand and battery state. Inside, a rotating electromagnet (the rotor) spins within fixed windings (the stator). The alternating current produced is converted to direct current by a set of diodes called the rectifier.

A failing alternator shows up as a battery warning light, dim or flickering lights, a flat battery or charging-system codes. If it stops charging, the car runs on the battery alone until the voltage drops too far and electronics shut down. Typical signs of a failing alternator are a battery warning light while driving, dimming lights, a whining noise or a battery that is repeatedly flat. A workshop can test charging voltage in a few minutes with a multimeter.