If you’ve ever wondered how your car’s battery stays charged, wonder no more. I cracked open a junkyard alternator and recorded the wonders that lie within in the second installation of our new video series, David Dissects.


The best way to learn how something works is to take it apart, so I’ve been heading to the junkyard, grabbing a bunch of awesome car parts, and tearing them down on my workbench. Last time, I showed you the guts of a turbocharger; this time, witness the magic of the alternator.

I’ll let the video above do most of the talking, but here’s the gist of how it all works: your alternator sends current directly to your battery via a stud on the outside of the alternator housing usually called the “B terminal.” Inside that alternator housing are two main components: a rotor and a stator. The rotor (shown below), which sits on ball bearings and is spun by the engine’s accessory belt, consists of finger poles placed over top of a bunch of copper windings making up the field coil.

The Rotor

That field coil on the rotor receives 12 volts from your battery through your ignition switch, and then through brushes connected to the housing that ride on the spinning rotor’s slip rings. As current passes through the field coil’s windings, a magnetic field is generated in what is essentially just an electromagnet.


You could use regular permanent magnets instead of an electromagnet (as this guy did with his home-brew generator), but then you wouldn’t be able to vary the alternator’s output—which is a problem, since your vehicle’s electrical load isn’t always steady, and neither is the rotor’s RPM (which changes with engine speed, since it’s driven by the accessory belt).

The Stator.

The changing magnetic field from the spinning rotor induces a current inside the three stationary stator windings (shown above). That current, which is alternating current, then goes through a rectifier, which contains diodes. These diodes act as a sort of “one-way valve,” changing the alternating current into direct current to feed the battery.

Another important component in a charging system is a voltage regulator, which keeps alternator output voltage between 13.5 and 14.5 volts to keep the battery charged up, regardless of electrical load (i.e. regardless of whether you turn your lights, radio or heated seats on), and regardless of engine RPM.

Inside the alternator casing.

The voltage regulator reads your battery state of charge through its “sensing terminal,” and uses that information to adjust the amount of current it sends to the field coil in the rotor. The more field current in the rotor windings, the stronger the magnetic field, the more alternating current it induces in the stator, and thus the more direct current that comes out of the rectifier to charge the battery.



So yes, alternators may seem fairly complex, but breaking it open helped me visualized just how straightforward it all is. You’ve basically got a magnet (whose strength can be varied based on battery state of charge) rotating inside of a stationary set of coils. This induces a current in those coils, which gets changed to DC via some diodes, and shipped off to charge the battery.

What junkyard wonders await us next time?