Your engine runs on air, fuel and spark. The spark is the center of it all, and for that we need electricity. Your battery supplies electricity, but only enough to get you a few miles down the road. We need more. That's where the alternator comes in. The alternator continually charges the battery so that we never have to worry about that whole "running out of juice" problem. Your battery is 12 volts, but to keep the battery 100% charged and run all of your car's electrical doo-dads at the same time, the alternator has an output of between 13.5 and 14.8 volts. We'll learn more about that in a second. The alternator has three main components: The Stator, Rotor, Diode and a voltage regulator. When the alternator belt or V-belt spins the pulley on the alternator, the rotor inside the alternator spins ... fast. The rotor is basically a magnet or group of magnets that spin, with all that speed, inside a nest of copper wires. These wires are called the stator. I won't go into all of the details about why a magnet spinning within a bundle of copper produces electricity, but it does. (If you want some more technical details, check out this great article on How Electric Motors and Generator Work from my buds at Alt Fuels.) The next step in the chain is a diode assembly that changes the electricity from AC to DC current that your battery can use. There is a final step in the chain, the voltage regulator. In modern alternators, this is a built-in component. Back in the day voltage regulators were big black boxes that had to be bolted somewhere under the hood and wired into the system. The voltage regulator is basically a gatekeeper that will shut off the flow of juice to your battery if the voltage goes above a certain level, usually 14.5 volts. This keeps your battery from getting overcharged and cooked. That's it! As your battery is drained, current is allowed to flow back into it from the alternator and the cycle goes on and on.