How Does Your Alternator Make Electricity?

Your Charging System Explained

That lump with a pulley at the end is an alternator. Look important?. Zach Zupancic/Flickr

Your car's alternator is what gives the engine its spark, the headlights their light, and your heater its blow. It's the electricity generator your vehicle uses for every electrical gizmo, and these days that's a lot of little electrical gadgets to power. You thought your battery powered all of those things, right? The truth is your battery only does one thing: it starts your engine. Once the engine fires, the alternator takes over and provides juice for everything.

To be fair, if you are using any of your electrical accessories -- like the radio -- with your engine off, the battery is there for you.

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.

If it's good enough for you to know that the alternator in your car or truck is charging the battery and providing electrical energy to run your accessories, you can stop reading now.

Hopefully this isn't enough for you and you'd like to know how exactly your alternator does this. In the old days, cars used generators, which were far less efficient than alternators and accomplished the task of battery recharging and accessories powering in a slightly different way. If you're ready to geek out a little bit on alternators, read on.


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.

When your alternator is going bad, you'll notice a reduced capacity for electrical use, often resulting in things like dim headlights. But these clues won't last long, because a partially charged battery usually has enough power to operate things like headlights and power windows, but will fail the next time you try to start the vehicle. There is also usually a dash board light, also known as the battery light because it's often shaped like a little battery, that will alert you to an alternator that is not providing enough charge to keep the system up. You can also check your charging system.