What Are High Output Alternators?

Alternators power your car's electronics, but how much is enough?

High output alternator for a car

Jason Young / Flickr / CC BY 2.0

When a vehicle ships from the factory, it comes with an alternator that meets the basic needs of a car's electrical systems. Although there is some wiggle room with the factory charging system, the addition of speakers, subwoofers, infotainment systems, and other energy-draining components can result in dim headlights, poor audio performance, and other problems. The solution to install a high output alternator.

Although there are a few ways to deal with a shortage of power (including additional batteries and stiffening capacitors), a high output alternator is the only way to address the problem. These high-powered units put out higher amperages than factory alternators and are available from aftermarket manufacturers, rebuilders, and OEMs.

What Qualifies as a High Output Alternator?

Since factory alternators aren't uniform in terms of power output, the term high output alternator is going to be relative to the original amperage rating of a vehicle. To qualify as a high output unit, an alternator needs to provide more amperage than the factory unit that it replaced. That means there is a big difference between a high output unit that provides 100A at idle speeds and a water-cooled unit that provides upwards of 350A. There's also a difference between simple re-winds and units that are remanufactured from the ground-up.

Why Factory Alternator Amperage Output Isn't so Hot

The first alternators blew older dynamo generators out of the water in terms of raw amperage output. However, the electrical systems that they had to supply in the 1960s were nothing compared to the electrical systems today. Many early factory alternators were only capable of putting out a maximum of 30A, which is a number that many modern stock units beat at idle speeds.

Today, a typical OEM alternator in an average passenger car or light truck can put out somewhere in the neighborhood of 50-80A. However, there's a variation from one application to another.

The rated amperage refers only to its output at 6,000 RPM, which is why a 108A alternator may only put out 40 or 50A at idle speeds.

Who Needs a High Output Alternator?

Stock units are designed to meet the needs of the electrical systems on the vehicles that the units ship with. Because most people don't make significant modifications to their vehicles, most drivers won't need a high output alternator. So, how can you tell if you need to replace a factory alternator with a higher-powered aftermarket unit?

One sure sign that an alternator is underpowered for its application is if it burns out too fast. If you go through alternators on a regular basis, your unit is probably running right up against the ragged edge at all times, which can cause undue wear.

Even if your vehicle is more or less stock, install a high output replacement alternator if you are in the shop for electrical problems regularly. Since some vehicles ship with multiple alternator configurations, you may be able to find a direct-fit, original equipment replacement unit.

When Enough Isn't Enough

Although a vehicle's electrical system is fairly complicated, you can get a good idea of whether you need a high output alternator by performing a few calculations. For example, wattage is determined by multiplying amperage by voltage, so an 80A alternator is capable of putting out:

80A x 13.5V = 1,080W

That's plenty of power for any factory sound system. However, if you add amplifiers, subwoofers, subwoofer amplifiers, and other power-hungry components (on top of everything from headlights to a cooling fan), it's easy to see how you might need a high output alternator.

It's also important to remember the difference between idle output and rated output. If the rated output of an alternator is 80A, it is only capable of delivering that much amperage when the engine is revved up. Both ISO and SAE standards use 6,000 RPMs to determine the rated amperage of an alternator, which roughly corresponds to 2,000 to 3,000 engine RPMs.