Understanding Alternator Output Ratings

Find out the amount of current that the unit is capable of providing

The output of an alternator is usually expressed in amperes, which is essentially just the amount of current that the unit is capable of providing to all of the equipment that’s hooked into the electrical system. This is an important figure due to the fact that OEM alternators are typically ill-equipped to handle additional loads from aftermarket equipment and upgrades.

Closeup of car alternator.
Robert Llewellyn / Photolibrary / Getty

When that happens, and your alternator output is unable to fully meet the needs of your electrical system, you can experience anything from dim headlights to severe drivability problems. Left alone, this problem will eventually lead to the alternator burning out altogether.

Of course, there’s a difference between an alternator’s amperage “rating” and the amount of current that it can provide at idle speeds, which is why it’s important to have a full understanding of how to read alternator output ratings if you have a lot of power-hungry aftermarket equipment installed.

While the output rating of an alternator gives you an idea of what it's designed to put out, the only way to see what an alternator is actually capable of is to test it. To that end, you can measure the actual output of an alternator under a simulated load, which allows you to get an idea of what it is capable of putting out in real-world conditions.

Alternator Output Ratings and the Real World

The term “alternator output” refers to two distinct, yet related, concepts. The first is the alternator output rating, which is the amount of current that a unit is capable of producing at a specific rotational speed. For instance, a 100A alternator has a “rated” output of 100A, which means that it is capable of providing 100A when the alternator shaft is rotating at 6,000 RPM.

The other thing that alternator output can refer to is the amount of current that a unit actually produces at any given time, which is a function of the physical capabilities of the alternator, the rotational speed of the input shaft, and the momentary demands of the electrical system.

Understanding Alternator Output Ratings

When you hear that an alternator is “rated at 100A,” it can mean a handful of different things depending on where you received the information from. The only time that this is actually a meaningful figure is when an alternator manufacturer or rebuilder uses the term “rating” in its intended capacity, which is defined by international standards documents like ISO 8854 and SAE J 56.

In both ISO 8854 and SAE J 56, alternator testing and labeling standards indicate that the “rated output” of an alternator is the amount of current that it is capable of producing at 6,000 RPM. Each standard also indicates a range of other speeds that an alternator needs to be tested at and defines “idle output” and “maximum” output in addition to “rated output.”

Although alternator manufacturers, rebuilders, and suppliers typically refer to the rated output in promotional materials, both the ISO and the SAE require a format of “IL / IRA VTV,” where IL is the low, or idle, amperage output, IR is the rated amperage output, and VT is the test voltage.

This results in ratings that look like “50/120A 13.5V,” which are typically printed or stamped on the housing of an alternator.

Interpreting Alternator Output Ratings

Let’s take the example from the previous section and examine it:

50/120A 13.5V

Since we know that both ISO and SAE standards call for a format of "IL / IRA VTV" actually pretty easy to interpret this rating.

First, we’ll look at IL, which, in this case, is 50. That means this alternator is capable of putting out 50A at the “low” test speed, which is either 1,500 RPM or “the idle speed of the engine,” depending on which standard you’re dealing with.

The next number is 120, which is “IR” or the amperage output at the “rated” test speed. In this case, this alternator is capable of putting out 120A @ 6,000 RPM. Since this is the “rated” test speed, this number is usually used for the alternator’s rated output.

The last number is 13.5V, which is “VT” or the voltage that the alternator was held at during the test. Since an alternator’s output can vary both up and down from 13.5V in real-world situations, it’s actual output limits will vary from the idle and rated numbers.

Alternator Output Supply and Demand

With all of that in mind, it’s also important to understand that the output of an alternator is tied to the demands of the electrical system in addition to its inherent capabilities and the speed that its input shaft is rotating at any given moment.

In essence, while maximum alternator output is dependent on the rotational speed of the input shaft, the actual output is load-dependent. That basically means that an alternator will never generate more current than is called for by the momentary demands of the electrical system.

What that means, in the real world, is that while an underpowered alternator can cause problems by not meeting the needs of your electrical system, a substantially overpowered alternator represents a lot of wasted potential. For instance, a high output alternator might be capable of putting out upwards of 300A, but it won’t actually provide more amperage than a stock 80A unit if that’s all the electrical system ever tries to draw.

Do You Need a Higher Output Alternator?

In most cases, alternators are replaced due to normal wear and tear. Internal components simply wear out, so the best case of action is to replace it with a new or rebuilt unit that conforms to the same output ratings. There are cases where it is more economical to rebuild an alternator instead of buying a new or rebuilt unit, but that is a different discussion.

There are also cases where an alternator may burn out due to excessive demands over a prolonged period of time. This usually doesn't apply to vehicles that have factory car audio systems and no other additional equipment, but it can quickly come into play as you pile on more and more power-hungry equipment.

In cases where an alternator seems to burn out faster than expected, and the vehicle has a powerful aftermarket amplifier, or other similar equipment, then a replacement with a higher output rating may fix the problem.

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