Alternators are typically rated based on the amount of amperage they can output at various speeds, but the subject of alternator ratings is much more complex than those numbers might imply. A number of national and international standards organizations each provide their own standards and specifications, which is why it can sometimes be difficult to determine exactly what an alternator rating means.
While the International Standards Organization (ISO) specifies testing methods and general requirements for alternators in ISO 8854:2012, the IEC has its own specifications for rotating machines like alternators, and some organizations like the Society of Automotive Engineers (SAE) have adapted ISO 8854 for their own purposes. So while alternator power output ratings are often given in a format of “IL / IRA VTV,” it may be useful to know the method by which those ratings were determined.
Cooperation Between Standards Organizations
The international standards landscape is complex, and both international and national bodies have their own standards for alternators in general and road vehicles in particular. This complexity is further confused by the fact that there is a certain level of cooperation between various standards organizations. For instance, although ISO 8854 was prepared by an ISO technical committee, all ISO standards are drafted according to joint ISO/IEC Directive Part 2.
ISO 8854, which specifies general requirements and test methods for alternators that are used in road vehicles, has also been adopted by some national standards organizations. One example of this practice is how the British Standards Institute uses BS ISO 8854 instead of its own unique standard.
Other organizations, like the SAE , have chosen to base their standards on ISO 8854. SAE J 56 is largely based on ISO 8854, but it has been modified to reflect historical precedents and market requirements in the United States. For instance, in order to avoid confusion, SAE J 56 notes that “cut-out speed, n0” was historically referred to as “cut-in” in North America.
Alternator Testing Procedures
Standards for alternator testing procedures are given in documents like ISO 8854 and SAE J 56. Although they differ somewhat, they share a lot in common. For instance, they both specify an ambient air temperature of ±23°C. Both standards also require a standard test voltage of 13.5V, and they both have provisions for using other voltages. One area where these standards differ is that SAE J 56 allows for slightly less accurate test equipment than ISO 8854 requires.
Understanding Alternator Ratings
Although the testing procedures and equipment may vary, alternator ratings are typically given in the format of “IL / IRA VTV,” where IL is the current provided at idle, IR is the current provided at 6,000 RPM, and VT is the test voltage. It is important to note that ISO 8854 specifies that IL is measured at “the idle speed of the engine,” while SAE J 56 specifies 1,500 RPM.
What this means is that an alternator rating of “50/120A 13.5V” means that the alternator is capable of providing 50A at 1,500 RPM and 120A at 6,000 RPM. Furthermore, it means that the rating was arrived at by using the standard test voltage of 13.5V.
Interpreting Alternator Ratings
While reading an alternator rating is relatively simple, there are a few important factors to consider. For instance, alternator test procedures specify that a variable load has to be used in order to maintain a constant 13.5V output. However, an alternator in real world conditions will put out a range of voltages depending on the load and other factors.
It is also important to note that the test speeds refer to the rotational speed of the alternator shaft, which does not spin at the same rate as the engine. That means the idle and maximum outputs of an alternator may differ somewhat from the rating. This is why it’s so important to allow for some deviation when choosing an alternator based solely on the output ratings.
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