I am currently working on a project that requires a 24v alternator, which is powered by a mower engine.

This engine in particular runs around 3,000 rpm, give or take, and does not exceed 3,200 (based on a few quick tests with a cheap tach).

The main reason for this question is that I am trying to understand the power output relationship between the alternator, pulley, and engine.


I did a bit of research and see that alternators, which conform to ISO 8854, are rated as IL/IRA VTV. Where IL is the idle amperage output, IR is the rated amperage output, and VT is the test voltage.

However, when looking at alternators online, these metrics are often left out, which leads me to believe I may be incorrectly interpreting their meaning.

For example, take this Delco 10SI (1102916), which is rated for 40A @ 6,000 rpm.


If you know the engine runs at 3,000 rpm, with a max of 3,200 rpm, and will never hit 6,000, can you just linearly extrapolate and say @ 3,000 rpm the alternator will provide half (20A)?

Or, given the alternator rating in the example above with 40A @ 6,000 rpm, do you gear your alternator pulley in such a way that maps the running engine rpm (3,000) to the alternators rated rpm (6,000) to provide the 40A regularly?


From an application standpoint, the 24v alternator will be used to charge 2 12v 55Ah (wired in series @ 24v). The expected engine run-time is around 1 hour. The goal being to charge the batteries, as the engine is running, for that hour duration. I am trying to figure out the basics in terms of the alternator output power, in relation to engine RPM. Then I can I can size the alternator and accompanying pulley appropriately to charge the batteries.

  • Usually alternators are driven at two or three times engine speed. Just look at the pulley diameters.
    – Solar Mike
    Commented Jun 8, 2020 at 16:36

1 Answer 1


Most alternators, and this one appears to be fairly typical, have a mostly linear power output from about 1000 shaft RPM until the max. In your case that is 6000 RPM. After that the curve is pretty much flat.

What's not clear since it's not spec'd is the slope of the line. You would get a good estimate of that by measuring the output at 1000 RPM. Then you would have a starting point to draw an output "curve" from 1000 to 6000 RPM.

Beyond that, it's just guesswork since that particular unit doesn't list any characteristics other than the 6000 RPM max output.

  • Thanks @jwh20. I tried to up-vote your answer, but do not have enough rep. I will do some testing tonight, but had one last question. If the crankshaft is expected to spin @ 3,000 RPM, should the diameter of the pulley on the alternator be updated such that it will spin @ 6,000 RPM, or is 6,000 RPM a max value? In other words, would running the alternator @ 6K RPM, for say, an hour, burn it out, or is that it's sweet spot where it is expected to run? solar_mike had recommended 2 or 3 times the engine speed, so I just wanted to double check.
    – Birts
    Commented Jun 9, 2020 at 13:18

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