# Alternator output power relationship

Overview:

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.

Example:

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.

Questions:

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?

Background:

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. Commented Jun 8, 2020 at 16:36