Alternator Sizing - Amp requirements

Mattax said
Thanks for posting that sheet.
Let's look over the numbers for everyone who is interested.

The current output graph and table show the same information.
Notice the output power was only at 12._ Volts!
This may be a standard for Denso or the rebuilder, but its not the test standard in the service manuals for our cars.
The maximum power the alternator can produce at any given rpm is the Volts multiplied by the Amps.
So if the output is being regulated to be 14.2 Volts, the maximum current will be somewhat less than shown in the table.
if you need to know, this will get it pretty close.
P (Watts) = I(amps) x V(volts)

In the upper test result table there is a line indicating voltage set point and another called secondary regulation. I am not sure what those tests are on this alternator. My guess is it was some sort of check that it can tolerate output regulation of 15 Volts?? On an alternator with internal regulation, presumbably one of those is a test for the regulator on/off.

Some things I find odd:

Ripple current but no ripple voltage.

Field Current draw of 0. The rotor must draw current. On an alternator with internal regulator there is zero current supplied externally once the alternator is turning fast enough. But regardles there is still a field current - just that the power is split off internally.

Thanks for the clarification. You are correct in that that curve represents amperage at 12.8 V. That must be how the rebuilder tests them. Here's a zoomed in version of the curve:

Here is the curve directly from Denso (below). I'll try to attach the full pdf of their data sheet for those interested. It's a bit hard to read since a lot of it is in Japanese, but you can clearly see the output curves. The top line is at 13.5 V. I don't know what the lower line is - maybe the regulated voltage?

An important note to point out is that these curves represent amperage at the respective alternator shaft rpm. With my pulley ratio, I'll already be at ~2,000 shaft rpm at idle.