# Is alternator rating at pulley rpm or engine rpm?

I just got a new alternator for my jeep that says 160amps at 2k rpm. Is this rating for the pulley rpm on the alternator - or is it the rpm of the engine?

Also, if the pulley is spinning slower than that RPM how can I approximate the output when it is not at 2k rpm?

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Why are you worried about it? – NitroxDM Jan 8 '13 at 4:45

Alternators aren't built for one engine so the manufacturer can't possibly know the gearing / ratio between the alternator and the engine. Therefore the RPM listed in the spec will be the RPM for the alternator.

To calculate the alternators RPM you will need to find the diameter of the alternator pully [a] and the one connected to (presumably) the crankshaft [c]. Then do the following:

Ratio = [c] ÷ [a]

Alternator RPM = Engine RPM * Ratio

I have no idea how to accurately estimate power output, but I can tell you it will be probably be curved not linear.

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Thanks. I know that most alternators are build for specific vehicles and that is why I was not sure if the listed rating would be for that vehicle or for the alternator. – Sponge Bob Jan 5 '13 at 5:11
Actually the output is pretty linear.That is why they stopped using generators sometime in the early sixties. – mikes Jan 5 '13 at 22:39
Just hook up a Induction Meter and then you will know what the power output it. Most shops will help you out if you don't have one. – NitroxDM Jan 7 '13 at 21:56
@mikes I always thought both were pretty curved and alternators came in for general efficiency etc. – Dan Jan 7 '13 at 22:16
@NitroxDM Won't that only show current draw at that moment, rather than the maximum possible? – Dan Jan 7 '13 at 22:17

Ok I'm going to a assume that your Jeep is set up for wheeling and has a big wench, 6 KC lights up top, and rock lights in on each wheel. So you have a large electrical load and don't want to kill your battery. Here is what you do.

1. Install the alternator while listing to the radio (we want to run your battery down a little.)
2. Hook up a Induction Meter to the battery.
3. Start the Jeep and make sure you see current going in to the battery.
4. Turn everything on, I mean everything the A/C, lights, radio, wench, everything.
5. If there is still current going into the battery, your good. Move the meter to the back of the alternator. That's not the max, but the draw of all your stuff plus the power going to the battery.
6. If there is current going out of the battery. Move the meter to the back of the alternator. That is the max output. Time to buy a bigger unit. Or install a second one. ;)

Another thing you could do.

1. Hook up a meter the alt.
2. Hook up a battery load tester.
3. Start the Jeep.
4. Turn up the load on the battery tester and take note of the max output on the alt. Just don't hold the load for more then a few seconds!

If you don't have a meter or a load tester. Let it be known I don't recommend this approach and you are at your own risk if you do this. Computers and regulators really don't like this and might let there smoke out.

1. Install the alternator.
2. Start the Jeep.
3. Disconnect the battery.
4. Turn everything on, I mean everything the A/C, lights, radio, wench, everything.
5. If the Jeep is still running you are okay reconnect the battery. If not you need a bigger alt.

Hope this helps.

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