Alternator output

[Spanky]

I am using a 100 amp (advertised) alternator, and I have minimal accessories on my bucket (Lights, horn, elec. fan). I recently installed a simple cutoff switch for my fan, since the temperature switch (third one I've purchased) in the intake manifold failed (again). So now I can turn my fan on and off with the toggle switch. Observation: when engine is running with no fan, volt gage reads about 14 volts; turning on the fan, it drops to just under 13 volts. Do I need a bigger alternator?

 

[Gerry]

No

 

[Indycars]

Alternator output in dependent on RPM and alternators don't put out nearly as much at low rotor rpm vs higher rpm. To get the alternator rpm, you will need to know the pulley size of the crank and the alternator. My ratio is 3.0, therefore my alternator turn 3 times faster than the engine. At idle for a stock SBC is about 600 rpm, therefore the alternator is turning 1800 rpm. You are not very high on the curve at 1800 rpm and the output is considerably lower than the rated output.

Do you need a bigger alternator, probably not. Unless you find your engine is overheating when the engine is idling for a long time like in stop-and-go traffic. The fan is not turning as fast as it could when it's receiving 13 volts, as opposed to 14 volts.

What model of alternator are you using ???


.

 

[Spanky]

What model of alternator are you using ???

It's a Tuff Stuff 1 wire GM model 10SI, made in the USA. Admittedly, I did check it at idling. Next time I'm at cruise speed, I'll recheck it.

 

[Indycars]

You might want to check voltage at idle with headlights active.

Assuming your alternator is working properly, you need to have sufficient output at idle up
to cruising rpm with all your possible loads active. That's when it's output is it's lowest and
when you want to have good bright lights while charging the battery.

I'm using the newer GM alternator CS-144 with a better low rpm output, but you know me
....... To much is just enough ! ( You certainly have other options, I have more loads than you. )

Alternator Selection


.

 

[409T]

I am using a 100 amp (advertised) alternator, and I have minimal accessories on my bucket (Lights, horn, elec. fan). I recently installed a simple cutoff switch for my fan, since the temperature switch (third one I've purchased) in the intake manifold failed (again). So now I can turn my fan on and off with the toggle switch. Observation: when engine is running with no fan, volt gage reads about 14 volts; turning on the fan, it drops to just under 13 volts. Do I need a bigger alternator?

No.

 

[Spanky]

Thanks, guys! I feel better now . . .

 

[PotvinGuy]

You're cool, Spanky. It's normal for system voltage to drop as loads are added. Especially true for 1-wire alternators, where the voltage sensing is done at the alternator, and there can be a significant drop in the wiring to the loads. Just curious, where is your volt gauge connected? At the ignition switch? And voltage gauges are not exactly lab-quality. Might check the voltage at the alternator with a digital multimeter. Would expect around 14.5; might have to rev the motor a bit.
Oh, I also have a 100amp alternator and don't have any problems. And I have lots of loads!

I drew this to illustrate where voltage can be lost in grounding, but the principle is the same for the +12. Depending on where you measure the voltage you will get different numbers.

 

[rubicon]

When I first saw the diagram, I thought the arrow was pointing at where the load was sitting. My mistake.