Reading Alternator Output graph

[thebearded1]

I received this testing report with an aftermarket alternator I purchased for a project. Just want to make sure i'm reading it right and figured someone on here would probably know. The graph shows the Amp output vs RPM, this is the rpm of the alternator not the engine correct?
Since the graph doesn't start until about 1300 rpms I assume that's the speed when this alternator starts charging right?

Thanks!

 

[Dana67Dart]

Yes and sort of


If you measure your crank pulley diameter and alternator pulley diameter.
Then take the engine RPM x crank pulley dia / alternator pulley dia

Just numbers
1000(engine RPM) x 6.5 / 2.5 = 2600( alternator RPM)


As for when it starts charging. That would depend on the load. No load it might be providing 12 to 15 v at much lower rpm but might not be able to maintain that voltage with a load.

 

[thebearded1]

Yes and yes


If you measure your crank pulley diameter and alternator pulley diameter.
Then take the engine RPM x crank pulley dia / alternator pulley dia

Just numbers
1000(engine RPM) x 6.5 / 2.5 = 2600( alternator RPM)

Cool this should work out for what I need then. It's sold as a 35 amp alternator but should produce about 75 amps at cruising rpms, if i am interpreting the graph correctly

 

[Mike69cuda]

Since the speed of the alternator would be dependent on the size of the crank pulley, and they don’t know that when testing an alternator by itself, I would assume that the rpm is the speed of the alternator.

The alternator is making some current anytime it is rotating, the graph is just the range it was tested in.

If you want to know how much current it is making at engine speed, you can figure it out as follows.

If you have a 2 1/2 inch alternator pulley and a 7 inch crank pulley (like mine), the alternator would be spinning 7/2.5 = 2.8 ( the ratio of the pulleys) so the alternator spins about 2.8 times faster than the engine. So if you divided the rpm numbers on the chart by about 3, you would be close. You can measure your pulleys and get an exact answer for your engine.

 

[thebearded1]

Since the speed of the alternator would be dependent on the size of the crank pulley, and they don’t know that when testing an alternator by itself, I would assume that the rpm is the speed of the alternator.

The alternator is making some current anytime it is rotating, the graph is just the range it was tested in.

If you want to know how much current it is making at engine speed, you can figure it out as follows.

If you have a 2 1/2 inch alternator pulley and a 7 inch crank pulley (like mine), the alternator would be spinning 7/2.5 = 2.8 ( the ratio of the pulleys) so the alternator spins about 2.8 times faster than the engine. So if you divided the rpm numbers on the chart by about 3, you would be close. You can measure your pulleys and get an exact answer for your engine.

Yup this will be going on a slant six and I have the same pulley diameters so I should hopefully be generating about 57 amps at 800 engine rpm and about 76 amps at cruising at 2100 engine rpms

 

[Dana67Dart]

Cool this should work out for what I need then. It's sold as a 35 amp alternator but should produce about 75 amps at cruising rpms, if i am interpreting the graph correctly

Depending on the pulley ratios.

And if you are pulling 75 amps you MUST redo your stock wiring to handle the load.

 

[67Dart273]

The fact that this sort of info is not more readily available is troubling. When I worked at the "big parts store" we had a HUGE Delco catalog that had curves for endless models of Delco alternators.

And of course it's measured at the alternator shaft. The maker of the alternator cannot possibly know what the pulley ratios are on whatever it's installed

 

[pt70]

Yup this will be going on a slant six and I have the same pulley diameters so I should hopefully be generating about 57 amps at 800 engine rpm and about 76 amps at cruising at 2100 engine rpms

Chrysler rates their alternators @ 1250 ENGINE RPM. Using the bottom part of your chart, it shows the output of your alternator is around 35 amps @ 1250 rpm and ~60 amps at cruising RPM. The amperage at idle RPM is considerably less. The ammeter of my old 1970's Dodge would go negative with the headlights on AND the heater on high, at a low idle speed. And immediately go positive when I hit the gas. This is normal.

The newer chrysler [nippo] alternators have an 8 diode system that seems to put out more amps at idle.

 

[thebearded1]

Depending on the pulley ratios.

And if you are pulling 75 amps you MUST redo your stock wiring to handle the load.

Yup I used the size of the pulleys to determine the alternator rpm. And this is getting all new wire so that's why I was trying to figure out what amps this thing could put out so I could size the wire appropriately. Also not going to use an amp meter or bullhead connector in the car.

I may hook up an ammeter temporarily just to verify the output

 

[Mattax]

1. The car ammeter does not show alternator output. It shows if the alternator is charging the battery; or if pointing to discharge, it shows the amount of current the battery is supplying to run stuff. When the battery is providing power to run things, the alternator is not providing that power. In that sense its an 'alternator gauge'.

2. Yes the shop manual test is usually done at 1250 RPM, 15 Volts. However the Chrysler rating is usually not the same as the shop manual.
An excellent point is this is a maximum output test. Same type of test as the alternator manufacturer is showing in the chart, except at a different voltage and presumably using alternator rpm.

3. At very low rpms, an alternator can suck more power than it is producing. The system will be at 12ish Volts because power will be coming from the battery. Some will flow through alternator's rotor as the regulator tries to get the system voltage up.

I can't fully decipher the test sheet, but it looks as though it may be a self energizing alternator, or has a built in regulator. The section at the top doesn't match the table and graphed output, so not sure what each is representing.

4. Alternator Output never exceeds the demand. The way to read the bottom graph is when the alternator is at 1270 rpm, it is capable of producing 37 amps (at 13.4 volts?). The power capability is amperes multiplied by volts.
That's the maximum power at that rpm.
The actual output will depend on what is using electricity.
If its just the engine running, and its a points ignition, then the current out will be about 2 amps plus whatever the alternator needs.
On the other hand if its an EFI with fuel pumps etc and the car was just started, then its going to be all of that stuff and the battery recharging.

 

[thebearded1]

1. The car ammeter does not show alternator output. It shows if the alternator is charging the battery; or if pointing to discharge, it shows the amount of current the battery is supplying to run stuff. When the battery is providing power to run things, the alternator is not providing that power. In that sense its an 'alternator gauge'.

2. Yes the shop manual test is usually done at 1250 RPM, 15 Volts. However the Chrysler rating is usually not the same as the shop manual.
An excellent point is this is a maximum output test. Same type of test as the alternator manufacturer is showing in the chart, except at a different voltage and presumably using alternator rpm.

3. At very low rpms, an alternator can suck more power than it is producing. The system will be at 12ish Volts because power will be coming from the battery. Some will flow through alternator's rotor as the regulator tries to get the system voltage up.

I can't fully decipher the test sheet, but it looks as though it may be a self energizing alternator, or has a built in regulator. The section at the top doesn't match the table and graphed output, so not sure what each is representing.

4. Alternator Output never exceeds the demand. The way to read the bottom graph is when the alternator is at 1270 rpm, it is capable of producing 37 amps (at 13.4 volts?). The power capability is amperes multiplied by volts.
That's the maximum power at that rpm.
The actual output will depend on what is using electricity.
If its just the engine running, and its a points ignition, then the current out will be about 2 amps plus whatever the alternator needs.
On the other hand if its an EFI with fuel pumps etc and the car was just started, then its going to be all of that stuff and the battery recharging.

I will just watch my digital voltmeter then once I get everything hooked up to ensure this alternator is meeting the needs. It's an off topic car but is powered by a slant six. Points ignition, elec rad fan and headlights are the only big things drawing power so hopefully I should be good.

Yea the top chart confused me too. It is a one wire internal regulator alternator.

 

[Mike69cuda]

Seems to me I was reading on here or other places that the parts store alternators were pretty much all the same for the 60s & 70s. They are all square backs & they just ground one of the field wires for the 69 back applications.

Dana67dart has a really good point on the wiring danger. If you ask for a 35 amp and get a 75 amp (because that’s all they sell no matter what you ask for) then that could be disastrous for older oem wiring. It is pretty hard to tell the amperage of an alternator if it is not obviously marked.

The parts stores probably figure that if they are giving you a “better” part than you paid for, you shouldn’t care.

 

[pt70]

From a 1972 dodge van manual; there are 4 alternators listed. 26A, 35A, 45A, and 51A. This is how the part's stores would know what alternator you have. The higher amperage alternators have a double belt pulley. The ratings are based on 1250 ENGINE RPM @ 115 volts.

I replaced the alternator on my late 1980's truck. The specs that came with the rebuilt Nippo alternator stated the maximum output current is 90 amps and the 'idle speed RPM output' is 28 amps.

Problems occur when a larger amperage alternator is swapped into a car designed for a smaller amperage system. Chrysler charging systems worked well for many years, then corrosion came along. And tinkering.

 

[Mattax]

From a 1972 dodge van manual; there are 4 alternators listed. 26A, 35A, 45A, and 51A. This is how the part's stores would know what alternator you have. The higher amperage alternators have a double belt pulley. The ratings are based on 1250 ENGINE RPM @ 115 volts.

It would be great if it was that clear and simple, but its not.

The 1976 shop manual shows test outputs a little lower than their rating.

Pictured next appears to be the Dodge truck test specs for a '78 amp' revised squareback alternator. It was posted on the Ram truck forum a few years back.

It happens that Nacho posted a test that came with a parts store '78 Amp' revised square back alternator.

It can produce 78 amps at 13.2 Volts when spun to 5000 rpm or greater.
With a 2.5:1 pulley ratio at 1250 engine rpm this alternator is capable of 65 amps at 13.2 Volts or 858 Watts. If the field is saturated, at higher voltage it will be capable of less current. 858 W/15V = 57.2 Amps.

And Dana recently bought a squareback labled as '60 amp' that had a field draw and output curve like this '78 amp'.



Lets look at 1972, which is a good year to work with.
From dealer book for Darts

Test specs in the Plymouth Chassis Manual

We see in 1972 the '34 amp' rated alternator was supposed to be capable of 39 amps when tested per shop manual.
While the other alternators are supposed to test the same as their rating.

Finally, lets look at 1966 because a TSB was issued that year.
The sales booklet shows "30 amp" standard for 6, and "35 amp" standard with v-8

However the 1966 TSB issued to help service and parts departments we see the factory ratings for the standard alternators given as 30 and 37 amps.
https://www.hamtramck-historical.com/images/TSBs/1966/66-78_page2.jpg

Finally the shop manual shows a test output of 26 amps for standard alternator on 6, (rated '30 amps)
and 34.5 amps for the standard v-8 (rated either 35 or 37 depending on where we look).
Presumably the 51 amps special equipment refers to the '60 amp' alternators.

Interestingly, the Dodge and Plymouth Parts books show that when it came to replacement parts or alternators, they generally consolidated down to one or two 'ratings'.