Alternator Sizing - Amp requirements

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mopowers

mopowers

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I'm trying to decide on an alternator for my big block 66 Dart project. I'm thinking a toyota Denso 90 amp should work, but what do you guys think? I'll be running an electric fuel pump, Taurus cooling fan, MSD ignition, line lock, head/tail lights, etc.

I know the right way to figure this out is to add up the amp draw and size the alternator accordingly, but how do I know how much power a particular alternator produces at idle?

Any idea if a 90 amp alternator would get it done for me? Thanks guys. This electrical stuff is like black magic to me.
 
Somewhere buried in the technical books you and I can never find are RPM vs output curves for alternators. You might check with some of the better aftermarket outfits who sell that sort of thing

If you have not, be certain you deal with the "bulkhead connector/ ammeter bypass" if you have not done so

Also, it is my personal preference AGAINST "one wire" set ups. They make it more difficult for battery disconnect situations, and they require a MUCH larger charge wire because the sensing is on that wire

You are right tho, adding up and estimating draw is the "right" way
 
67Dart273 said:
If you have not, be certain you deal with the "bulkhead connector/ ammeter bypass" if you have not done so
Click to expand...

That's one thing I don't have to worry about. I've already welded up the firewall and have done away with the stock bulkhead connector. The stock gauge cluster is LONG gone and I will not be running an ammeter at all. There is not a factory wire left in the car and I will be wiring it from scratch.

I'm just struggling to figure out how to size the alternator for what I need. Say I add up the total amp draw of everything I'm running. Do I then to to ensure the alternator can produce that much power at idle??? I know "perfect world" scenarios and "real world" scenarios are different. I'm just trying to get a general idea on what will do the job.
 
I'd say "it depends." If this is a street car and you do a certain amount of low speed, especially night cruising, they, yeh, you will need a lot of low RPM power

Is this a monster with a really big fuel pump? If not, sounds like your fans are the greatest draw, and in the Sac area, you likely need them a lot at low speed
 
The big issue is the wiring and connections. Overload it and you can burn that pretty car to the ground...
 
When adding up, try to find "running current". I suspect many spec's such as radiator fans are for starting current, which drops greatly after the motor is spinning. At a high level, if your battery stays charged over a week of driving, your alternator is doing its job. One way to know is to put it on a 10A smart charger every Saturday morning and see how long before the charger shows "full". Another approach is to copy a modern car with similar loads, since the designers surely put in the smallest alternator which suffices, as every penny counts.

With a bigger alternator, you likely need bigger than factory wires, and probably bypass the alternator route into the cabin (to dash ammeter) to return to the battery. Search "MAD Bypass". I put large diodes in the underhood shunt route so current still routes thru my ammeter until the diodes start conducting it direct to BATT+.
 
I recommend a call to Powermaster. They can share alternator output curves and you can pick from there. As @67Dart273 notes above, 1-wrie alternators need a big charge wire. I designed up my own wiring like you are planning to and since I have a 1-wire alternator, I ran a 4-ga wire from the alternator all the way back to battery in the rear. (I have it fused on both ends in case of a short anywhere!) With a 95A alternator, I can tell you that at 1000 engine rpm, and with 2 electric radiator fans (approx 35A total), 1 trans cooler fan, and all headlights/brake lights on, the voltage measured at the battery matches the voltage up at the back of the alternator so the 4-ga wire is apparently doing the trick in terms of minimal losses. I share this just to say the 95A rated output is sufficient for the loads I note.
 
67Dart273 said:
Is this a monster with a really big fuel pump? If not, sounds like your fans are the greatest draw, and in the Sac area, you likely need them a lot at low speed
Click to expand...

The fuel pump is just a Mallory 140. I just called Holley, and they said it draws 5 amp max, while the ignition I'll be running draws .7 per 1,000 rpm - so lets say 7 amps there. Looks like a line lock draws about 2 amps. After some searching, looks like the fan I've got pulls about 35 amps on high speed, which is about 50 amps total so far.

I'm just unsure what the load is of high beams/brake lights.

BillGrissom said:
When adding up, try to find "running current". I suspect many spec's such as radiator fans are for starting current, which drops greatly after the motor is spinning. At a high level, if your battery stays charged over a week of driving, your alternator is doing its job. One way to know is to put it on a 10A smart charger every Saturday morning and see how long before the charger shows "full". Another approach is to copy a modern car with similar loads, since the designers surely put in the smallest alternator which suffices, as every penny counts.

With a bigger alternator, you likely need bigger than factory wires, and probably bypass the alternator route into the cabin (to dash ammeter) to return to the battery. Search "MAD Bypass". I put large diodes in the underhood shunt route so current still routes thru my ammeter until the diodes start conducting it direct to BATT+.
Click to expand...

Thank you! The car will have all new wires, as I'll be wiring it from scratch eventually. There will be no bulkhead connector, or ammeter.

Demonx2 said:
I recommend a call to Powermaster. They can share alternator output curves and you can pick from there. As @67Dart273 notes above, 1-wrie alternators need a big charge wire. I designed up my own wiring like you are planning to and since I have a 1-wire alternator, I ran a 4-ga wire from the alternator all the way back to battery in the rear. (I have it fused on both ends in case of a short anywhere!) With a 95A alternator, I can tell you that at 1000 engine rpm, and with 2 electric radiator fans (approx 35A total), 1 trans cooler fan, and all headlights/brake lights on, the voltage measured at the battery matches the voltage up at the back of the alternator so the 4-ga wire is apparently doing the trick in terms of minimal losses. I share this just to say the 95A rated output is sufficient for the loads I note.
Click to expand...

This is great information. Thank you! Do you run an electric fuel pump as well? Which alternator did you go with?
 
Remember that 90 amps is the max output. For street use, the output at idle is at least as important. That 90 amp alt may only put out 50-60 amps at idle speed...
I used a CS-130 alternator that's good for 105 amps... also electronic ignition, Holley electric pump, Ford Contour elec fans. Keeps up fine, I can't find the test sheet with the amps vs. rpm curve though - but since my idle speed is 1200 rpm it seems to be turning the alt fast enough :)
 
mopowers said:
The fuel pump is just a Mallory 140. I just called Holley, and they said it draws 5 amp max, while the ignition I'll be running draws .7 per 1,000 rpm - so lets say 7 amps there. Looks like a line lock draws about 2 amps. After some searching, looks like the fan I've got pulls about 35 amps on high speed, which is about 50 amps total so far.

I'm just unsure what the load is of high beams/brake lights.



Thank you! The car will have all new wires, as I'll be wiring it from scratch eventually. There will be no bulkhead connector, or ammeter.



This is great information. Thank you! Do you run an electric fuel pump as well? Which alternator did you go with?
Click to expand...
I have a Magnafuel QuickStar 275 electric pump. I have not measured current but it is a bunch...I think its around 6-8 amps. As for the lights, I used LEDs for the marker lights and backup lights to minimize current. You can add up the lighting currents for the bulbs you pick but even at 12v, there's plenty of margin. I'm using a #75191 Powermaster 95A single wire alternator - there may be other/ better ones but I'm happy so far with this one. Put it this way...if I had it to do again, 95A would still be my choice for size. Hope this helps!
 
The issue is output at idle, low RPM. You want something that will easily cover the amp draw of your car at idle.

Where power is pulled for non OEM stuff and headlights is the another area to concentrate on.
 
^^And more MAX output does not necessarily mean "more" at low RPM^^ This is why I suggested trying to dig back to the root of a supplier. Somplace there are curves for that output vs RPM
 
Well, after some research, it seems I should be looking into something in the 120 amp range. Depending on the power curve, this should produce plenty of power at idle for what I'll be running.

I've been looking into a reworked 90 amp Land Cruiser model wired for 120 amps, or a John Deere 120 amp Denso alternator. My only concern with a 120 amp rebuilt Land cruiser alternator is that since they were designed for 90 amps, maybe the extra windings etc. may cause overheating issues? Is this a real concern??

Alternatively, the JD model is a stout piece - just pretty large.

As far as wiring, I'll likely be sending power from the alternator to a central distribution block in the cab for cab-located accessories, w/ a separate feed for the headlight relays and anything else under the hood.
 
mopowers said:
I'm trying to decide on an alternator for my big block 66 Dart project. I'm thinking a toyota Denso 90 amp should work, but what do you guys think? I'll be running an electric fuel pump, Taurus cooling fan, MSD ignition, line lock, head/tail lights, etc.

I know the right way to figure this out is to add up the amp draw and size the alternator accordingly, but how do I know how much power a particular alternator produces at idle?

Any idea if a 90 amp alternator would get it done for me? Thanks guys. This electrical stuff is like black magic to me.
Click to expand...
90 amp alternator wont cut it for a street driven car with all that load. Its idle and low rpm cruzn that will hurt you.
Will 90 amp do it, maybe.
Will you run the guts out of making it run hot, yes.
IMHO I would find a single wire of 140 amp that turns on at the lowest rpm.
This is one case you dont want to scrimp on.
 
mopowers said:
My only concern with a 120 amp rebuilt Land cruiser alternator is that since they were designed for 90 amps, maybe the extra windings etc. may cause overheating issues? Is this a real concern??
Click to expand...

My gut says "yes" this is a concern, but it might just depend on "how" it was done. The problem with low RPM high amp/ power output is the low RPM frequency. Because of "the way AC works" lower frequency AC equipment requires different magnetic design and HEAVIER transformer cores at low frequency. A great example is common 120V AC "land" equipment, vs WWII aircraft 115AC which operated at 400 cycles. The transformers and motor cores used at 400hz is MUCH smaller and lighter than 60hz

What this means in alternators is, that at lower frequency (lower RPM) the thing can NOT be as efficient and WILL cause more heating.

I don't know a definitive answer, except to hold the builder's hands to the fire so to speak. I'm with 512Strocker. If you want to be sure, and have dependable cruise, consider a larger one to start with.
 
Thanks guys. I appreciate the input.

After summing the potential loads, I know a 120 amp alternator will do the job just fine, even at idle. Heck, the fan I'll be using came originally in a car with a 130 amp alternator, but had a TON more electronics than what I'll be running. I'll probably just go with the JD unit since it's designed for 120 amps, though the rewired 90 amp unit would be a better fit.

I actually just heard from the alternator shop who would rework that smaller 90 amp alternator. They said that rewired 90 amp unit would likely end up at around 145 amp max. I'm just not sure I like the idea of a re-worked alternator putting out >60% more amperage than what it was designed for. Seems like a failure waiting to happen - albeit from a complete electronics novice's perspective. Either way, it doesn't sound like a good idea.
 
I ended up getting a great deal on a 90 amp Toyota Denso alternator. With the pulley ratio I'll be running, it should work out just fine for me. Here's the charge sheet that came with it.

upload_2021-4-4_20-11-36.png
 
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.
 
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.
Click to expand...

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:

upload_2021-4-5_6-44-12.png


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.

upload_2021-4-5_6-46-18.png
 

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That's great!
Just a quick look at the pdf right now.
My interpretation is Denso's rates their alternators at 5000 rpm shaft speed as the lower line crosses 90 amps at that rpm. Less certain about this, but the lower line may be 12 or 12.5 Volts, hence the 12/90. Or maybe its showing the low end of the current output when controlled to 13.5 Volts.

The circuit diagram looks to me like a self feeding arrangement. F is an internal field connection and the smaller B is a smaller output connection feeding the F through the IC. I think S is the external field connection or start up.

There's guys here who are much better at reading circuits. maybe they can provide some more insight.
The circuit as drawn has some similarities to one commonly used with GM self-exciting alternators that requires an alternator lamp. Don't let that be a concern. That's just one of several ways they can be set up.
I haven't done more than glance at Denso regulation setup some time ago. I know AndyF and others have posted about the wiring connections. I just havent had a need to pay much attention to it.
 
from the builder's test sheet at 2000 rpm shaft speed.
12.8 Volts x 100 amps = 1280 Watts

therefore at 13.5 Volts
1280 W/13.5 V = 94.8 Amps

and at 14.5 Volts
1280 W/14.5 V = 88 Amps

Now there may be a catch in this. It may be that reducing the output voltage also is reducing the voltage to the rotor. That means the rotor is creating a less powerful magnetic field at 12.8 Volts than itwill at 14.5 Volts. In which case regulating to a higher voltage could result in more output power. Without know a little more about the test hookups and the self energizing circuit, we don't know. Or at least I don't know.
I would say that if you derated the 100 amps at 2000 rpm to 88 amps, that would be a worst case scenario.
 
I actually called and spoke with a lead engineer at Denso when I got their data sheet. One thing I thought was interesting was that he mentioned that they rate their alternators very conservatively. The two data sheets that I've posted are essentially for the same alternator - only one's from the manufacturer and one is from a rebuilder. I guess it just shows the numbers in the test are simply a function of the parameters they are tested under. Kinda like flow benches and engine dynos...
 
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