# How to get two 12v car alternators to output 24v ?

#### xhdskip

Joined Dec 16, 2015
7
new guy here, wondering how to get two 12v car alternators to output 24v to hook to my 24v dc inverter.
Any help would be appreciated, and Thanks,
skip

#### #12

Joined Nov 30, 2010
18,223
First guess: Fool the two alternators into thinking they are charging two batteries by actually connecting two 12V batteries to them, then connect the batteries to the inverter, being very careful not to let any ground circuits happen between the two alternators.

#### hp1729

Joined Nov 23, 2015
2,304
new guy here, wondering how to get two 12v car alternators to output 24v to hook to my 24v dc inverter.
Any help would be appreciated, and Thanks,
skip
Does your car have two 12 V alternators? One side of the alternator goes to the case so you can't have the mounting mechanism be metal. If you are providing some creative source of mechanical energy be aware that the alternators need a few thousand RPM to put out a meaningful output. See if you can find data on the alternators you intend to use.

Picture attached, 5,000 to 7,000 RPM is typical. Figure car RPM at idle is around 1,000 RPM. Gear ration of crankshaft to belt at the alternator is about 1:5?
If you are planning on running this from a water fall or windmill plan on a BIG gear ratio, but can be done.

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#### alfacliff

Joined Dec 13, 2013
2,458
if you are usng an older alternator, one without an internal regulator, just connect the output to the "field" terminal, it should generate voltage determined by rpm, higher rpm, higher voltage, or make your own regulator for the alternator.

Joined Jul 18, 2013
23,632
If you want each of them to output 24vdc it is simple to make a switching regulator similar to the early ones that replaced the mechanical version, I have done it with a LM311 and a Mosfet.
Even used the same circuit on a 240vdc generator to reg the field.
Max.

#### MikeML

Joined Oct 2, 2009
5,444
You can get 28V out of a 14V alternator by changing the regulator. A 14V 60A alternator will put out ~40A at 24V, but it will take a higher rpm to the shaft.
Watch the internal heating inside the alternator; you may have to force some air through it for cooling.

If you need more current than one alternator can deliver, I would mod both of the them to output 24V and run them in parallel. That avoids having to electrically float the second alternator so that its case could be connected to the 12V output of the first alternator (the case of the alternator is common to the internal rectifier stack)

#### hp1729

Joined Nov 23, 2015
2,304
If you want each of them to output 24vdc it is simple to make a switching regulator similar to the early ones that replaced the mechanical version, I have done it with a LM311 and a Mosfet.
Even used the same circuit on a 240vdc generator to reg the field.
Max.
Good idea. Suggest a design that delivers about 50 Amps.

#### hp1729

Joined Nov 23, 2015
2,304
You can get 28V out of a 14V alternator by changing the regulator. A 14V 60A alternator will put out ~40A at 24V, but it will take a higher rpm to the shaft.
Watch the internal heating inside the alternator; you may have to force some air through it for cooling.

If you need more current than one alternator can deliver, I would mod both of the them to output 24V and run them in parallel. That avoids having to electrically float the second alternator so that its case could be connected to the 12V output of the first alternator (the case of the alternator is common to the internal rectifier stack)
Hmmmm!

#### inwo

Joined Nov 7, 2013
2,419
A full fielded alternator will put out 120vdc.
Once had a kit that converted standard alternator to run power tools.

Opened it up and it was a switch. That's it, just a switch.
Find the sense input of regulator and add 12 volts of drop.
Or see if the same brand alternator has a 24v regulator as a service part.

Joined Jul 18, 2013
23,632
Good idea. Suggest a design that delivers about 50 Amps.
I did a quick modify of the 240v regulator, being as the original was a generator and not an alternator it pulled itself up by the bootstraps, so you may have to initially use a 12v or 24v supply to get it going, (pull itself up).
Also you could eliminate Q1-Q2 for the Mosfet you need.
Only the field circuit is shown, the rectified AC is not.
Max.

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#### MikeML

Joined Oct 2, 2009
5,444
A full fielded alternator will put out 120vdc.
Once had a kit that converted standard alternator to run power tools...
That worked for an alternator out of a '66 Ford Galaxie
Dont try it with an alternator out of anything built recently.

Hint: the PIV rating of the diodes in the alternator's internal rectifier stack is not up to it...

#### xhdskip

Joined Dec 16, 2015
7
Thanks guys, don't know if my son-in-law has an adequate understanding of the attached files or not, hopefully he does,
but either way I appreciate the help and input. To answer a few questions both asked and unasked,
First, I'd already thought about just wiring the 12v alternators to the two batteries,
then connecting the batteries to produce 24 volts, but, I'm looking for a smaller, easier, less complicated pattern
than sending the two sets of 12v charges through the two 12v batteries which converts it to 24 volts so my inverter will take it.
I could eliminate a lot of the process IF I could just directly input the alternator charge into the inverter.
Is there a bridge rectifier or voltage regulator that would do it, and if so,
does anyone have a part number or can ya point me to a "simple" how to?.
I'm no electronics guy, but I'm not above learning new stuff.
I found this site by asking Mr. Google how to change a 12v alternator into a 24v alternator.
This site was the first one, so I clicked on it, and went to a forum page where I read a lot of stuff I really didn't understand.
So I thought, ask a question, heck, you know what you need, there's a lot of guys there smarter than you,
so see if you can gleen some knowledge and a simple answer to the problem that you can do.
That brings us to here and now.
any further assistance will be sincerely appreciated.
skip
P.S.
It's a CS model alternator like on a Chevy Tahoe, or GMC Yukon, or Denali
If that helps, thanks again.

#### tcmtech

Joined Nov 4, 2013
2,867
The most practical method for a non electrically experienced person would be simply find a properly sized light commercial 24 volt unit and go with that given that the common automotive alternators have their negative output tied to their case which makes electrically isolating one but keeping it solidly mechanically mounted more difficult.
Granted with some work the internal rectifier and regulator components can be taken out and physically mounted externally to isolate them from the alternators frame grounded design but that to involves a bit of knowhow to get it to work right.

As far as actual commercial 24 volt alternators go they don't cost much more than your common auto alternators, many are far less in fact, for what will be a superior designed and rated unit. The last 24 volt 100+ amp alternator I bought was a Delco 21si style and it set me back about $180. Thread Starter #### xhdskip Joined Dec 16, 2015 7 Thanks tcmtech, but I couldn't find a high output (say 250 amps) alternator that's also 24v. Tried Amazon and e-bay, any where else I could look and purchase one that you know of? Tried that right after I thought of running the two alternators tied to the two batteries to get my 24 needed volts. I'm trying to build a generator, so high output amps is important. Finally decided I would have to make what I needed, but I'm sure open to suggestions and help from those more knowledgeable than myself in these matters. Thanks again, skip #### tcmtech Joined Nov 4, 2013 2,867 If you need 250+ amps at 24 volts worth of power you will be time, effort and considerable money ahead to just skip the battery and inverter part and go with a normal 6000+ watt rated AC generator head unit. 24 volt 250+ amp alternators are out there but they are expensive and you could looking at over$800 for one whereas a common belt drive 6000 watt 120/240 VAC generator head for far less.

As for the 250 amp 24 volt method you can always put two 24 volt 140 amp units in parallel.