Alternator hack to charge 24v Battery Bank with 12v Alt - Electrician Talk - Professional Electrical Contractors Forum
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Old 12-25-2016, 10:19 PM   #1
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Question Alternator hack to charge 24v Battery Bank with 12v Alt

So i'm looking for a more efficient way bulk up my 24v batteries in the winter.

I'm thinking horizontal 4hp engine belted to one of the old GM 12 style alternators.

I was thinking of fooling the sense line into thinking there was a load with a 100 ohm pot in series with the stock wiring to get the output voltage up to around 30v. Would I leave the idiot light field series circuit intact in that case?

Internet research results have been vague, so I thought I'd ask if anyone had any ideas.

IE some sites suggest the alt output will assume whatever voltage is connected to the output, but don't mention if that's before or after the regulator. (The field controls current not volts out?)

I realize the field would need to have a separate 12v source. I wonder if the regulator would have to be tapped after the diodes but before the regulator - maybe it would be better to get a ford externally regulated one?
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Old 12-25-2016, 10:26 PM   #2
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why don't you just get a used 24v alternator ?
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Old 12-26-2016, 12:24 AM   #3
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Old 12-26-2016, 11:03 PM   #4
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If you get the type with an external regulator, you can apply current to the F terminal and it'll output any voltage up to about 150.

Many years ago a device was made to run 120 volt stuff from these alternators. Of course it was DC but it was still 120. I installed one in my uncles truck when I was about 15 or so. it would run a worm drive Skilsaw just fine.

A word about HP.......a basic 12 volt alternator will produce roughly 30 amps per HP input. This is actual HP, not advertised HP on a gas engine.

I know for a fact that you'll get about 25 - 30 amps @ 14 volts from a 3HP Briggs with the governor wide open. This was in the mid 70s, the engine ratings may have changed.

For any type of generator/alternator, HP in = watts out. In the example above, the 3HP Briggs would have produced about 12 - 15 amps @ 28 volts.

If there's a torque rating on the engine, 1HP = 1.5 ft.lbs @ 3600 RPM, 1HP = 3 ft. lbs @ 1800 RPM.

In my experience, this type of alternator is the most efficient at around 3500 RPM. Below about 3000, the output (and HP input) will drop off. Above about 4000, the output stays the same but the HP input increases.

I agree with the others though, getting a 24 volt model will be the easiest. You'll get about 15 amps per HP.
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