Building a 24v Generator to charge a 24v battery bank

Okay.  So we're thinking of upping the 12v system to a 24v system.  We have only 1 need for direct 12v power; a few 12v lighting strips.  So I figure we can meet that need by either using a low-amperage step-down transformer to supply 12v power from the 24v bank, or run a line from the charge controller's accessory port to a switch or series of switches that will supply 12v power to these light strips.  We have multiple MPPT charge controllers that are variable 12/24v and have 6v and 12v accessory ports.  Our panel setup is 810watts / 24v (really 45v of course).

The major kink with going to a 24v system seems to be charging the batteries with a generator when the sun's been gone too long.   I can't find any sources for high amperage (over 10amps, at least) step-up transformers, so I tossed out that idea.  I found some info on building a large transformer yourself but I don't want to do that.   We are actually about to build 2 generators (using 200cc motors) ourselves, and I thought, well, why not make one a 24v generator?

I haven't been able to find much literature on 24v generators.  My options, so far, appear to be the following:
1.  Use a 12v alternator, run the power through a regulator that bumps it up t o 24v, which the alternator should be able to supply- BUT this will likely burn up the alternator / shorted its life
2.  Wire x2 12v alternators in series to produce 24v power.  That's twice the alternator expense though, and complicates the generator build.  
3.  Use a 24v diesel engine alternator.  I can't find any info on doing this but I can't see where it'd be an issue?  I'd only need half the amperage rating as compared to a 12v alternator, so I wouldn't need a heavy-duty one, if I'm not mistaken.   Could it be as simple as swapping the alternators out or am I missing something?



Any thoughts?  Pro's/con's of 24v generator build?  Anyone tried this?

I haven't done this, but read up a bit, a while back.

A 24v alternator or a heavy duty 12v with 24v regulator should both work alright, the trick is not burning them up with excessive speed.

With the regulator swap, for clarity, it is not that you would be generating 12v at the alternator and then running it through the 24v regulator to step it up; replacing the 12v regulator (assuming it was there..) with the 24v regulator means the alternator will be putting out 24v. (nominal voltages..)

The remaining drawback is that your average 24v regulator is not going to do a great job charging deep cycle batteries. There are 3-stage regulators meant to do this much better. (Balmar, xantrex... mastervolt?) These are marine products, generally, so there is probably some sticker shock involved...

2 alternators in series does not seem like any sort of good idea.


This instructable has an interesting take on the matter. Note how hands-on the charging process is...

https://www.instructables.com/id/Off-Grid-LPG-Battery-Charger/


I ended up deciding an AC generator, while less efficient for batt charging, was sufficiently more flexible to be preferable. I can use it to run the tablesaw or compressor instead of firing up the huge shop generator. I also charge the laptop and up to 5 powertool batteries at the same time that I'm topping off the 24v battery bank.

Alternators provide wild 3 phase ac the regulator does the rectifying and voltage control. if you are going to build a alternator based generator get one geared from the work go for 24 volts. I purchased this one:
https://www.amazon.ca/gp/product/B0081SBDCM/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
I only ran it 4 times and have removed it and stored the parts as I needed the engine for a snowblower. It worked fine. I upgraded to a magnum inverter with a charger so didn't need it any more. Some cautions:  alternators are made to charge a small battery fast then slow down not always the best for deep cycle. At 40 amps I was charging a 400 amp hour battery bank  so that is a c10 rate, which is ideal. That kind of sustained amperage on an alternator is hard so they usually recommend a heavy duty or marine unit. Also you must turn a one wire fairly fast to get it to self energize. The older 2 wire units are good as well you can adjust charge amps with a resistor. They can turn a little slower but you need the rpms to turn the fan fast enough to keep it cool. Recommended RPM is usually in your specs.  Alternators are not good at 3 stage charging and loose efficiency as a battery gets close to full so think of it as a bulk charger only let the solar do absorb and float...
Here is the base article I used this one is a 12 volt 2 wire but 24 volt single wire is mostly the same. Its an oldie but a goodie from Home power magazine when it was more then just what gear to buy... All the other articles I've found online are derivatives of this one.
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CFkQFjAG&url=http%3A%2F%2Fwww.homepower.com%2Fsites%2Fdefault%2Ffiles%2Fuploads%2Fwebextras%2Fmark8.pdf&ei=m70uUf_aOYK8yAGihYGYAw&usg=AFQjCNFSSinc1_FuS_mj2lM9RAjsn6dzwA&sig2=WtkyXcTaagzdtdXrsZukog
You could also just buy an iota charger for the same money which does three stage charging and power it with a 1500 watt ac generator also pretty common and cheap. Home built is always more fun though!!!
Hope you build one.
Cheers, David

Large truck (Semi's) use 24V systems, including 24V alternators.

Also, when searching for solutions, it often helps if you use the correct terms.  "Transformers" only work with AC circuits, for example standard 120V /240V systems.  Transformers do NOT work with DC systems (batteries).

You use "DC to DC converters" to go from one DC voltage to another.  Specifically a "Buck" converter to reduce voltage and/or a "Boost" converter to increase voltage. Perhaps a Buck/Boost if you need one that can do either.

Final thought.  Alternators (cars or trucks) will happily produce over 100V.  As you know, the regulator controls the voltage, replace the regulator you can change the voltage.  Unless you get way over 100 volts, what damages alternators is pushing too much current through them not the voltage.

Thanks for all the input!

To clarify, we already use a charge controller on our generator to charge the batteries.  It handles the 3-stage charging.  Basically the generator supplies a high amerapge charge controller, like the solar panels would, and that is then used to charge the bank.  So the problem of 3-stage charging is taken care of.

On the transformer/regulator terminology; I learned it as regulator, but in all this research I've been doing I kept seeing the terms intermixed.  Even when shopping for DC regulators, many of them are also called transformers in the same item description.  So thank you for the clarification.

We've already purchased 2 12v alternators (before we started thinking about a 24v system).  We did our research on a model that was built to actually sustain its rated amperage.  So we're hoping to find a 24v alternator that is built tough enough and large enough to sustain a higher amperage output.  Ours is a 450ah bank, so we don't need crazy high amperage, 10-15 would suffice.  
It's sounding like going with a 24v alternator might be the best choice here.  We just need to figure out which unit can endure this application!  Any input or experience on that?  

David B;  thanks for the product recommendation.  I'm researching this model.  And you gave me a good chuckle!  I followed your link and tried navigating Amazon further and got in a tizzy "WHY DOES AMAZON THINK I'M IN CANADA".  Hahahahaha!  Thanks

If it's a 450AH bank, you'd want at least 45 amps of charging to keep it healthy.   In general, lead-Acid batteries work best when charged at their C/10 rate (capacity / 10), at least for the bulk charging phase.

Good to know, thanks Peter

We keep bouncing back the idea of running 2 alternators in series.  It seems like running 2 12v alternators to supply the 24v power would equate to less work per alternator, kind of splitting the load between the two so that it's easier work for them.  Does that sound reasonable?  

Is your current system 450AHr times 12V = 5400WHr = 5.4kWHr
If so your future system will be 225AHr times 24V = 5400WHr = 5.4kWHr
Can you clarify?

But lets go with your quoted 450AHr times 24v = 10.8kWHr
You are going to need a generator that is outputting  45A * 24V = 1080W (1.5hp)
So the engine should be 1.5hp*2= 3hp
The alternator efficiency is 60% and it would be best if the engine was only at 60% so more like 1.5hp*3=4.5hp


I see an alternator that does 24v and 40A
https://m.autozone.com/starting-charging-and-miscellaneous-electrical/alternator-universal/remy-agricultural-alternator/75978_0_0?review

I see lots of 12V * 140A alternators for sale here.
https://www.autozone.com/batteries-starting-and-charging/alternator

What is the max output of the MPPT controller?

If it's at all likely to receive more power than this, what is the expected behavior when it has excess incoming power; burn up, or self-limit?

The below post mentions getting more than rated power from alternators used as described with an mppt controller; by virtue of the lack of a 24v battery connected, the alternator ends up putting out notably higher voltage. This is likely perfectly fine for the mppt controller, as long as the resulting higher amperage and hence higher wattage don't become excessive... or, if the unit is equipped to handle excess power by limiting output.

https://forums.energymatters.com.au/wind-solar-misc/topic6042.html

S Bengi wrote:Is your current system 450AHr times 12V = 5400WHr = 5.4kWHr
If so your future system will be 225AHr times 24V = 5400WHr = 5.4kWHr

We currently have x4 12v 225AH batteries, which would be combined into x2 24v 225AH batteries.

S Bengi wrote:
But lets go with your quoted 450AHr times 24v = 10.8kWHr
You are going to need a generator that is outputting  45A * 24V = 1080W (1.5hp)
So the engine should be 1.5hp*2= 3hp
The alternator efficiency is 60% and it would be best if the engine was only at 60% so more like 1.5hp*3=4.5hp

The motors are 6.5hp, the heavy duty alternators we have currently are rated 100amp.

Dillon Nichols wrote:What is the max output of the MPPT controller?

If it's at all likely to receive more power than this, what is the expected behavior when it has excess incoming power; burn up, or self-limit?

The below post mentions getting more than rated power from alternators used as described with an mppt controller; by virtue of the lack of a 24v battery connected, the alternator ends up putting out notably higher voltage. This is likely perfectly fine for the mppt controller, as long as the resulting higher amperage and hence higher wattage don't become excessive... or, if the unit is equipped to handle excess power by limiting output.

https://forums.energymatters.com.au/wind-solar-misc/topic6042.html

Our current generator is actively wired through its DC plug port to a 60amp mppt controller.  All of our controllers allege that they disperse excess power through the controller body as heat.  I've never knowingly overloaded one, so how they truly function I do not know.  Our panels flow through 40amp controllers, though they're never handling that much.  They might if our panels produced their full potential, but our current setup does not allow for that.  We get about 50-70% efficiency from them at the moment.

Thanks for the clarification.
Can you send a link to the manual/documentation for the alternators that you currently have.

Also I am confused if it a alternator with AC sine wave power
Or if it is a generator with DC/rectified power.
If it is noisy AC it will be super hard to combine two in series, due to phase.
But it will be easy to use a simple transformer to change just 1 from 12V,100A to 24V & 50A.
And that would be 1200W which would provide all the power that we need.
You would have to change that it from AC to DC for the charge controller.
If it is in fact DC/generator vs an AC/alternator then it would be simpler to combine two in series.

Charge controller can deal with "excess" solar panel energy very easily.
But when it comes to Hydro/Wind/Engine DC power it does need a dump load.
I have never seen one with it built in but they are always releasing new things.
Can you send a link to the Charge Controller manual/documentation.
I love seeing what "new" products gets release.

Next will be setting up the 6.5hp engine to only output 1.5hp (1080W = 24V*45A)
What is the optimal RPM/torque for engine life vs fuel efficiency.
Hopefully you will not be using the generator too much for you to even worry about it.
How often do you see yourself needing to use the generator/alternator.
12 times a year (once a month) or is it more like 52 times a year (once a week)?

We've had the 24v system thing on hold, waiting on some new batteries to arrive; and for the 24v generator to be built.  In the mean time we've altered our solar setup and are getting excellent power coming in.  The highest we've seen is 600w~ out of a max 800w~, which is great considering where they're at and the time of year!

In the summer we rarely need the generator.  In the winter we need it very frequently.  But part of that is how sickly our battery bank is.  Hence getting new AGMs.  Our flooded batteries have been through both unattended frozen winter storage and extreme summer heat of 100º+.  They don't hold a charge for long anymore so we're having to help them along in the evening with the generator.  We almost leaned towards lithium ions, but I'm not ready to go into debt for superior batteries

There is a lot of excessive complication in this thread.

I would just purchase a charger inverter and be done with it.  Hook any old generator up to the inverter and it will charge your batteries, plus you have the bonus of being able to use 120 v AC equipment.  Also, You do not need any step up/step down voltage converter for any of your 12v dc equipment.

For example, you said you have 12 volt dc lighting.  Wire two in series and your done.  Connect the positive of one 12 volt light to your switched fused power and the negative of that light, to the positive of the next 12v d/c light.  The negative of the second 12v dc light then goes to ground.  Voila, you now have a 24 volt light, without buying anything.  The two loads must share the voltage equally in a series circuit, so each load only receives 12 volts.

I wired my shop lights this way.  12 volt LED strips that are 15 ft long are like 8 dollars, 24 volt strips that are 15 ft long were 100.  16 dollars and I can make a 30 ft 24 volt strip..... it really is that easy

Eric Hammond wrote:There is a lot of excessive complication in this thread.

I would just purchase a charger inverter and be done with it.  Hook any old generator up to the inverter and it will charge your batteries, plus you have the bonus of being able to use 120 v AC equipment.  Also, You do not need any step up/step down voltage converter for any of your 12v dc equipment.

For example, you said you have 12 volt dc lighting.  Wire two in series and your done.  Connect the positive of one 12 volt light to your switched fused power and the negative of that light, to the positive of the next 12v d/c light.  The negative of the second 12v dc light then goes to ground.  Voila, you now have a 24 volt light, without buying anything.  The two loads must share the voltage equally in a series circuit, so each load only receives 12 volts.

I wired my shop lights this way.  12 volt LED strips that are 15 ft long are like 8 dollars, 24 volt strips that are 15 ft long were 100.  16 dollars and I can make a 30 ft 24 volt strip..... it really is that easy

It seems these things can be as simple or complex as we want to make it :P

We have AC chargers to charge the bank with, but they won't do 24v and we don't want to buy special 24v battery chargers, hence running it through a 12/24 charge controller than will handle 3 stage charging.

The lighting isn't a problem because the charge controller accessory port is 12v.  So our 12v lighting runs directly off the accessory port and to a lights witch.  Even when we switch to 24v the 12v port will still work.  And we have 2 ports available to us if we need any other 12v accessories.  

We also just settled on 24v alternator.  

Hi Jen and Co.
Well it’s been 10 months and it’s December again so I thought I’d bump this thread and ask how it’s going??. What did y’all figure out or not figure out? Did you get a 24 volt alternator and use it? Are you using your old lead batteries?
I’m juggling similar decisions. Except I’ve been considering going from 24 volt to 48 volt. Am learning a lot about alternators and solar.  Thanks