Small(est) engine powered auto alternator

[Boss]

 DIY battery charger
I know this isn't anything new, what I would like to determine is what the minimal engine would be

Let's see if I can state my intentions concerning building an alternator, without appearing to be a armchair dreamer. First of all, what is the purpose of building a genset like this when it is easier to buy a cheap generator? Investing in the 12 volt inverter and using batteries changes our power needs. The inverter will change battery power to household power. Our generator only needs to charge the batteries.

Typical automotive alternator.
A little arithmetic  shows us the relationship between horsepower and watts where       
1 horsepower = 745.699872 watts
For our 12 volt system a standard automotive alternator delivers 90 amps
power formula shows P=VI   P in this case is Watts, V is voltage, and I is current.
12*90=1080 convert watts to horsepower I think it would go like this: 745.699872/1080 = 0.690462844 horsepower, Hmm seems this engine is over-powered for our little project.
Of course it is a good idea to add 20% to the horse power just to make sure the engine isn't struggling thus lowering efficiency,  1.2 X .6905 = .829 hp still under one horse power. The above engine is 5hp I think. So I'm looking for a weed-eater engine or something similar.



 Above is a DIY battery charger I found on the Internet.  Unless my calculations are way off this person used too large an engine.
P.S.  On my site one reader already pointed out that I left operating altitude loss out of my math. We are at ~7500 feet here in northern NM
Also when we get done using this little generator for charging the batteries for our remote stage storage system, I will swap the 12 volt alternator for a 24 volt alternator to charge my home system when the wind isn't generating power using our 11footer on a 50 foot tower

[dave ames]

 Hi Boss,

 sounds like a fun project. i have not had a go at putting one of those together yet.. have given it some thought though.

 saw somewhere a bracket kit available for the alternator that would help for those of us that are equipment challenged.

 looks like you got caught up in the oopsie flip with the maths. so we end up somewhere around 1.5hp.

 not so sure that the 5-6 hp would be that crazy far off?  the five on the lawn tractor seems happiest at about half power,  just sips the gas at that setting too.

 nothing wrong with going smaller. 20 amps is still a lot when you have zero!

have fun, dave

[SparWeb]

Careful with the math and doing it on a piece of paper first usually helps.

If it is an alternator that will produce 90 Amps, then if the voltage is regulated to 12V it will be putting out 1080 Watts.
At the same time, however, it is generating a huge amount of heat in the wire.  I don't happen to know the coil resistance of an auto alternator, but for the sake of argument, it might be about 0.5 ohm.  In that case the resistance losses alone will be I-squared-times-R = (90)*(90)*(0.5) = 4050 Watts
Also of concern is the voltage drop across the rectifier, which is probably 1 Volt, therefore I*V = 90*1 = 90 Watts.

The input power required for any machine must equal ALL of the outputs.

The total power required to DRIVE the alternator is 1080+4050+90 = 5220 Watts.  That is almost exactly equal to 7 HP.

The motor is undersized.  (Though anybody who can correct my assumption about the winding resistance could show they are better matched than this).

[Boss]

This is interesting, I was thinking of the auto alternator I had setup on an exercise bike and how it seemed like it was taking a heck of a lot of energy to spin it when the electromagnets were energized (excited)


Not sayin that a human is as powerful as a horse, still my estimate  seemed low at a horse. 

In that case the resistance losses alone will be I-squared-times-R = (90)*(90)*(0.5) = 4050 Watts
Also of concern is the voltage drop across the rectifier, which is probably 1 Volt, therefore I*V = 90*1 = 90 Watts.

All from heating wire?
Seriously lossey
those engines aren't cheap with a buy it now price of around $300US

[dnix71]

I have a battery charger jump start that uses a 2 1/2 HP Briggs edger, but it only puts out about 75 watts into a car battery and little voltage regulation.
The generator is a used brushed treadmill motor.  The 6.5 HP engine you show looks underpowered, but it's portable and puts out enough to charge a battery.

You could make a trickle charger from a weed eater motor, but that won't drive a car alternator. It might be more fuel efficient to trickle charge with a bicycle generator hooked up to a leaf blower.  http://www.bikeworldusa.us/Generator-12V-6W-Bicycle-Light-Power/M/B000OBWMGK.htm?traffic_src=froogle&utm_medium=organic&utm_source=froogle

You'll need a rectifier to go with it. One reviewer bought it to make a battery charger and then discovered it puts out unregulated ac.

[bj]

  I have built a few of these for farmers  around here, as they are great for dead tractor batteries that are miles away from home.  What I
have found is that under powered is not good for long life.  Basically these engines are designed for running at governor speed.  (usually 3600)
Slower than that, at full throttle, and they build heat.  Which is shortened life.  The Honda's are quite a bit more robust, which is why you
usually pay more.  Direct drive, and a 90 Amp alt will pull a 5HP down pretty bad, if the alt is maxed.  The Honda will take it for a while, Briggs
less.  China stuff, I can't say, haven't used one yet.
  Just what I have found, building them.
  BJ  Lamont AB Can

[Peter-]

Hi Boss -
I think yer on the right track. Doing the math shows that a 3-horse can power most any auto alt. Even the new 120-amp units. Problem is this, though: Small engines depend on having a lot of "flywheel effect" to keep it spinning. A car don't have that problem.  For example most mowers and weed whackers won't run real good at all without having a blade or whatever attached. With a load on them like an alt, they would just stall right out. So I would put a pretty big flywheel out of something like a 10-inch dia. piece of 1-inch plate steel and gear up the alt like crazy say 3-to-one. so the engine is like 2000 rpm and the alt is 6000 rpm. And let the flywheel keep it going during loads. I have a project like this right now, with an antique 5-horse Tecumseh cast-iron snow-blower engine. Just got it going after boiling out the carb.

[ghurd]

Doing the math shows that a 3-horse can power most any auto alt. Even the new 120-amp units.

However, placing a 35 or 60A auto alt to low batteries with a reasonable AH rating, on a 4HP motor, with something resembling an efficient gearing ratio, will stall it.
Peter "Dinges" posted relevant info.  I don't know how to find it now.
G-

[Peter-]

Hi again Boss - sorry to post twice in your same thread! But I think I got some more good info.

If you look close and do some comparison shopping online like I did. You can convert both the rated engine HP and the rated genny kW, both into kW. You will notice that the gas engines are almost always rated at about 2x the genny kW. There's a sound engineering reason for this: Engine HP does *not* directly relate to electrical HP.  It all has to do with torque. With no load and very little flywheel, it'll go all day. But when you add an electrical load, it subtacts torque. This tends to kill the ability to spin. The manufacturers of commercial units aren't gonna add a big flywheel, so they go over-rated on the engine.

With a diesel you got a bit more slack. But you're paying a lot more for the engine. So it's six of one and half-a-dozen of the other.

It's all a balancing act. Ideally I would get a fairly heavy flywheel and gear it up to a higher RPM. Or else get a really heavy one hanging directly off the crankshaft. Then I would run the genny off that. That way, you get a low-rpm engine that lives longer and doesn't use much. And the genny gets the RPM's it needs to do its thing without stalling out the whole bit.

Remember that the HP or kW of an engine is directly relate to its torque times RPM and you'll do OK.  If you can keep the engine RPM's down and the torque up it'll live a long happy time.

[Peter-]

Doing the math shows that a 3-horse can power most any auto alt. Even the new 120-amp units.

However, placing a 35 or 60A auto alt to low batteries with a reasonable AH rating, on a 4HP motor, with something resembling an efficient gearing ratio, will stall it.
Peter "Dinges" posted relevant info.  I don't know how to find it now.
G-

You are correct. I am trying to show and get a way around this.

[ghurd]

You are correct. I am trying to show and get a way around this.

You can not.

Between the first 60W being wasted in the electromagnets,
then transmission losses in the coils, rectifier losses, B-H curve losses in the stator laminates, bearing and belt losses, eddy currents in the CU and laminates...

As I see it, the gas/diesel motor can be connected to a Giant coreless PMA and use an accurate power curve, and a lot of expensive stuff.
Then thats as good as it gets.

Or use a mid 70s Chevy truck 35A alternator and a 3.5HP B&S from the neighbor's trash, with the right pulley ratio.  Maybe add a bit of caveman electric control.
G-

[Peter-]

"As I see it, the gas/diesel motor can be connected to a Giant coreless PMA and use an accurate power curve, and a lot of expensive stuff.
Then thats as good as it gets"

That is actually ideal in my book. There isn't any losses that you mention. But OTOH you have to lay out a bunch of $$$ and effort. For my own self, I was wanting a bog-standard 230-volt, (RMS) split-phase PMA for North American use at 60 cycles per second. I finally have an engine that might just do it. But now I have to come up with a bunch of mags, steel, epoxy, and wire. I think that an air-core PMA with an engine driving is the way to go in my case for sheer economy. But you have to spend a bunch to get there in the first place. So I'm taking my time.

I don't know how any of it will hold up. I can speak strongly in favor of DevCon industrial epoxy but it's like $140 a quart. Or a liter if you prefer. That stuff is like both steel and nickel and sheds heat + wear in the same way. I used it to line electrode holders in furnaces.

Ghurd, I would much rather just an inverter to put 230 VAC that I could bolt into my car and just leave it idling. And then transfer switch or just plug it in. Problem is, that it seems nobody makes that for the US market. It's a huge hole in the market. We have enough public power fluctuations up here in the norteast, to consider things. Let alone the expense. I've gone so far as to consider the new MOSFETS and building my own. gov't be damned. So I am hauling out all my old uni books and looking again.

It's OK, no worries.

[ghurd]

But now I have to come I would much rather just an inverter to put 230 VAC that I could bolt into my car and just leave it idling. And then transfer switch or just plug it in. Problem is, that it seems nobody makes that for the US market. It's a huge hole in the market. We have enough public power fluctuations up here in the norteast, to consider things. Let alone the expense. I've gone so far as to consider the new MOSFETS and building my own. gov't be damned. So I am hauling out all my old uni books and looking again.

It's OK, no worries.

What you want is not that hard to get, if you scrap the car alt and the concepts related to a car alt.
I am working on one now, and it is pretty simple.

"that I could bolt into my car and just leave it idling"
Careful.  Idling cars are not a lot different than idling 3.5 B&S.
Turn on your car head lights, with a warmed up motor, and watch the tach. 
All power comes from somewhere.
Thinking a turning car alt makes power with no effect on an idling 200HP motor is a pipe dream.

[Boss]

Or use a mid 70s Chevy truck 35A alternator and a 3.5HP B&S from the neighbor's trash, with the right pulley ratio.  Maybe add a bit of caveman electric control.
G-

, Glen you are one funny dude

[Peter-]

What you want is not that hard to get, if you scrap the car alt and the concepts related to a car alt.
I am working on one now, and it is pretty simple.

"that I could bolt into my car and just leave it idling"
Careful.  Idling cars are not a lot different than idling 3.5 B&S.
Turn on your car head lights, with a warmed up motor, and watch the tach. 
All power comes from somewhere.
Thinking a turning car alt makes power with no effect on an idling 200HP motor is a pipe dream.

You are wrong, period and full stop.
Cars idling are very much different: They have much more mass as expressed by the flywheel rim.
I have turned on my car headlights with both a cold and a warm motor. There is *zero* difference at the tachometer.

You need to remember that HP is a product of both torque and RPM. The car alt is only a very tiny fraction of what is available at the flywheel of a 4-cylinder in my case.

[dnix71]

Peter, how do you boil down a carb to clean out the varnish? I usually wire out with copper electrical strands. Can you boil out a weed eater carb without ruining the rubber parts or do you have to disassemble an remove them first?

I have thrown away several older machines because the carbs eventually get gunked up.

[WoodSpinner]

Disassemble the carbs and soak the metal parts in carb cleaner, then blow out all the passages with spray carb cleaner.  Thats how I do it.

John

[hiker]

if your going to use a 3hp engine to drive a car alt--i always put a bigger pulley on the alt--and a smaller one on the engine..
that way your engine wont lug down under load--sure you wont get the max amps from the alt!! but it will save the engine.....
cant seem to acess my old files --other wise i would show a picture......
to regulate the field currant a simple way is to use 20watt tail lite bulbs--just use from one to three in parelle--hooked to the
field coil lug[nut].....the more bulbs the more power out[amps]....if your alt dosent have a field coil lug--just go inside and hook directly to the field bruches..in other words hook up a wire from the batt output on the alt--to the field bruches--with 1-3 bulbs in parelle for desired output.  you can add a switch for each bulb--to govern the output[amps].
works on a exersize bike conversion as well--just use one bulb--makes for a lot easier peddling........

[blownoiler]

I've been experimenting with the same sort of system, working out how I'm going to drive a 24 volt 55 amp automotive alternator. I was using a Toyota 4 cylinder diesel car engine at idle speed, but I clagged it using veg oil as a fuel, (veg oil just won't work at low load/revs) Right now I'm using a g200 honda engine, around 5 horsepower. I tried  a 1 to 1 pulley ratio first off, but the motor would just stop as soon as I excited the alternator. A change of pulleys (smaller pulley on the engine) has partially solved the problem, but to get a decent charge rate the engine is working very hard.....not good for long engine life! I live in a built up area, with neighbors both sides of me so my system has to be very quiet which further complicates matters. I'm thinking that a ford 4.1 litre six cylinder idling on L.P.G. might be a good system(L.P.G. is half the price of petrol here), and intend to lean out the gas mixture as much as possible, and advance the ignition timing to a more efficient setting. The only problem that I can foresee with the six cylinder is a changing idle speed as the load on the alternator varies, an aftermarket cruise control could be used to keep the motor at a reasonably stable fast idle, but they cost around $300., so I'm not sure if I want to fork out more dollars for a system that might not work. Any suggestions ?

[Boss]

What it sounds like is nothing beats a Listeroid with an Axial Flux alternator, am I right?
These little engines can not do the job when it comes to an auto alternator.
Makes me wonder why the auto industry hasn't snapped to the excessive horsepower being wasted to run the alternator.
Seems like they could cut fuel use back by running AF alternators
Thanks for all the input gang, this saved me a lot of work.
Now we are thinking we'll build a hay bale shed behind the stage for whatever generator we come up with, my electrician son came up with fifty feet of three wire #2 direct burial cable, bless his heart.
We'll set up a panel of sorts on each end, perhaps a one of the (3) blade switches on the stage side with one  weather proof receptacle.
I have a 50 amp (decent duty cycle) battery charge which we'll plug in there.
The issue as I see it now is going to be to make sure that DJs and musicians do not interconnect the output from the generator with the output from the pure-sinewave  inverter, I should think that would be bad.
Thanks for the timely input everyone

[jlt]

Using a standard auto alt. with a built in regulator is not the way to go. they will try to charge at max amps until batts are full.              and that will stall the engine .A better way is to bypass regulator with Resistance control of the field.I used a head lamp switch out of an old caddy. and used the interior light dimmer to control the field.

[bob g]

if you get to where you want the ultimate in charging efficiency, let me know

you are never going to get their with a car alternator using either the oem regulator or some sort of field reostat
charging a 12volt battery, the best efficiency is typically in the range of 50% (average) to a max of about 55% (rarely seen)

i think the axial flux air core might fair a bit better, however

over the last few years i have been working with heavy duty automotive (truck/lorry) alternators i have proven one can
attain ~80% efficiency at very high power levels, if one does it right.

you will need a much better control scheme than what is afforded by the oem regulator or a field reostat or a pwm homebuilt job.

controllers such as balmar, xantrex, amplepower, hehr, sterling and a few others provide very accurate control that is 3 step, with temp
compensation and alternator temp sensing as well, they do several things exceptionally well, particularly the manner in which they control
both amps and voltage setpoints.

you cannot control amps and volts at the same time using the oem regulator, reostat or a simple pwm regulator, you can only do one or the other
but not both effectively.

if anyone is seriously interested let me know, i will direct you to a pdf file outlining the process involved for reapplication of a heavy duty alternator for either 24 or 48volt charging.

i have it posted on our forum, which is non wind related, but don't want to link to it here without permission from the leaders of this fine forum

i will however email it to anyone interested

fwiw

bob g

[TomW]

if anyone is seriously interested let me know, i will direct you to a pdf file outlining the process involved for reapplication of a heavy duty alternator for either 24 or 48volt charging.

i have it posted on our forum, which is non wind related, but don't want to link to it here without permission from the leaders of this fine forum

i will however email it to anyone interested

fwiw

bob g

Bob;

Posting such a link is perfectly acceptable.

We frown on posts that lure users elsewhere for an answer or to sell them something.

Tom

[esc]

Here is one that I made.

I used a 3hp Briggs and Stratton lawn mower engine from a "thrown away" mower and an old Chevy alternator I had laying around.

I cut the top out of the lawnmower frame and used it as a template to cut the engine holes in the steel that I used for this projects frame.

I added ~1/2" spacers between the B&S and the frame to get the crank pulley and alt pulley to line up.

I used a very small crank pulley (~2") It limits max power but this also limits the heat the alternator will generate and keeps teh engine from stalling when the alternator is switched on.

It works well...but I have never used it very much.

[Boss]

if you get to where you want the ultimate in charging efficiency, let me know

I do, unfortunately my budget is sucking air right now because I pinched a nerve, but that is getting better, worked Friday. That didn't ruin me, so maybe I'll be okay. I've always loved the CH&P idea. I've been homebrewing Biodiesel since 2005, possibly can own this:

This is a Kohler 4-cylinder diesel 18.5 KW with a bad controller board see: http://fieldlines.com/board/index.php/topic,143197.0.html

[Fused]

"if you get to where you want the ultimate in charging efficiency, let me know"

I dont think any DIY person needing a inexpensive generator for back up to a battery bank is going to worry about top spec efficiency so long as it does the job. Those who are off grid and live off their battery bank for power might be the only one to actually see the difference in a super efficient generator vs maybe one that just does the job.

I used a roto tiller engine built to sit idling or run wide open nothing but a pulley attached to its shaft, no counter weight flywheel needed.

http://www.windbluepower.com/category_s/27.htm

In no way am I helping or supporting this company windblue......but it does have a way to build your own delco 10si PMA that "can" be used on some home made do hicky generators. I am one that could not go out and buy a generator for $500 and only use it 6 times a year.

Hope it helps
Fused

[bob g]

Fused and others:

you guys make a good case for a simple back up charger, i forget sometimes that there are very distinct differences in the
"needs" of one group vs another.

certainly someone that is a hobbiest has no concern for ultimate efficiency

as opposed to one that is living offgrid with perhaps 5 or 10 grand in battery capacity that must be maintained.

my background or rather interest lies in microcogeneration, wherein efficiency is of major concern, also
should something bad happen down the road that leads to short supply of gas or diesel (or both) everyone's attitude on
the subject of efficiency will come sharply into focus.

having been a member here for many years now, i realize that the membership is very wind "centric" and looks at engine driven
power as a necessary evil that should be avoided for a number of reasons, some practical, and  some political.

having also been a member of various slowspeed engine forums, i also found out in short order they are very engine "centric"
and have not issues with running an engine 24/7/365 if necessary.

i find myself somewhere in the middle, where my belief is if one is to run an engine driven genset he should do so as efficiently as possible
and not waste anything such as coolant and exhaust heat. Once one starts down the path of research and development of microcogeneration
it is very easy to get caught up in the efficiency game, most especially when you find you can reach the same efficiency that the big boys get with
similar sized units.

bottom line, if you only need battery charging a handful of times a year to back up a windgen or solar, by all means do so with whatever is expedient, but realize that there might be a time where after a big storm you do not have that capacity and have to rely on your genset
and fuel may or may not be as available. so it might be prudent to avail oneself to how best to increase output and efficiency of your genset.

anytime you can increase output and increase efficiency you are doing something useful in my opinion. with such a system it is not unusual
to cut the engine run time to a fraction of what it would be using other methods, and to do so in a manner that is safe for your batteries and something you can walk away from knowing that nothing bad is likely to happen in your absence.

looking back i did mention my comments were directed to those looking for maximum efficiency ...

"if you get to where you want the ultimate in charging efficiency, let me know"

this does not mean there is no place for a simple charger, quite the contrary i would also recommend having one no matter
what high efficiency system one has in place.

bob g

[bob g]

here is the link to the pdf

because of concerns i don't want to get into, anyone wishing to download the pdf will have to join the forum
in order to do so.

for that reason, i would suggest only those most interested in the topic do so, anyone that has little or no interest
in the subject matter will probably find the requirement unreasonable.

http://www.microcogen.info/index.php?topic=157.0

since that paper was published there has been more work done with a 48volt nominal version, early tests have been
very encouraging at ~50% capacity 60amps at 57.6 vdc, the goal is 130-150amps at 57.6volts and i think it is attainable.

after all testing is concluded i will be publishing part 2 of the pdf, at which time i will also go back and clear up a few lose ends
in the part 1

after all that is done, perhaps by late summer i will be publishing the results of the use of an automotive alternator for high
voltage transmission over distance using commonly available transformers used in 60hz applications and the results of their
use at 500-600hz for 3phase step down applications.

none of this is applicable to windpower in my opinion, but rather of interest to folks for other power systems.

thanks
bob g

[blownoiler]

This might be a silly question Bob, but from what I now understand about auto alternator efficiency ,  would it be more efficient to run 2 100 amp 12 volt alternators at 50% load (to charge a 24 volt battery bank) than to run a single 24 volt 55 amp alternator at an amperage that is close to it's total output?  Also, how much (in U.S. dollars) would I expect to pay for say the Balmar controller?

[bob g]

this is why i reference the white paper over at microcogen.info

there is so much misconception out there as it relates to the use of auto alternators, and the paper clears up
most of it fairly well.

basically there is no reason that a 12volt alternator can't be used to charge at 24 (28.8vdc) or 48 (57.6vdc) provided
you have suitable rectifiers and drive it at the required rpm.

what is not widely know or appreciated is you can get much more power and efficiency from such an alternator

the wrong thing to do from an output and efficiency standpoint is to buy an automotive 24volt alternator, the efficiency
will be similar to a 12volt alternator charging a 12volt battery.

the issue comes down to a set of compromises that the auto engineers must make that we do not, that being they
have to have an alternator that will produce significant amounts of power at engine idle all the way up through cruising speeds
that sets the design up to be a jack of all trades and master of none.

what is outlined in the white paper is the specific application of an alternator running at a specific design speed, and with a specific
high side output , and quite frankly we don't care if it produces "any" power at engine idle (unless of course that is the design speed
of the genset).

in the example given a typical heavy truck alternator the prestolite/leece neville 110-555jho which has an oem spec of 160amps cold
at 14.2volts at ~6000rpm. when the alternator is hot it will derate to approx 130amps give or take. the output therefore when hot
is approx 1850 watts.

the very same alternator being managed with the balmar controller driven at 4850rpm, will charge a 24volt bank at 28.8vdc at 100amps
hot, and do it all day long. however if used as a truck alternator it would produce no power at engine idle, but who cares? the engine
is set to run at a specific speed which be virtue of pulley ratio allows the engine to run at 1800rpm and the alternator to run at 4850rpm.
therefore the reapplied alternator is good for 2880watts.

(its also fair to note this alternator is good for 8krpm continuous.)

so, in stock trim doing what it was designed to do, and considering the compromises taken the alternator produces 1850 watts, reapplied
and controlled by the balmar it will produce 2880watts, that is an increase of ~56% in power.

the interesting thing is the efficiency goes from approx 54% charging into the 12volt battery all the way up to 80% (+/- 1%)
that is an increase of approx 26 percentage points.

now if we were to just go out and buy the typical 24volt counterpart of the oem 12volt unit, we would get the 28.8 without much trouble, but
we would get approx 1/2 the hot rated amps or in this case 65amps, which brings us interestingly right back to ~1850watts and 54% efficiency, but

we get to have a good charge rate at engine idle, but that is not important to us in this application.

now using the same alternator (12volt nominal) and replacing the rectifiers with higher PIV, non avalanche diodes, and running it at 48volt nominal
it will produce approx 54volts and 80amps (this is the limit at 4850rpm, which is what i am hard ratio'd at with my trigenerator), this is not quite enough voltage and the current would also go to closer to 100amps without issues if i were to turn the speed up to approx 6500rpm.

even at 54 volts and 80amps that works out to approx 4300watts output, which is well over twice the output power (232%) and the efficiency improves to a bit over 80%.

as for the balmar controllers, they are expensive, at around 350bucks retail, but can be found for under 200 on ebay from time to time, also the xantrex units will do the same job and are somewhat less expensive.


anyway that is the short story, if you want more, go check it out.

currently i am working with a leece neville alternator with a simple modification of the stator, going from delta to wye, adding water cooling to the stator, its goal is 130-150amps at 57.6volts with an efficiency of over 80%, the alternator started out life as a 12volt nominal unit, but has had
the rectifiers changed, the stator reconnected, control via balmar, and running at ~6krpm.  field power at ~50-60watts max.
even at 130amps and 57.6 volts that works out to approx 7500watts, and that is a serious amount of power from a 31 lb alternator.

to answer your first question, yes running the 12volt alternator at half output will be much more efficient that running it at or near full rated output, heat is the big killer of efficiency in my opinion, and is borne out in testing.

bob g

[blownoiler]

Thanks for the info Bob, I know nothing about electronics, and am on a steep learning curve right now, hence the questions with possibly obvious answers to you and most others on this board. My next question is  could I use an automotive 24 volt regulator on my 12 volt alternator to charge my 24 volt battery bank? 

[Watt]

What you want is not that hard to get, if you scrap the car alt and the concepts related to a car alt.
I am working on one now, and it is pretty simple.

"that I could bolt into my car and just leave it idling"
Careful.  Idling cars are not a lot different than idling 3.5 B&S.
Turn on your car head lights, with a warmed up motor, and watch the tach. 
All power comes from somewhere.
Thinking a turning car alt makes power with no effect on an idling 200HP motor is a pipe dream.

You are wrong, period and full stop.
Cars idling are very much different: They have much more mass as expressed by the flywheel rim.
I have turned on my car headlights with both a cold and a warm motor. There is *zero* difference at the tachometer.

You need to remember that HP is a product of both torque and RPM. The car alt is only a very tiny fraction of what is available at the flywheel of a 4-cylinder in my case.

What kind of car do you have?  Is it a 70's vintage or is it late 80's or 90's or newer with fuel injection and computer controls?

You imply idling that gas engine is nearly free and so is the wattage used by electrical accessories and such.  First, is you alternator so new as it is field controlled by your ecm?  Is the alternator driven by resistance from a load such as a bulb?  Or, has your alternator fully charged you battery and now at float stage?  Or, the more relevant the question, have you started your vehicle and swapped the battery for a very low charge state battery and then watched the tach?  A WATT is a WATT.  Once your engines mass has delivered in the way of energy, to the load, it's mass is no longer usable.  meaning for every action, there is an equal but opposite reaction.  That engine now has to deliver the power to sustain the new load.

If that is not a good enough explanation, put a vacuum gauge on your engine.  Put a load on that engine and watch the vacuum gauge when it starts towards zero, that is your throttle delivering more air and therefor fuel.   That car engine idling is very inefficiency, just to keep it idling takes a huge amount of energy. 

Seems you need to remember HP is a product of BTU.  Huge mass is great for getting a load moving or the initial start of huge energy requirements but only for very short burst but none can be more of a load than that available as energy. 

[peter altas]

Hello,

Check out www.christieengineering.com.au

This is the kind of stuff they've been selling for years in Australia.

I spoke with them about my own project.  I sourced a 200cc Kawazaki FG200 that is key start and converted to run on Propane.  Want to get a 24V 45A alternator coupled to it and get my inverter to auto start it when a preset battery discharge level is reached.

They recommended direct coupling the alternator to the motor to reduce torque losses.  They also sell the Bernini Be-One auto start controllers which are hard to fine here.  You'll need either a momentary switch to excite the field, or they recommended either a small resistor or an auto type bulb to do the same job for an auto-start setup.

To couple the two together, I'd recommend a jaw-type coupling that has a urethane type spacer.  It'll take the stress out of a metal-metal coupling and is a bit forgiving with misalignment (mine has 3 degrees of compensation).  If I can't get an alternator with a 3/4" keyway on the shaft, I'll get it milled out to suit the coupling.

The guys there (Michael and Peter) were happy to discuss my project with me and even supplied Bosch alternator part numbers for my 24V project.

Good luck, very doable project.