Is this alternator suitable for battery charging?

[BioHazard]

I'm trying to build an ultra-light battery charger with a 25cc 2 cycle engine that makes about 3hp @ 12,000 RPM. I need a light weight alternator to couple to the engine. I'm hoping to keep the entire thing under 15 pounds fueled up and ready to go. I've been thinking about using one of these Denso "racing" alternators:
http://cgi.ebay.com/ebaymotors/100-NEW-ALTERNATOR-CHEVY-MINI-STREET-ROD-1-WIRE-50-AMP-_W0QQcmdZViewItemQQhashZitem1e6041ed91QQitemZ130463952273QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

Will a "one wire" alternator like that sufficiently charge (let's say at least 90%) a lead acid battery? Or is there a better external charge controller I could buy that would work with that alternator? Can I get a simple regulator with a voltage sense wire?

Additionally, what is the top speed I can spin an alternator like that? Can I direct drive it @ 12,000 RPM?

[TimSR2]

The one wire alternators can be tricky. Their regulator circuit uses the SLI battery's voltage dip on cranking as a 'turn on' signal. , and turn themselves off after a period of inactivity.

You'd be better off with a conventional alternatorso  that you control the power to the field connection manually

[BioHazard]

Hmmm...could that be modified to work with an external charge regulator?

I wonder if any of these regulators would work with that unit?
http://store.alternatorparts.com/regulator-20.aspx

I may end up with a larger size alternator, but I'm really trying to keep this as small and light as possible. The engine only weighs about 3lbs...

[BigGreen]

I did what you are trying to do years back with a Delco 94A 12SI and a chain saw engine of unknown size.
http://www.madelectrical.com/electricaltech/delcoremy.shtml
This is a 3 wire integrated regulator that requires being connected to a battery for excitation to get an output and a disconnect switch to prevent battery drain when not in use. The wiring info is out there somewhere.

What I learned: They are inefficient, alternator got extreemly hot after about 10 minutes, once you hit a certain speed higher speeds didn't increase output. I didn't have a chance to measure rpm's or temps, the engine failed after running it a few hours and project over.

Dave

[BioHazard]

Yeah, I'm a little more familiar with the older Delco type, I've got a bunch of vehicles with them. Definately a lot bigger though. I like the internal fan the Denso type alternators have. I may have to go with a spendy marine type alternator controller to get what I really want...

Does anybody know any other options for "small" alternators in the 50 amp range?

[TimSR2]

You can't go wrong with a GM 10si alternator  in 38 to 55 amp 3 wire format. You can spend a great deal more money trying to get a tiny bit more efficiency.

If you want to externalize the field control this is the unit that has the most support,  both commercial  and  homebrew solutions.

[BioHazard]

Can a 10si alternator withstand being direct driven at 12,000 RPMs? Or will I have to use a pulley?

[Ronmar]

Can a 10si alternator withstand being direct driven at 12,000 RPMs? Or will I have to use a pulley?

I think you will have a problem finding any alternator that can withstand being direct driven to 12000 RPM, unless it is something off an aircraft turbine application:)  I think you need to keep the rotor in the 3000-4000 RPM region...  You might shave a few pounds with the 2 stroke, but that high RPM wears on things, especially your nerves:).  Plus there are the hassels and losses of whatever reduction system you come up with.  If I were doing what you are doing, I would probably opt for the smallest 4 stroke I can find.  A 100A alternator is going to deliver under 1500W, so around 3 HP would probably do it quite comfortably, with perhaps a little HP left over to incorporate a larger additional blower to enhance alternator cooling.  Perhaps something like this?

https://www.surpluscenter.com/item.asp?item=28-1695&catname=engines

My .02 

[TimSR2]

Can a 10si alternator withstand being direct driven at 12,000 RPMs? Or will I have to use a pulley?

The turn on rpm for a 10si is about 2000 rpm, any less than that and you don't get excitation.  In normal chevy v8 the crank is about a 8 inch pulley, and the alternator is a 2.5  , so about a 3 to one ratio. 5000 engine rpm would equal 15000 on the alternator.. 12000 should be no problem.   

Load control is a problem in your application. The little weed whacker motors don't want to run flat out full load or they will melt down . Undersize your alternator to about half of your engine's WOT output or it may have a very short life.  A 55 amp alternator at 14.5 volts is about  a horsepower output, but these are pretty inefficient devices and it will cost you 2 hp direct drive to get your 1 hp output. That would be way too much load for that engine on constant duty.

[BioHazard]

I definately don't expect the engine to last all that long. I am using a Zenoah "model" engine, and replacement pistons/jugs are extremely cheap. I'm not sure if I'll have to rev it all the way up to 12,000 or not, that's just where it makes it's peak torque. It will go on to 18,000 RPM if you really want.

I want to build a similar device that will be quieter/larger as well, but I want this one small enough to fit into a backpack.

[Crofter]

There is a big jog in the wear curve that corresponds to the point where you get stress reversals on the con rod. Below this point the rod is always in compression. Above this point you get into bearing instability (element skidding) A bit higher yet and you are into ring flutter territory and there is an even sharper jog in wear. It depends a lot on design of parts just where they they start to hurt from Revs.  The Zenoahs have a good rep in the RC model field but they dont get the hours flying that you will log up on generator service.

[mobile_bob]

efficiency of the alternator will go into the toilet once it gets over about 6k rpm
reactance losses really start to get higher over that rpm, and i would expect the alternator to be well
under 40% efficient at 12krpm

50amps might be hard to achieve at that rpm level, with the available power the engine can deliver.

this might be useful as a limited use emergency charger to back pack in somewhere to rescue a dead 4x4, however
i would expect the life span to be measured in a few 10's of hours

bob g

[BioHazard]

i would expect the life span to be measured in a few 10's of hours

I would be ok with that so long as all that needs replacing is the piston/jug. Part of this is that I just want to see how long the engine can last. I don't know much about model flying but I imagine the helicopters and planes with this engine run balls out until the fuel runs dry.

I didn't realize the alternator would be less efficient at high speed...though they do sell a nice v belt pulley that fits on this engine. If I come up with something useful I also want to make an air compressor out of one of these little engines. Like most of my generators, not because I actually need it, but because I'm bored...

[BioHazard]

This has sort of been my inspiration, the Coleman "UltiMite" generator:
http://www.drive.subaru.com/01_04_Winter/Ultimite.htm
(now discontinued)

They are getting 90 amps for charging from a 40cc engine. Don't know how long it lasts but I would guess it would be somewhere in the hundreds of hours?

[TimSR2]

Just buy it. If it has a Subaru/Robin engine it will probably run for the rest of your life.  The nice thing about 2 stroke gas engines is that they can stand 'out of service' happily for a couple of years without trouble. All their insides are pickled in oil, like a diesel.  

(Edited: Oh too bad these are no longer available!) 

My Robin powered 2 stroke lawnmower just died. I've been mowing a quarter acre lot with the same mower for 16 years, and it is now 25 years old! Robin makes great 2 strokes.