I *think* an alternator draws power off the belt at a rate proportional to the electrical load.

Can anyone confirm this for sure?

Reason I'm asking, is I am ordering a 1 wire v belt alternator for my go kart, to run headlights, winch etc, and because the engine is small, I don't want a huge power loss.

On the other hand, it would be nice to have the extra amps to charge up quick in the winter after cranking.

35 amp sounds ideal, but 100 amp aren't that much more....

If it isn't going to waste power, I'd get the big one.

edit: I have a car battery for the vehicle.

 

Yes you are correct..The alt basically charges the battery so battery low high alt amperage. I would go with a small unit small battery and you prob will be fine. What horsepower is your ride?

 

It's a l100 yanmar clone, diesel air cooled. About 9 - 11 hp maybe a touch more, depending on governor and fuel screw. I don't plan on messing with the fuel or RPM's, but I plan to add a small oil pump and oil filter and oil cooler.

This guy has a 13 hp yanmar in a car, with 2 ft/lbs more tq than mine:
#aid=P8YXIgKJcaA
And heres an almost identical engine to mine:
http://www.utterpower.com/10hp_chevy.htm

Gotta love diesel!
Mine should run a little different, I have a 4x4 toyota 1/4 ton truck trans, 5 sp manual, 2 sp transfer case, and axles, but the engine is reduced with a belt drive (also acting as belt clutch), then reduced again with a chain drive to the trans input shaft, so 3600 RPM in 5th gear should hit maybe 60-70 kmh.
The idea was to build a small offroad vehicle that was cheap to operate, and so far the project is spot on.
Since the gearing is so low, it should run like a tractor in 4 low.
Nothing will stop it, but it won't be burning the tires off haha.

It's a 2 seater, 48" wide and about 11 ft long, so about the size of a golf cart with a truck box on it.
But with big axles and tires protruding outside the body width lol.

So far the frame is done, parts laying everywhere, planning to get it done real soon. Got all the joints and linkages for suspension, coilover shocks, everything is here, just haven't had much time.

It should weigh roughly 1000 lbs all done.
Because of the small engine, gotta keep the power losses low haha.

Heres a couple pictures, and in advance, yes I know I'm not a great welder.
https://www.flickr.com/photos/794270...7631686513718/

 

Heres another 10 hp diesel vehicle in action lol :

 

Huh? Can you explain what this means?

To the original question, will a 1 wire alt require less scavenging effect than a 3 wire unit?

 

I thought the only difference was one has the voltage controller on the firewall, and the other is integrated?

 

If that's the only difference, why not use a 3 wire in your kart? I thought your question boiled down to not using extra energy to power your cart without the scavenging effect of amp draw. Could you increase the size without affect?

I'm not sure the answer given by 99f above was clear or understandable.

 

you need to consider the charge rpm of the alternator and weight that against the idle and run rpms of your engine....most alternatoes do not kick in unless thaey reach 1200 rpms...An oversized alternator will act like a brake......the need charge rate to re-charge a batter that can start a 15HP engine is very small...my 8000 watt generator used a 15hp motor and the charge circut to recharge the battery is 10 amps....so you certainly do not need a 100amp alternator....find one for a small 4 cyclinder auto engine.

 

I was planning to use the 1 wire just for simpler wiring, thats all.

Basically the 30 amp should work, but I thought why not go bigger?
The bigger one should make more power, especially at lower RPM's, and if used in the winter lots, it's nice to think the battery will have a better chance at staying full all the time, even with using accessories like lights, winch, heated seat covers, etc.

This would however be a problem, if the larger alternator is going to waste a bunch of energy just spinning freely.

I guess my question is that I know either one will take mechanical energy, and convert a % of that into electrical load, but will a higher amp alternator use significantly more just spinning without a load?

 

The drive pulley is smaller than that on a car or truck, but the engine idles around 1500 RPM, so I think it should be about spot on as far as spinning alternator speed should be.

The engine itself has a small charger on it, 5 or 10 amps. I suspect it is intended only for re-charging a starting battery, so since I plan on running a rather large battery, and quite a few accessories, I don't even plan on hooking that integral charger to anything. I suspect it is for charging a 5 - 10 amp hour lawn tractor battery, not a 70 or 80 amp hour car battery.

As for the size, if it says "30 amp" or "100 amp", we know those are theoretical maximum numbers from a test bench at maximum speed, in real world use I don't think you'd get close unless the engine RPM's are super high.

The small car alternators are like 30 amp.
If in normal use it's putting 15-20 out, thats not much power.
I think the seat warmers use 6 amps each I think, fan for windshield another 2 amps, Whats a 55 watt light take, 5 amps? Theres 6 of them that are going on, mind you all this stuff would rarely be on all the time, but it is kinda looking like a 30 amp alternator would be working hard all the time.

Maybe just downsize the lighting lol.