How many amps can I pull?

[BioHazard]

I just scored a 79cc, 2.5hp engine from Harbor Freight for $59. I would like to attach a 10SI Delco alternator to this engine.

How many amps @ 12v should I be able to produce with this engine? Will I be able to spin an 85A alternator...or is that asking too much?

Also, could I direct drive it at 3600 RPM rather than using a belt and pulley?

[mobile_bob]

you might do around 30-40 amps continuous, however
the regulator is going to be your enemy

when the battery to be charged is low, it will start to full field the alternator, which in turn
places a very heavy load on the little engine.

you really need a current limit regulator/controller either of which is expensive and hard to implement with a 10si delco.

direct drive is going to be your best bet in my opinion.

bob g

[BioHazard]

Really...that low? I have a 10SI already that I wanted to rebuild, and replace the stator with one sized right for the engine. (Maybe a 37 amp?) With a 3 wire regulator I should be able to use a rheostat to control the field...shouldn't I?

This is basically what I want to make:
http://www.alten-dc.com/products/item/107-12-volt-80-amp-gas-powered-battery-charger.html
They're claiming 80 amps but that engine has a bit more power.
(can anyone identify what type of alterantor they are using there?)

[mobile_bob]

if you can control the field and set it up for direct drive, you might be able to pull more amps

only one way to find out!



bob g

[SteveU.]

By the case style and the black rear unitized removable brush holders and regulator assemblies these unit are either Robert Bosch alternators or Chinese clones of the same. Unlike an SI Delco alt these Bosch are easily convertible to externally regulated. 1970-71? first year era 10SI's cased units have no internal regulator and were for existing equipment wired for external regulation. Internally regulated have the two blade regulator terminals side by side inline with each other. Externally regulated have the "field" and "relay" blade terminals parallel with each other. Most rebuild shops have these hollow factory internal terminal spacer blocks laying around. If you are sourcing small cased Delco's, the plastic vacuum fanned 12SI's have a much lower cut in and lower RPM output capability. There is much more metal in the rotors and the stator lamination  stack is thicker. Of course use what you got always.
Regards
Steve Unruh

[BioHazard]

Is there an alternator I can get for less than about $100 that might work better? How would I regulate an externally regulated alternator? Is there a cheap regulator I can buy to use for now, and then get a fancy 3 stage charge controller in the future?

[mobile_bob]

you might consider this regulator

http://cgi.ebay.com/ebaymotors/Sterling-Power-Marine-Advanced-Alternator-Regulator-B-_W0QQcmdZViewItemQQhashZitem4aa23cbcd8QQitemZ320549469400QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

they have a number to call, and you might check with them about the regulators use in your application, as to whether or
not it can tailor the output amps to the available power of your small engine.

at 109 bucks it is close to your price point, and if it will do what you need done...?

i have many different controller regulators, from balmar, xantrex, amplepower and hehr, all of them have a way to tailor the output
amps to match the available power of the prime mover, however i have no experience with sterling so i can't tell you what is possible with
their products.

bob g

[Ronmar]

Watts are watts reguardless of the source.  2.5HP should be good for 1250KW.. P over I X E tells me that 1250W divided 14 VDC equals 89A of current draw.  The 2HP/KW rule usually covers you for generator efficiencies, but with auto alternators being supposedly less efficient, this number may be way out to lunch:)

[rl71459]

Cant you just use an 30 thru 40 amp version of the alternator to closer match the engine?

In my drag racing days... I built a compressor/alternator rig using an old Ford Reciprocating AC compressor and a GM Internal reg alternator. Used a 2 Groove alt pulley on a Clinton 2hp (I think) with one belt going to the Compressor and another to the Alt. I made a Throttle Solenoid out of a Chevy starter Sol (Pulled a Cable for High speed) and used the contacts to switch between Alt to compressor when the pressure switch called for it. It had a CO2 tank for air. It worked great! I had people at the track's copy it more than once.

Use went something like this... After a run (1/4 mile) then return to the pit's I would connect the rig cables Via Welder Jacks to the car. and start the engine, Running at the "Low Speed" (Sol not pulled in)
it would charge the battery in the car, As the battery recovered you could hear the engine load drop.

If the pressure switch called for air it (Via Contacts) shut off the alternator (Dropped Field) at the same time pulled the sol (High Speed) and the contacts in the sol would energize the clutch on the Compressor.
once the tank was full the pressure switch dropped out the solenoid (Low Speed) and re-enrgized the alternator.

I used that thing for many years... Never let me down at the track. Sometimes when needed I would use it for Charging a car up in remote location. It was handy!

Rob

I now remember it had a switch so you could run it unloaded when warming up the engine with the cables still connected.

also the pulley arrangement was a 1:1 ratio due to the fact that the engine pulley was indeed a alternator pulley.

as I'm describing/writing this it sounds like it should not work very well... But it worked great! can it be that the small alt and slow rpm were working in my favor? does the alternator slip at low speed? Whatever the reason it worked good for many years of use. 

[mobile_bob]

here is the math

2.5 hp = ~1862 watts

1862 watts x 45% (alternator efficiency and drive losses at max output) = 838 watts

838 watts / 14 volts (approx charging voltage)= 59.86 amps

that is about 60amps maxed out, if the engine is in perfect running order, at sea level, on a cool day, running good gas
with your mouth held just right, however... do you think you can run the engine at max output for very long? or would you want to?

the engine will also have to be running at full rated rpm to achieve this output, not many of these engine's are designed for such duty.

probably better to derate to around 70% which would give you about 41.9amps continuous capability, which is probably more inline with
reality in my opinion for a useful unit that can be depended on to run at that level at the drop of a hat and do some useful work.

the real issue with trying for more power is related to how the alternator oem regulator (if you use that), it will try to immediately go to
max output before the engine has had a chance to startup, get to speed and become stable. the alternator will make it nearly impossible to start
and get up to stable operation. if you use a programmable controller you can set the warm up time to allow for a no load startup and time for the
engine to come up to speed, run long enough to get to operating temps and stabilize before the alternator is gradually brought on line either in steps of a soft ramp up.

i am not saying it is impossible to get more than 40amps or so from a 2.5hp engine, just that it might not be as reliable and difficult to get started
and into operation.

bob g

[Tom Reed]

I've got a 62 amp 10 si belted up to a Tecumseh 5hp engine with a 5" pulley on the engine and 4" on the alternator. If the battery is low it will frequently kill the engine when the field is connected. It will usually stay running if rev'ed to 3600 rpm. After it's connected and charging the rpm's can be reduced to around 1800. I made it to extend the run time on our household backup battery/inverter system. Its also good for a fast charge on a car/tractor with a dead battery and it's been used to charge RV batteries with good results too.

[BioHazard]

you might consider this regulator

http://cgi.ebay.com/ebaymotors/Sterling-Power-Marine-Advanced-Alternator-Regulator-B-_W0QQcmdZViewItemQQhashZitem4aa23cbcd8QQitemZ320549469400QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

they have a number to call, and you might check with them about the regulators use in your application, as to whether or
not it can tailor the output amps to the available power of your small engine.

I like that! If that would work with the Delco alternator I already have it would be perfect...

Cant you just use an 30 thru 40 amp version of the alternator to closer match the engine?

Well, that is the main question. I don't want to use an alternator so big that it will kill the engine, but at the same time, I don't want to cut myself short either...

[BioHazard]

After a bit more research I can see that this engine is a clone of the Briggs and Stratton 2.4hp Vanguard engine...about $400!  Here's the HP/torque curve that Briggs supplies...

[BioHazard]

do you think you can run the engine at max output for very long? or would you want to?

the engine will also have to be running at full rated rpm to achieve this output, not many of these engine's are designed for such duty.

Say what? Nearly all cheap generators run at a fixed 3600 RPM, whether you need the speed or not! I finally fired up my engine tonight and it ran so smooth even at 3600 that I didn't even have to bolt it down to anything. Very impressed with it...not obnoxious sounding at all.

Looking at the Briggs HP curve they recommend a max load of about 2hp or 1.5kw.

Is there a good rule of thumb that can be used with alternators and amps per HP, similar to how we always estimate 500 watts/HP with a normal generator?

[mobile_bob]

the rule of thumb for gas engine driven alternators is 25amps @ 12 volt nominal per hp of engine, that being for a good engine
well tuned, and with the proper drive ratio

in my opinion that is perhaps a bit optimistic in real life, my thinking is 20amps per hp is more likely over the long run.

that might have more to do with the engine oem hp claims being at times more than optimistic,

if your engine will do 2hp continuous, then 50amps is possible if you drive it correctly.

bob g