Does anyone know how much power an alternator robs?

[blykins]

I was just curious...I had my engine dyno'd with the alternator turning. I wonder if it robs much power....

[DAVID GAGNARD]

blykins;

Depends on the condition of the battery and what the alternator has to put out to keep up with the demand... Anywhere from 1 to 3 hp I'd say.... Have proof,way back 20 years ago,a buddy and I had a hunting camp way back in the woods,no electricity.... We used a car battery and 12v bulbs for lighting,then we got a "bulldozer battery",weighed about 125 or more pounds,would last all weekend for lights.... BTW,I forgot to mention this camp was only accessable by water,so everything had to be brought in and out by boat,big pain....Got tired of lugging the hernia maker in and out to recharge it,so I got the bright idea of taking a GM alternator,mounting it on a stand,connected to a lawn mower engine to the alternator with a fan belt to recharge the battery at the camp and leave the battery and homemade charger there....First time used a 3hp Briggs and Stratton,worked real good on car batterys,but when the "big" battery was almost dead and we started up the charger and hooked it up it would "kill" the 3 hp B & S motor in about 2 secounds..... Put a 5 hp B & S motor and it worked great,used it for years....

On a regular auto battery the 3hp motor would pull it and charge it up with no problems even when it was stone dead,so I'd imagine a good battery would pull less than 2 hp from an alternator.....

David

[petek]

If the battery was charged then the only draw on the altenator would be the rest of the engine electronics (coil, ECU, etc.). David's estimate is probably pretty close.

Want to get closer? Figure out how much amperage the engine's electronics are drawing, figure a 60% efficiency in the altenator, and convert the amps to watts (13.4v x amps) and then convert the watts to hp (Watts x 0.00134).

Doing it backwards: 1hp = 746 watts, 746 watts -> 55.7 amps.

We've ignored the efficiency of the altenator, but I'd say that 1hp is pretty close and it probably wouldn't be more than 2 hp.

Pure SWAG!

[Cracker]

for my engine builder and he said it really is negligible. Under 3 hp for sure!

Cracker

[DAVID GAGNARD]

I have a 60 amp alternator,and using Petek's formula that 1hp would equal 55.7 amps,than at a full load my alternator would only draw approx. 1 hp.... I do not think it has ever been on or at a full draw so the loss is definetly negligable.....

Petek,thanks for the technical side,always wondered exactly how much the draw was.....

The GM alternator used in my "recharger" was either 100 or 125 amps,it's been too long ago to remember,but I remember when we hooked it up to the "bull dozer" battery when the battery was totally dead,it really pulled hard on the 5hp motor for about 1 minute then the rpms of the motor came down as the governor let off and then we could run the motor about half throttleto continue the charging...also,the motor was an old 5hp very well used that I'm sure was not putting out an honest 5 hp,probably more like 3 to 3.5 hp....

David

[G.R.]

If you are running the quarter for the finals, disconnect it, make sure you battery is at full charge, otherwise 1-3 hp loss isn't worth fretting over

[RICK LAKE]

Blykins Depending on the charge of the battery,condition, the amount of load on the alt, how many amps it puts out, and the wiring being in prefect shape, you are looking at 3- 10 hp. Load has alot to do with it. Fuel pumps,lights,igntion system,ECU, relays, coolant fans, radio amps, A/C for the softy guys. Themunber is on the soft side. It may go as high as 15-18 hp. under full output load. Rick Lake

[rdorman]

There is an old hot rodder trick that even Nissan put on my 300ZX. Put a vacuum switch in the alternator curcuit that will shut down the alternator under full throttle. What ever the loss, this puts and end to it when you don't want it.
Rick

[petek]

Ya know, I ignored mechanical inefficiencies in the analysis... losses in the belt, pulley, bearings in the altenator ... but if you've got a fully charged battery, I can't imagine that there is enough horsepower loss to worry about.

Heck, even if we were talking on the upwards of 5hp, wouldn't it just be easier to loose a few pounds than to disconnect the altenator!

[Mr.Fixit]

With today's high output ignitions, the couple of ponies the alternator uses is comparable to the ponies you won't make (if alt is disconnected) because your MSD is running at 11.5 V instaead of 14 V.

[DAVID GAGNARD]

Mr Fixit is dead on,go to any dirt track and look at the front running cars,they all run alternators,MSD claims you will actually lose more hp without an alternator and if your battery is not fully charged than running an alternator. The ignition systems of today need high constant voltage to perform properly....

David

[JWheaton]

Petek,

I have now learned more information. Learning and remembering may be 2 different issues though.

Can you answer this question correctly?

What is the air-speed velocity of an unladen swallow?



Jamie

[RICK LAKE]

J Wheaton Is that question talking about a bird or person Rick Lake

[JWheaton]

LMAO!!!


I was thinking bird.

The other vision is not pretty!

[petek]

African or European swallow????

[JWheaton]

What, what, what ----- aarrrggghhhhhhhh!

And so Sir Richard continued on his quest

[petek]

Massive thread drift ... of the Python type. Gotta love it!