View Full Version : How many amps does a 65 amp alternator put out at idle?

04-22-2003, 02:26 AM
I am getting a new engine and want to add an extra big alternator to powere lots of lights I am going to have on the boat. My thought is that I can troll at idel at night with lights on all over the place. But someone told me that alterntors generate next to nothing at idle and that their rated output is at 3000 rpm? Is that true? What do they generate at 500 rpm?Also, what guage would tell me if the batteries are being charged or discharged when the engine is running, ie whther the alternator is keeping up with teh current draw from the lights?

04-22-2003, 10:15 AM
That's correct. Sort of. Most alternators in the 55-100A class put out a reduced amount amperage at idle, but usually more than sufficient to power whatever you might have up and running draw-wise.

To properly analyze your needs you will need either an induction-type ammeter that you can clamp over the output feed of the alternator while you run your accessories to load the voltage regulator OR a very accurate voltmeter. I say very accurate because the factory-supplied gauge that resides in your dash could very well not be any damn good at all. Mine wasn't.

What you need to do is to verify that the gauge is accurate with a DVOM at the batt terminals and then throw on all the accessories that you will be running at your idle speed.

Does the Voltage drop below 12V? No? Good - then your alternator is adequate. This means that the alt is putting out sufficient amperage to keep the voltage at a value that says it will not be borrowing power from the battery over and above what the alt can output at that specified RPM. Not dropping below 13V would be a better situation of course, but with the small alternator you have that might not be possible.

I myself use a Delco-type 70A in my boat, I always have the radar, 10-inch FF, pilot and 2 GPS's running and so far I've only gone thru 1 Alt in 6 seasons. But of course if I'm anchored up all day with all but the radar running and no motor on to supply voltage to my small Group24 Batts, then I can really hear the alt singing on the way home as it works hard to put the missing amperage back in the aux batt. Not really the best way to go, but like I say, so far 6 seasons and 1 alt, so maybe not so bad either.

Two ways to make it better (for both of us) -

Plan A/ - put a smaller pully on the alt to make it spin faster at a given engine rpm. This normally works, but you have to watch that you don't overrev the alt when you at high cruise speeds. This band-aid solution might also require a smaller alternator V-belt to compensate for the smaller pulley diameter.

Plan B/ - the better way to go - would be to drop a larger capacity alternator in the boat. Typically you can find alternators up to about 100A with the internal regulator and 1-wire output feed, higher than 100A and you need to start looking into a separate stand-alone V-regulator as the heat generated by a high-Amp alternator's diode bridge and the v-regulator itself would greatly shorten the life of that alt.

There are any number of good high-quality manufacturers out there of high-Amp alternators and related gear - Balmar, Powerline and Jack Rabbit Engineering all come quickly to mind and all have their own highly detailed websites for you to explore the various solutions till you find one that works for your needs and wallet.

Personally, I think Jack Rabbit Engineering is the best of the lot, based on numerous conversations I've had with their applications engineers at various boatshows over the years.

Rgds, Leprechaun

Lolo Kane
04-22-2003, 10:18 AM
I have heard that the "High Amperage Alternators" actually put out LESS amperage at idle than a normal alternator. You might look into having your alt rebuilt with the specific goal of putting out some amperage at idle. This is what I intend to do. You also could put a smaller pulley on it and get more RPM at idle. Our engines rarely see more than 4000 RPM and most alternators can handle >6000 engine RPM.

My old Lowrance FF had a great feature, it would display battery voltage. But at the end of it's life thats about all it would do. Now I have a fancy new Furuno FF and a Navman Chartplotter, niether of them display battery voltage. I guess I'll have to install a voltmeter guage in the dash. The factory guage is useless.


04-22-2003, 10:38 AM
Alternator output is most definitely determined by alternator speed. Typically a 12 volt DC unit will begin making power around 1000 rpm's. Remember, this is alternator-not engine speed. The curve is not straight either. Generally 1200 rpm's = 10amps, 1500rpm's = 20 amps, and 2000rpm's = 35 amps, max output is reached at 4500 rpm's (51 amps). This is on a 51 amp Prestolite alternator that is typically used on marine engines.
When selecting a larger alternator check real close your pulley reduction between the crank and alternator. You can play games here. Get as close to max output at 4000 rpm's as possible which will allow more power down low. You may also want to check a dual output alternator. This will give a leg for house and a leg for starting batteries. There is one other solution that our sailing counterparts use. It is an adjustable regulator. This allows for you to adjust the alternator output at different engine speeds.
Last make sure you will not be putting too much side load on the crankshaft. If the load is ballanced you should be fine but as you get to the larger units it does have some affect.
Also get the measurements on you mounting points. The thickness of the flange and relationship of the adjusting screw to the mountiong base.
Lot of mumbo jumbo but hope it helps.


04-22-2003, 11:33 AM
Just a reminder --> The bigger the alternator, the more power (gas) it takes to run it. Add up your current (no pun intended) needs, future needs, a safety margin, and buy the alternator w/ the output you need.

I run all LCD electronics and don't have the power needs of someone running big CRTs, a lot of pumps, lights, and other current hungry 'appliances'.


04-22-2003, 11:46 AM
Good info.
It breaks down to this:
watts=volts x amps
746 watts = 1 horsepower = .746 kilowatts = 33,000 ft. lb/min. = 550 ft. lb./sec
Man these old Detroit Diesel Bible come in handy.


04-22-2003, 06:48 PM
Thanks for this info. I talked to the shop that is going to install teh new engine and he said that I should start out with the stock alternator becasue he has never seen a 26' boat that needed a bigger one. If the time comes that I need a bigger one, I can always change it out. But thanks for the great info and Lep esp on the guage.