How big an alternator can I get on an outboard?

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Jan 8, 2009
2
Macgregor 26 Montreal
I have a MacGregor 26s which until recently had a 9.9 HP Honda 4 stroke with a 10 amp alternator coil on it. It was stolen off my boat a couple of weeks ago, so I am looking for a new motor. We have a mechanical fridge on our boat, and a 50 Watt solar panel which doesn't quite keep up with the fridge.

I had 2 problems with my old motor. The first was that it didn't seem to have a regulator. Our GPS lets my monitor the voltage on the house battery, and I have noticed that when the outboard is on for a while the voltage is way over 14 volts, and have even seen it as high as 15. I have been told that this can cook the electrolyte for our battery, which is now shot after 2 seasons. The second problem, is that I would like to charge my battery faster, particularly at lower RPM.

My question is how big an alternator have you seen on a 9.9 hp or less 4 stroke outboard? and secondly, is there a way to regulate this output to keep from overcharging my battery?
 
Mar 1, 2012
2,182
1961 Rhodes Meridian 25 Texas coast
You do realize that most alternators charge at 14 - 14.2 volts? They HAVE to in order to force a charge into 12 volts. Check your car with a voltmeter while it's running.

I think you just need a good regulator, separate from the engine.

And how large an alternator you can get is highly dependent on the outboard you buy
 
Jan 10, 2009
590
PDQ 32 Deale, MD
One battery? You may find that a second battery is a very cost effective way to add overnight range, cheaper than solar and alternators. You need the second battery for the night, anyway.
 
Jan 22, 2008
169
Beneteau 343 Saint Helens, Oregon OR
I had an 8 HP Honda 4 stroke that was listed at 5 amps but when measured was really about 2.
 
Jul 23, 2009
857
Beneteau 31 Oceanis Grand Lake, Oklahoma
I think I've seen Yamaha or maybe Honda list a 12 amp alternator.

I don't see why you couldn't use a solar charge regulator. I also don't think you need to.

Troy
 
Jul 23, 2009
857
Beneteau 31 Oceanis Grand Lake, Oklahoma
Over discharging is very bad for your batteries. I wouldn't wory about 15 volts as long as it's not for days at a time. Make sure the battery is not getting too warm. When I want to spend more time away from the dock I bring a small generator with me. I highly recommend the Honda 1000 inverter generator, not a cheap copy. My little Honda generator runs a 900w hot plate, battery charger, fridge and a fan all at the same time.

Troy
 
Jun 17, 2007
402
MacGregor Mac26S Victoria Tx
A couple of thoughts

Typically, people find that outboards only will marginally help to charge their battery bank because of two reasons. The "rated" amps are at wide open throttle. Most of the time it will be putting out far less. The second issue is how long to you actually run your outboard?

As far as your fridge unit - you need to get some numbers together. How many amps does it actually draw? (or what does the manual list? - not as reliable) How long does it actually run when operating?

For example, a customer wanted to know the same thing you are asking. We calculated his unit ran 50% of the time in the summer. We knew what the rated draw was for his model. We then calculated how much sun was available in his location (So FL) With this information, we calculated he needed about 160 watts of solar to replenish what his fridge was drawing.

It helps to have some numbers when working these calculations.

I have a MacGregor 26s which until recently had a 9.9 HP Honda 4 stroke with a 10 amp alternator coil on it. It was stolen off my boat a couple of weeks ago, so I am looking for a new motor. We have a mechanical fridge on our boat, and a 50 Watt solar panel which doesn't quite keep up with the fridge.

I had 2 problems with my old motor. The first was that it didn't seem to have a regulator. Our GPS lets my monitor the voltage on the house battery, and I have noticed that when the outboard is on for a while the voltage is way over 14 volts, and have even seen it as high as 15. I have been told that this can cook the electrolyte for our battery, which is now shot after 2 seasons. The second problem, is that I would like to charge my battery faster, particularly at lower RPM.

My question is how big an alternator have you seen on a 9.9 hp or less 4 stroke outboard? and secondly, is there a way to regulate this output to keep from overcharging my battery?
 
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Apr 8, 2010
1,606
Frers 33 41426 Westport, CT
I don't see why you couldn't use a solar charge regulator. I also don't think you need to.

Troy
be careful, (in most cases) when a solar regulator decides the batteries are charged, it disconnects the panel, which is the same as disconnecting the wires. that would blow the diodes on most alternators. you need an alternator designed for an external regulator hookup.
 
Jul 23, 2009
857
Beneteau 31 Oceanis Grand Lake, Oklahoma
Fourpoints

You are correct a solar charge regulator may damage the alternator if it simply disonnects the supply from the load. The correct regulator would be one that shunts the extra voltage across a load resistor.

Outboard motors use a permanet magnet alternator. They don't have a field coil to regulate the output. The faster it spins the more power/voltage it puts out. They usually have a set of diodes to convert the AC to DC and a sunting regulator.

I don't see the need for a second regulator.

Troy
 
Jun 4, 2004
33
-catalina -22 fresno, calif.
it takes about 2hp to run a 12v 60amp alternator, so smaler is better for a small engine
 
Jan 8, 2009
2
Macgregor 26 Montreal
I would gladly give up 2 HP for a 60 amp alternator. I don't need 60 amps of charging, but I bet I would hold 10 amps at idle. My Honda 9.9 could keep me at 5 to 5.5 knots at half throttle, and I've heard of people with 5-6 HP engines on their MacGregor 26s.

My fridge draws 4 amps when running, and it runs about 30% to 40% of the time. If I am only running the fridge and my LED anchor light at night, then i should only drain 30 or 40AH of a 115 AH battery, so I don't think I am discharging my battery too much. I think a second battery is a good idea, but there isn't much space for 2 batteries under my sink. I have room under the V berth for 2 batteries, so I may relocate them there. Better weight distribution too.

I bought a new solar charge controller that will let me shut off the fridge load at a higher voltage, and also prevent it coming back on until a minimum charge has built up.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
I would gladly give up 2 HP for a 60 amp alternator. I don't need 60 amps of charging, but I bet I would hold 10 amps at idle. My Honda 9.9 could keep me at 5 to 5.5 knots at half throttle, and I've heard of people with 5-6 HP engines on their MacGregor 26s.

My fridge draws 4 amps when running, and it runs about 30% to 40% of the time. If I am only running the fridge and my LED anchor light at night, then i should only drain 30 or 40AH of a 115 AH battery, so I don't think I am discharging my battery too much. I think a second battery is a good idea, but there isn't much space for 2 batteries under my sink. I have room under the V berth for 2 batteries, so I may relocate them there. Better weight distribution too.

I bought a new solar charge controller that will let me shut off the fridge load at a higher voltage, and also prevent it coming back on until a minimum charge has built up.
We have the same boat.



http://purplesagetradingpost.com/sumner/macgregor/outside-13.html

63 amps from 5 HP. You don't need the 5 to keep it running but it sure helps in starting even when you turn the ...



... the alternator off while starting or it will stall the motor. I've never seen it put our more than about 23 amps as we try and keep the batteries above 12 volts.

We have a 9.8 HP Tohatsu that is suppose to put out 6 amps, but I don't think it does that at 1/2 to 2/3 throttle which is all you need with the Mac S.

If you do have sun then...



http://purplesagetradingpost.com/sumner/macgregor2/outside-33.html

... 180 to 200 watts above will take care of the fridge and quite a bit more. Our portable fridge...



http://purplesagetradingpost.com/sumner/macgregor2/inside-22.html

...uses 30-40 amp/hr a day also. Put the solar on if there is sun where you cruise.

Also....



.... add the second battery. It will help for a day or two, but then you need to be able to charge it also. We did that and a lot more to the interior of the Mac.....

http://purplesagetradingpost.com/sumner/macgregor/inside-index.html

Good luck,

Sum

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Jun 5, 2010
1,107
Hunter 25 Burlington NJ
I know a guy with an H25 who installed a Toyota Corolla 25-amp alternator on top of his electric-start 9.9 outboard. He mounted it right onto the flywheel-- even made a coffee-can-looking hood over top of it. The problem with automotive alternators is that they are NOT spark-protected; so for an inboard this is an explosion waiting to happen. But the same may not be true for an outboard on the transom. (Try this at your own risk-- I'm not recommending it.)

I am, however, looking for an alternator to do this for my outboard but I'd prefer a marine one. They are much more expensive, however.
 
May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
^ yep, I've seen a franken-outboard with a delco 60amp alternator with brackets to the powerhead.


-ugly and dangerous... but it threw some amps.
honda eu1000 or 2000 FTW!
 
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