Outboard charging rate?

Stu Jackson · May 3, 2012

You'll usually get a maximum of 6A at WOT from an outboard.

Try this and search around for more info on your engine.

http://www.marineengine.com/parts/parts.php#manuals

sailingandsuch · May 3, 2012

Not sure what the wires are, but I will say that charging from an outboard sucks so I have been told. The alternator doesn't put out much unless you are running you motor wide open.

I just bought a new outboard for my boat and thought I definately needed an alternator on it but realized it was practical at all.

Hope you figure out what the wires are for.

Stu Jackson · May 3, 2012

finding41 said:
Thanks. I was on that site just before I posted. (That's how I figured out what my motor was, It's all white...) It won't tell me unless I buy the book. I'm way too cheap for that!

Cheap? Never!

Frugal? Yup. Means you're a true sailor.

That said, are you saying that you can't get enough detail from searching through the parts or you just haven't been able to find it...yet...?

jimg · May 3, 2012

If your Johnson motor has electric start, which I didn't think the 7.5's did, then it has a charging system. If not, then not. The wires are probably for a prior owner installed kill switch.

Joe · May 3, 2012

You need to check the specs for your motor.... check OMC site or here's a parts source that has info.... http://www.boats.net/parts/search/BRP/JOHNSON/1983/parts.html

Unless you have electric start or need a battery to run the motor you will have magneto, not an alternator. The two wires are most likely a 12 volt running light power source. Put a multimeter on it, or touch it to a 12 volt bulb to see if there's any output when the motor is running. The tell is the P.O. having no idea.... I would be surprised to find an alternator on a that small a motor.

The alternator on my 9.9 tohatatsu 2 stroke only puts out 5 amps... so it would take quite a long time to charge up my battery bank.... but it uses so little juice to start I don't need a dedicated starting battery.
The newer 4 stroke hondas all have 12 amp alternators...

FourPoints · May 4, 2012

The round object to the left of the picture is probably a rectifier, which will convert the 20-40v AC output of the charging coil into 10-20v DC current for powering your running lights, or trickle charging your batteries. The previous statements about output in the 5-6 amp range are accurate, some motors output as much as 7 amps, but most are around 5 or 6 at WOT.

For a little more info about outboard charging systems and voltage regulators look at my regulator project last year.
http://sailingit.com/blog/boat-projects/outboard-voltage-regulator

Also Joe, your 2 stroke Tohatsu almost certanly does NOT have a true alternator, it probably has the same 80w charging coil wired to an unregulated rectifier output that mine has/had.

tommays · May 4, 2012

IMHP most sailors do not run the outboard long enough to be a very usefull way to charge the battery

FourPoints · May 4, 2012

tommays said:
IMHP most sailors do not run the outboard long enough to be a very usefull way to charge the battery

As a primary charging source no, unless you are delivering the boat or there is no wind.

I ran into a problem where my outboard started OVER charging my batteries, after I added solar and a 3 stage charge controller. That's when I added the rectifier regulator so that it would stop running up to 16+ volts.

kenn · May 4, 2012

Our 5 HP Suzuki has that "magneto" coil that allegedly puts out up to 80 watts into 12v lighting, so yeah, just over 7 A, theoretically.

I say allegedly cos I haven't got measurable output from the thing yet, but I haven't really tested everything.

twistedline · May 4, 2012

I recently measured the output from my 1984 Johnson 9.9 electric start. I used a DC clamp meter.

At idle it was virtually nothing and slowing increased to about 4amps as rpm increased.

Better than nothing.

FourPoints · May 4, 2012

4-5amps is what I would really expect from a ~80w charging coil / magneto.. it's the same problem that solar has and why MPPT charge controllers are so useful.

the 80w charge coil is 80w, but at roughly 20 volts (full throttle), or 60w at roughly 15v (3/4 throttle). This is because it is not a real alternator, but just a magneto coil so its output is dependant on engine RPM.

When you attach it to a 12v battery it never comes up to it's ideal voltage, so you get the same aperage, but at the lower voltage so it's really much less wattage.

80w @ 20v = 4a
4a @ 12v = 48w

read this for more info.
http://genasun.com/aboutmppt.shtml

I actually looked into installing a MPPT controller to take the output from the engine to get the missing 40% of power back, but didn't find any that I was confident would work without damaging the rectifier diodes.

Joe · May 5, 2012

FourPoints said:
Also Joe, your 2 stroke Tohatsu almost certanly does NOT have a true alternator, it probably has the same 80w charging coil wired to an unregulated rectifier output that mine has/had.

Hmm... that makes sense... except... I'm reading my Service manual spec page... for the 9.9 through 18 hp models it shows all are equipped with: magneto energized CD ignition system and a 12v-80w alternator. The electric start models are also equipped with a 12v-6a rectifier. The manual start models are not equipped with rectifiers, but are available as an option.

So my questions are: what is a "true" alternator? Is the magneto the same as the alternator on small OB's. And, if the rectifier output is unregulated, what keeps the system from overcharging the battery?

Well...... now I've read on down the page and alot of my questions were answered, but if there's anything else you might share it would be appreciated. Thanks for your help.

FourPoints · May 5, 2012

Joe said:
So my questions are: what is a "true" alternator? Is the magneto the same as the alternator on small OB's.

Someone may correct me, but to me a "true" alternator will output a constant voltage regardless of what RPM it is turning (within the designed operating range of the engine of course) It does that by means of exciting a 2nd coil controlled by a regulator to alter the magnetic field to increase or decrease the output based on load and RPM.

Joe said:
And, if the rectifier output is unregulated, what keeps the system from overcharging the battery?

Our outboards with the magneto coils use permanent magnets under the flywheel rotating past a coil, and the output of the coil is directly related to the RPM of the engine. Engine at idle speed will only output 6-8 volts, rev it up to a normal low end cruise speed and it outputs around 12-13v (aka starts to charge), and around WOT it's around 18-20v

What stops the battery from over charging is the simple fact that if you are not charging the battery any other way, it's unlikely to ever get fully charged from the ~4amps of output you are getting from the engine, and the voltage will be limited by the battery the same way a hard wired solar panel is. If you disconnected the rectifier output from the battery when the engine is running WOT, you would see the voltage instantly jump from 12.xv to 18v+ (at least until the rectifier diodes blow, I don't recommend testing that yourself).

I only discovered the problem after installing the 3 stage charge controller for my solar panel (it didn't appear prior to that when I was using the cheap sunforce shunted controller because the batteries never fully charged with it). Since the batteries were already fully charged and unable accept the additional charge from the engine, the voltage started to rise to the output voltage of the magneto output, and I was seeing 16+ volts through the entire electrical system.

JohnShannon · May 5, 2012

Four, what a great idea. Should be OK if the magneto output can run unloaded, nothing the MPPT tracker does should harm it.

Joe · May 5, 2012

Thanks Brian.......

FourPoints · May 5, 2012

JohnShannon said:
Should be OK if the magneto output can run unloaded

I know you cannot disconnect the battery when the engine is running without damaging the rectifier diodes, but I'm not sure about running without a battery attached. I think you can safely do that as long as it is detached from the onset.

JohnShannon said:
nothing the MPPT tracker does should harm it.

most MPPT controllers will disconnect the "panel" from the battery when the battery is charged. Disconnecting the rectifier from the load while the engine is running will damage the diodes. You need a controller with a divert circut or one that routes to ground through some resister to prevent blowing the rectifier... I searched long and hard did find one that claimed to be safe to use, but it required attaching something like a hot water heater to the divert circuit to prevent blowing the rectifier, which I don't have.

chp · May 6, 2012

My Mercury manual states NOT to run the engine without being connected to a battery or you will blow the rectifier. I just replaced mine since the pins were corroded and one was missing. After testing the rectifier it was dead.
I debated installing a new one for the reasons already mentioned. On a sailboat the motor is usually only used for a short period of time, however after a few trips we had to motor for hours and with a fridge and other things running I figured any extra power will help.

Outboard charging rate?

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