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Power-User

Join Date: May 2007
Location: Sweet home Alabama
Posts: 143

# Outboard motor battery charging

06/20/2007 7:26 PM

Another EE question from a non EE. I have an old outboard motor (75HP). It starts and runs once started. The problem is that it does not charge the battery when running. I understand there is no generator and the charge is "backwards" through the magneto that makes the spark into the battery. We currently have to charge the battery every time we use the boat and I suspect the starting efforts deplete the charge.

Several questons are:

1. Is the battery ever used for the spark? If not and the spark comes from the magneto than the magneto must be working. Is this logic incorrect?

2. If that is true, is there any way to test if the charging device is working or if a disconnect can be traced?

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Power-User

Join Date: Apr 2006
Location: USA
Posts: 255
#1

### Re: Outboard motor battery charging

06/20/2007 10:05 PM

I have a small boat-electric start. I have always assumed (that is your warning) that the electric starter becomes the generator once the motor runs under gas power. There may be some current limiitng circuit, but I would believe the scheme is as simple as possible.

You could have a number of other reasons your battery is losing charge. If you have a DC volt meter (They do sell cheap ones) measure the volts of your charged battery proir to starting motor. Probably be around 12V. Then measure volts with motor running, volts should read higher if charging is taking place, say 13.5V. There is a good possiblity something else is draining your battery or the battery is just old- Check lights, radios, or burnt wire-shorting to metal.

A good battery does not have to be charged every time it starts the motor.

Guru

Join Date: Jan 2007
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#2

### Re: Outboard motor battery charging

06/21/2007 4:17 AM

Any connection that re-charges a starter battery will place a load upon the engine.

A 6hp 2-stroke outboard on a boat normally moored at these co-ordinates has pull-cord start. Though it has a magneto and electrical wiring, the wiring is contained entirely within the engine compartment. Another vessel nearby has a 10hp 4-stroke outboard with electric start. There is a trickle-charge circuit taken off the engine for 12V lighting within the cabin in this particular case, and it operates while the engine is not running, so there is an alternator or a dynamo of some sort located within the engine compartment. With electric lighting and electric starting on a boat, it is common (and good practice) to provide 2 sets of batteries on separate circuits, as losing the lights is less of a problem than losing the ability to start the engine. The style of battery for lighting will be different to the style of battery for starting; the lighting battery is designed for deep discharge whereas the starting battery is designed to supply a lot of current for a shorter time.

So mechanisms and arrangements vary. If they are not available at a local boat chandler's outlet it is possible to obtain copies of manuals for certain outboard motors over the internet, and this was certainly the case with the 6hp.

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Guru

Join Date: Aug 2006
Posts: 4540
#3

### Re: Outboard motor battery charging

06/22/2007 12:44 AM

In general, (and this is a gross generalization) magneto systems have two coils under the flywheel: an ignition primary coil and a charging coil. (Sometimes both of these are encapsulated into a single piece.) There is also a secondary ignition coil (or HT coil) outside the flywheel, that looks like a small version of the ignition coil on a car.

The current from the charging coil goes through a rectifier and into the battery. On some outboards, there is no voltage regulation, on others there is. In yet other outboards, the system is more refined, with an alternator that works more like that in a car. There are all sorts of possibilities, so you really need a schematic to see what's going on in your engine. But in any case, I am not aware of any system which uses a single coil in the magneto to supply both ignition and charging.

Almost all electric start outboards have separate starter motors, very much like the starter motor on a car.

As a pure guess, I'd suspect wiring first, the rectifier second, and the charging coil third. The charging coil can be tested with an ohmmeter, as can the rectifier. The charging coil should have a fairly low resistance (maybe 3-10 ohms) and the rectifier(s) should conduct in one direction but not the other. Wiring can be checked visually and with an ohmmeter.

Ignition is separate and unrelated.

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Power-User

Join Date: Feb 2007
Posts: 274
#4

### Re: Outboard motor battery charging

06/22/2007 8:26 AM

ALL ELECTRIC START OUTBOARD MOTORS HAVE ALTERNATORS!

Spend \$20.usd and get a repair manual for your motor then you can trouble shoot it and fix it.

Guru

Join Date: Aug 2006
Posts: 4540
#5

### Re: Outboard motor battery charging

06/22/2007 10:09 AM

What you say re all electric start motors having alternators is technically true, although the original poster might not recognize his charge system as having one. If he looks for something hanging from the side of the powerhead that looks like a car alternator, he won't find one -- the stator is under the flywheel, there is no adjustable strength field winding, the AC is generally single phase instead of three phase, etc.

Some outboards in this size range could have either of two charging systems: 9 amp and 35 amp. The first could be substantially undersized for typical uses, such as running instrumentation, stereo, etc.

In any case, your advice to get the manual is excellent. If the poster is going to do any work whatsoever on the motor, it is money very well spent. In fact, even if he doesn't do the work himself, it enables him to better evaluate the diagnoses of mechanics who might work on the engine.

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Guru

Join Date: Feb 2006
Posts: 1775
#9

### Re: Outboard motor battery charging

06/24/2007 1:11 AM

Regards.

And all ALTERNATORS! have embedded rectifiers.

You see even the Bikes having dynamo [ALTERNATOR] in rear wheel hub have built-in rectifier to charge the cells.

Charging circuit may be at fault & needs check

Guru

Join Date: Aug 2006
Posts: 4540
#10

### Re: Outboard motor battery charging

06/24/2007 3:04 AM

And all ALTERNATORS! have embedded rectifiers.

This is false. Most older and many newer outboard motors have external rectifiers. Most motor cycles and motor scooters also have external rectifiers, with the alternator coils located within the engine cases, and the rectifier mounted to the frame. Automotive alternators, on the other hand, have exclusively internal (embedded) rectifiers.

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Anonymous Poster
#6

### Re: Outboard motor battery charging

06/22/2007 4:50 PM

You have a stator on the outboard running through a rectifier (voltage down to 12 volt)then voltage regulator

If the motor runs...your stator pack...power pack is OK...I'm guessing your rectifier or voltage regulator is fried...

find the rectifier...if your putting out 13-15 volts...regulator is toast

Power-User

Join Date: Jul 2006
Posts: 301
#7

### Re: Outboard motor battery charging

06/23/2007 4:24 PM

If you have to charge your battery before every time you start your engine I suspect the battery is no good as the energy required to start your engine is only a small portion of what should be available after a full charge.

After you charge your battery the next time, shut off the charger, measure the voltage immediately. It should be >13v. Then remeasure the next day and it should be at least >12.65. If it is, start your engine, then shut off your engine and then remeasure. It should still be in the vicinity of 12.6. If you are measuring voltages closer to 12v or lower, then your battery is probably shot.

Guru

Join Date: Aug 2006
Posts: 4540
#8

### Re: Outboard motor battery charging

06/23/2007 6:13 PM

Very good suggestion.

Also, you can do a check check to see that the charging system is working by measuring the battery voltage, then starting the motor, and then remeasuring the voltage with the motor running at mid rpm with the boat in neutral. Voltage should be higher than it was before starting, and can range up to about 14.2 volts, a common regulator setting. If you appear to be charging but the battery quickly drops back to 12.5 or fewer volts after the motor is shut down, then you can conclude that the battery is shot.

Also, if the charging system appears to be working, and the battery drains from one use to the next, it is possible the the rectifier is partly shorted, and therefore draining the battery. (There could be other continuous drains as well.) If you are comfortable working around the battery and have an ammeter, you could check for current flow out of the battery when the boat is not running, and all accessories are off. (A couple milliamps is fine, but a 100 milliamps would be too much.) You could also simply disconnect the ground cable, and see if the the battery is OK next time you want to use the boat.

If you are unfamiliar with this stuff, be certain not to inadvertently cause a short from one battery terminal to the other -- which has the potential for burns, fires and explosions.

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Anonymous Poster
#11

### Re: Outboard motor battery charging

07/05/2007 12:31 PM

Can you guys help - I am trying to check the charging circuit on a 25hp Mariner outboard. Putting a voltmeter across the battery with the engine not running shows 12.6 volts. With the engine ticking over it shows the same voltage. Does the engine need to run at high revs to charge?

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Join Date: Aug 2006
Posts: 4540
#12

### Re: Outboard motor battery charging

07/05/2007 12:43 PM

Often, small engines need to be at least a little over idle to show a charge. If you are only checking at idle because the boat is out of the water, bear in mind that even idle speed can damage the water pump -- so best to check in the water, or on a stand, running in a barrel of water. But in any case, charging should be noticeable just slightly off idle -- with a good battery, you could expect 13V or greater with the engine at "no wake" speed, and up to about 14.5 at high speeds.

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