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Author Topic:   Powering Lighting Loads Directly from Engine Charging Circuit
BeaverGuy posted 08-01-2005 07:55 PM ET (US)   Profile for BeaverGuy   Send Email to BeaverGuy  
My 15 HP Honda has 12 volt battery charging current circuit. Is it possible to use this voltage to power navigation lighting when the engine is running without a battery? The owners manual says the motor's 12 volt output is intended for battery charging only and that lights and electrical accessories for the boat should be connected to the battery. I currently use navigation lights powered by dry cell batteries but they are unreliable.
jimh posted 08-02-2005 09:55 PM ET (US)     Profile for jimh  Send Email to jimh     
I would imagine you could do this. The current from the outboard cannot tell what load it flows through. The caution against it is probably more for concern of having too much voltage from the charger output.

Invest in a small lead-acid battery, like those used on small engines such as motorcyles, etc. This will work better than running the navigation lamps directly. The charger voltage output will probably be about 14 volts or more, and this will tend to burn out your lamps. A battery across the charger output will tend to stabilize the voltage somewhat.

I suggest you measure the output voltage before connecting the lighting loads. It may be higher than I think and could cause you to burn out all the incandescent lamps.

Jerry Townsend posted 08-03-2005 05:15 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
If you somewhat directly connect to the engine's alternator, be sure to include an in-line fuse (2 - 5 amp)in the hot line will work. This could prevent the alternator diodes from failing in the event of a problem in your lighting circuit. -- Jerry/Idaho
JOHN W MAYO posted 08-03-2005 11:18 PM ET (US)     Profile for JOHN W MAYO  Send Email to JOHN W MAYO     
This would probably work as you described, but I would not do it. I have done it in trucks without a battery.
Reason not is, I bought a Switch to switch between battery 1-2-off, the instructions of installation, it warns not to turn the switch off if the engine is running to prevent damage to the charging system.
jimh posted 08-03-2005 11:28 PM ET (US)     Profile for jimh  Send Email to jimh     
John--Very good point! If there is no battery present and the lamps are all turned off, the charging system may very well be quite upset about running into an open circuit load.

So get a small motorcycle battery. You can buy one at WALMART for $30 I bet. Much cheaper than a new alternator!

Chuck Tribolet posted 08-03-2005 11:54 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I agree with JimH, especially because some of the small
motors don't have voltage regulation. They will get a
pretty high voltage without a battery to pull it down. That
will give your bulbs a short life.

Put that motorcycle battery in an acid-proof box. I've seen
Tupperware and Pelican boxes used. And keep it upright.


Cicada posted 08-03-2005 11:55 PM ET (US)     Profile for Cicada  Send Email to Cicada     
I guess I need a little clarification. It was also my understanding that the charging circuit needs to be connected to the battery to prevent damage due to an open circuit. If you were powering a device like a light bulb that failed with an open circuit you'd then fry the works. Wouldn't the same thing happen with a fuse?

I guess I don't know what makes a diode fail.

If the open circuit is in fact a problem then maintaining the contacts on motors that are mounted and dismounted from the boats becomes important.
I have a kicker with a charging system that is off the boat as often as on and I need to keep an eye on the contacts for the pigtail plug or corrosion can form rapidly. When the kicker is mounted there is a direct path to the battery.

There is also a "Zapstop" that I've seen mentioned that mounts on the motor to prevent damage (which again I thought was from an open circuit). I don't know if this would act in lieu of a battery, load or fuse but it may be an option.


JOHN W MAYO posted 08-04-2005 10:01 AM ET (US)     Profile for JOHN W MAYO  Send Email to JOHN W MAYO     
If you want to do it, go ahead, but also be prepared to spend money on your charging system.... IF it fails.

It seems more prudent to me to use the system as it was designed for.

As a pilot, we have to follow certain peramiters in flight.If we exceed those, such as -speeds past redline-. -G- forces greater than what the plane designed for, things may happen ....such as a wing may come off.
I am not going to be a test pilot. There was an airpane I have heard about years ago that -pulled the overspeed warning circut breaker- to eliminte the warning horn nose and then continued to exceed the red line speed,.....plane came apart, everyone made there last fight.

Your situation is not that critcal of course, but maybe you get my reasoning.

I agree with Jim, get a battery. Yesterday I talked to a guy who bought a 12v lawn mower batter for $14.00.

I am only speculating, but I do think it would last for a little while, but how long is the question.

Good luck.

Chuck Tribolet posted 08-04-2005 10:30 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Think of electricity as water flow. Current (Amps) is the
volume of water. Voltage is the water pressure.

Fuses fail due to too much current.

Bulbs fail due to too much voltage (and the passage of time).

Diodes are a one-way valve. They can fail due to too much
current in the direction they are made to flow, or too much
voltage in the direction that they are made to NOT flow.

You can blow the diodes in the alternator by disconnecting the
alternator from its load suddenly. It takes the voltage
regulator a fraction of second to get the voltage regulated
back down, and during that fraction, the voltage can get
WAY big. If the battery is hooked up, the excess energy gets
dumped into the battery before it has a chance to drive the
voltage up. Been there, done that, DUH.

Zapstop works by taking all voltage over some threshold (say,
18V, which the diodes can handle), and dumping it to ground.
It's like that air-filled pipe stub that keeps your pipes from
banging when you shut off the water.

Net: need a battery. If you've got a battery switch that
allows you to disconnect the alternator, a Zapstop would be
a good idea.


Cicada posted 08-04-2005 07:19 PM ET (US)     Profile for Cicada  Send Email to Cicada     

Thanks for the clarification. Nice explanation.


BeaverGuy posted 08-08-2005 02:30 PM ET (US)     Profile for BeaverGuy  Send Email to BeaverGuy     
Thanks for the replies - I didn't understand that it could ruin the alternater in the engine - I'm going to add a battery!!

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