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Old 04-07-2015, 09:49   #1
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Outboard Motor Without A Battery?

I have a Mariner 25 horsepower two stroke engine on my dinghy.

This old engine, if the ambient temperature is over 20C, starts if you look at it crossways. It has electric start which is totally unnecessary, with a car type battery which is just useless extra weight in my davits.

Do I have to have a battery connected to it to run, or will this ruin the alternator? I'd like to just toss the battery and forget about it.

Ignition is obviously by magneto as a dead as a doornail battery doesn't not inhibit its starting at all.

Grateful for tips.
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Old 04-07-2015, 10:10   #2
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Re: Outboard Motor Without A Battery?

No, you don't need the battery. You don't want hot leads laying about though, so you will have to either disconnect them or cut them off short.

Mark
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Old 04-07-2015, 10:50   #3
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Re: Outboard Motor Without A Battery?

Quote:
Originally Posted by colemj View Post
No, you don't need the battery. You don't want hot leads laying about though, so you will have to either disconnect them or cut them off short.

Mark
Agree: Most OBs are sold with electric start as an option. Just disconnect the external wire at the terminals and store it.
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Old 04-07-2015, 11:18   #4
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Re: Outboard Motor Without A Battery?

I'd be cautious about disconnecting it and running your engine. I'm not an authority on this subject and don't want to bother my outboard mechanic friend on the holiday but have always heard that you can damage the electronic parts inside the motor if you run it without being attached to a battery. I realize you don't care about ruining the charging parts but I'm not sure that's where the damage will end? I once looked into doing this on an auxiliary trolling motor I had on my old Grady White and was advised to remove the fuse that enabled the charging system rather than just disconnecting the terminals, but I don't know if that was just for that particular (Mercury) motor or if they are all made that way.

I share your concern about lugging around the extra weight and agree that most motors start easily enough if you just give the cord a good yank so electric start isn't a necessity. The compromise I chose was to buy an electric start motor to make it more "wife friendly" and a small, motorcycle AGM battery in small plastic battery box that's screwed to the transom about halfway up, so the terminals are further from water level should the 500gph automatic bilge pump fail, but still just below the top of the transom so I can leave the battery in the box if I transport the RIB upside down on deck. Dinghy motors in the 20-40hp range are still smaller than most motorcycle engines so a size 24 or 30 AGM motorcycle battery has plenty of oomph( technical term for cold cranking amps), weighs much less and is smaller than a full size car battery, and cost under $100. I also added a charging port located under the seat for cellphone or IPad or mobile VHF radio or rechargeable flashlight, and plan to add a cheap depth finder as well so it's nice to have those 12volts available when I want them.
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Old 04-07-2015, 11:25   #5
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Re: Outboard Motor Without A Battery?

Have you considered RFTM? Most likely available on the internet if yo don't have one. It might tell you if you'd do any damage by having the engine run without a battery. Does it have an alternator inside? Most likely without an alternator no issue, but would be helpful to check the manual.

Quote:
Originally Posted by Dockhead View Post
I have a Mariner 25 horsepower two stroke engine on my dinghy.

This old engine, if the ambient temperature is over 20C, starts if you look at it crossways. It has electric start which is totally unnecessary, with a car type battery which is just useless extra weight in my davits.

Do I have to have a battery connected to it to run, or will this ruin the alternator? I'd like to just toss the battery and forget about it.

Ignition is obviously by magneto as a dead as a doornail battery doesn't not inhibit its starting at all.

Grateful for tips.
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Old 04-07-2015, 17:42   #6
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Re: Outboard Motor Without A Battery?

There isn't an "alternator" in these outboards as traditionally understood. Just a bridge rectifier and some controlling circuitry. There is no damage at all from running it disconnected from a battery.

All of these are sold from the start as manual/electric - meaning connection of the electric part is fully optional.

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Old 05-07-2015, 03:29   #7
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Re: Outboard Motor Without A Battery?

Quote:
Originally Posted by colemj View Post
There isn't an "alternator" in these outboards as traditionally understood. Just a bridge rectifier and some controlling circuitry. There is no damage at all from running it disconnected from a battery.

All of these are sold from the start as manual/electric - meaning connection of the electric part is fully optional.

Mark
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Old 05-07-2015, 08:42   #8
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Re: Outboard Motor Without A Battery?

If my memory still serves me. The problem happens when connecting or disconnecting the battery while the motor is running. A tiny spark will happen, usually although not always and this spark will hurt the ignition system.

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Old 05-07-2015, 08:54   #9
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Re: Outboard Motor Without A Battery?

Motorcycle battery! For the times when you have so many guests/crew in the dink that you have no room to pull the chord without wacking someone.
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Old 05-07-2015, 08:57   #10
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Re: Outboard Motor Without A Battery?

Well, as Stu suggested...

http://www.mercury-marine.eu/cache/D...cumentFull.pdf

Don't know if that is your exact model, but this is what it has to say on page 46:

Quote:
Battery Installation
MOUNTING BATTERY

Follow the battery manufacturer's instructions carefully. Mount
battery securely to the hull in a place free from splashing water.
NOTE: Electric starting outboards must have the battery cables
connected to a battery whenever the engine is running, even if
started manually, as damage to the charging system could result.

BATTERY CONNECTIONS
Connect a red cable to the positive (+) terminal first and a black
cable to the negative (–) terminal of the battery. Put a red cap on
the positive terminal. When disconnecting battery cables, be sure
to disconnect the black cable first, and then the red cable.
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Old 05-07-2015, 09:37   #11
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Re: Outboard Motor Without A Battery?

My Merc 15hp must be attached to a battery. I ran it for a couple of years without and burned out the rectifier. There is no regulator on mine. Not sure at what HP they start to put on regulators or if that even makes a difference.
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Old 05-07-2015, 09:50   #12
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Re: Outboard Motor Without A Battery?

That surprises me. Our electric start Honda and Suzuki will not be damaged by operating without a battery. The electric start Toshiba that we almost bought also will not be damaged (I have the manual for it).

This is explicitly stated in the engine manuals.

I guess I assumed this translated across all brands, but it looks like the Mercury may not be as convenient or robust.

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Old 05-07-2015, 10:05   #13
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Re: Outboard Motor Without A Battery?

At a guess it has to do with how the alternator is excited. Both Honda and Mercury manuals leave the CD unit as a black box, but Honda shows a possibly battery driven excitation circuit (so removal of the battery also remove excitation), while the Mercury manual shows only rectifier connections to the battery, thus implying excitation from the magneto side (in which case the alternator would always be excited). That's a guess, impossible to tell from the diagrams I found.

As suggested, if you don't want to use the electric start could always put in a 2-3kg 15Ah sealed battery just to help the voltage regulation do its job.
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Old 05-07-2015, 12:17   #14
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Re: Outboard Motor Without A Battery?

dockhead-
I'd suggest a small (12AH or 17AH) agm battery. Size and weight of two bricks and power for your running lights if need be. Easy enough to grab and move.


IIRC an outboard "used to" always use a magneto, not an alternator. But it could equally have a generator or alternator, some of those companies have legacies with motorcycle engines as well and generators ruled there.


When in doubt...RTFM, or use the small sealed battery. Why risk burning up expensive parts? Heck, if you're not going to use it, even an 8AH battery would be "protection" in tiny package.
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Old 05-07-2015, 12:40   #15
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Re: Outboard Motor Without A Battery?

I use a 13 lb odyssey AGM. So far so good. My owners manual says I need a much bigger battery, but I thought that was looney.
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