Outboard Motor Without A Battery?

[Cadence]

[QUOTE=Dockhead;1862057]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.[/QUOTE

Try a motorcycle battery, Smaller less weight and starts scooters larger than that OB.

[HopCar]

I was going to suggest a much smaller battery but I see several people have beaten me to it. You'd be surprised how small a battery will spin a 25 hp engine. My Lehr 15 hp has a little LiFePo battery under the hood. It's about the size of three packs of playing cards. They use the same battery in their new 25.

[jtsailjt]

Originally Posted by Dsanduril

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.

I think you're right, and the fuse I mentioned earlier is between the magneto output and the rectifier so if you simply remove that inline fuse (less than 30" of time), then you can safely run the motor without having an excited rectifier with nowhere for it's output to go. I always seem to end up with Mercury's so that's where my familiarity ends, but since the OP asked the question about a Mariner, which is essentially a saltwater Mercury in gray clothing, I suspect that the same is true of it as the black motors that actually say Mercury on the side, and if he wishes to save his charging system for possible future use, he should remove that fuse before running the engine with no battery connected.

[Dockhead]

[QUOTE=Cadence;1862948]

Originally Posted by Dockhead

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.[/QUOTE

Try a motorcycle battery, Smaller less weight and starts scooters larger than that OB.

That was my first thought. I actually ordered one last April but it did not arrive before my departure.

[chp]

Originally Posted by jtsailjt

I think you're right, and the fuse I mentioned earlier is between the magneto output and the rectifier so if you simply remove that inline fuse (less than 30" of time), then you can safely run the motor without having an excited rectifier with nowhere for it's output to go. I always seem to end up with Mercury's so that's where my familiarity ends, but since the OP asked the question about a Mariner, which is essentially a saltwater Mercury in gray clothing, I suspect that the same is true of it as the black motors that actually say Mercury on the side, and if he wishes to save his charging system for possible future use, he should remove that fuse before running the engine with no battery connected.

I didn't think about disconnecting the wire from the magneto to the rectifier. I'm assuming that could work without doing any damage. One other thing about having a rectifier only charge system and batteries. There is no control over the charging. I have solar panels to charge my batteries and also use the motor, which is not usually run long, however sometimes if I have to motor for hours I've watched my battery voltage get pretty high. Even though the charging system is only 6 amps max. it will boil your battery. I would look into the above method and talk to an outboard tech about any consequences.

[FamilyVan]

Or better still than a motorcycle battery- an airplane battery.

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[Dockhead]

Originally Posted by FamilyVan

Or better still than a motorcycle battery- an airplane battery.

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That sounds like £££££££££££££

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