Electrical Output from Outboard Motor

[patbratton]

I have not yet connected the alternator output from my Mercury outboard to my battery. We have dock power and I keep a NOCO Genius 10 connected to the battery all the time. I could wire the outboard thru a normally closed contact on a 120 vac relay or a large diode to isolate the motor when the shore power is connected but I don't want to take the time to do so if it is not necessary. We have a refrigerator on the boat that I would like to be able to keep running. I also want to make sure the navigation lights can stay on if we run out of wind on the other side of the lake at sunset. This has been a class B priority up until now. We have a Hunter 240 with a 5 hp Mercury Propane Sail Power outboard. Access to the battery is poor. I have not found this question anywhere on this site.

 

[isaksp00]

I'd think the output from the outboard would be 12 v dc, and not 120 ac. My 8hp Honda puts out about enough power to top off the battery for say nav lights but not much more. I don't think you want to attach the motor output wire anywhere near the ac circuits.

 

[JBP-PA]

Those little outboard outputs do not really produce much power. My 7.5 HP can theoretical produce 50W at full throttle. It produces practically nothing at low throttle. It's just a magneto and a rectifier, not really an alternator.

 

[Crazy Dave Condon]

The older motors by Tohatsu for example the 5 hp kept the lights on only without charging up the battery. I would suggest an battery indicator to read the charge of the battery. Remember AC and DC are separate systems

 

[DaveJ]

I think the OP understands the difference between AC and DC, with the use of an isolating relay. When AC is on, the DC contacts would be ‘open’, isolating the two voltages. Is it a good idea? Not sure, but it would work.
Cheers
dj

 

[Orbweaver]

IMHO, the isolating relay is not necessary. Your shore power will be rectified and regulated with that charge, and the battery will absorb any large transients before any damaging voltage can get to the OB. If it makes you feel better it certainly won’t hurt. You do want to verify that the outboard output voltage is regulated if possible. A lot of small OBs skipped the regulation and simply rectify AC to DC. Run at higher rpm and you might see 16+VDC output, which is too much for the battery to handle without damage.

 

[patbratton]

IMHO, the isolating relay is not necessary. Your shore power will be rectified and regulated with that charge, and the battery will absorb any large transients before any damaging voltage can get to the OB. If it makes you feel better it certainly won’t hurt. You do want to verify that the outboard output voltage is regulated if possible. A lot of small OBs skipped the regulation and simply rectify AC to DC. Run at higher rpm and you might see 16+VDC output, which is too much for the battery to handle without damage.

I have found a parts website for the 5 hp Propane Mercury outboard. It shows an alternator which looks like a dual coil pack that goes in the flywheel assembly. The fly wheel is different for motors with the 12 volt output. It must have a magnet inside. There is a rectifier assembly with a fuse and a large heat sink. I had a Piper Warrior once with an over voltage relay that would send anything over 14.5 volts directly to ground. This may be similar. The output is listed in the marketing literature as 12vdc at 4 amps. It is sold as having the ability to run navigation lights. I will forgo any immediate decision as for as connecting the output to my battery. Thank you for your feed back.
Good luck with your Hunter 23.5. I tried to buy one that was at Lake Allatoona from a man who was moving Jekyll Island and wanted something bigger and heavier. It was a very nice boat but he sold it the day I planned to drive over and look at it. I wound up having to drive from Atlanta to Jupiter, Florida to buy my H240.
Pat

 

[Benny17441]

It is not worth the effort, the power output of the small outboards is negligible. It is intended to run navigation lights in a skiff and not much else.

 

[Crazy Dave Condon]

@Benny17441

You summed it correctly as it was explained to me by Tohatsu.

 

[AaronD]

So, dumb curiosity question (possibly demonstrating a terrible ignorance of magnetos and alternators):

At what outboard size do manufacturers include an alternator that will make usable power at something less than full throttle? I see Tohatsu / Mercury lists a 12A on their 15 and 20 HP outboards, but it wasn't clear from a cursory reading of the specs if that's just a larger version of the 6A magneto on their smaller models or a 'real' alternator that might charge a battery at sailboat cruising power.

Not that I'd put a 15 on my C-22 Just curious.

 

[Crazy Dave Condon]

It is called experience as a dealer for Tohatsu that the small charging system runs lights only as explained by Tohatsu. I know the charging system for the 8/9.9 ho engines will charge a battery

 

[walt]

A small outboard charger like that may be specified for 4 amps at full RPM so lets say you get 2 amps at half RPM. If you run the outboard for 1/2 hour at 2 amps, that is 1 amp hour.

2 amps will continuously run a lot of the tiller mount autopilots
1 amp hour will run .3 amps of LED lights for 3 hours
1 amp hour will run many smaller chart plotters for two to three hours.

Getting by on low power requires more understanding and monitoring but some people do this. You would need to decide if that level of charging is important or not.

I think the initial question was on the lines of can I connect the outboard charging wires to the battery while an AC 12 volt charger is also hooked up and charging the battery? You can connect the outboard charging wires to the battery and leave them connected all the time. You dont need a relay or switch. I have my outboard charging system plus a solar charge controller plus a 110V to battery charger all connected to the battery all the time and now for 15 years. Often two of the three will be charging the battery at the same time. Never a problem. All the charging devices are designed to work this way.

What do you need to be careful of..

You have to run the outboard for a LONG time to have this issue but there is some chance of the outboard charging taking the battery voltage past say 15 volts. Just occasionally monitor battery voltage. This may only be something to care about if you were to run that outboard for many hours.

Second, never disconnect the outboard charging from the battery while the outboard is running. Doing this will have a good chance of damaging the outboard charging system rectifier. Example, let say you have run the outboard for 6 hours straight and the battery voltage hit 14.8 volts. You need to still motor more and dont want any more battery charging. Just shut the outboard OFF before disconnecting the outboard charger from the battery. FYI, you possibly could connect the outboard back up to the battery while its running but its easier to just remember to never connect or disconnect the outboard to the battery while the outboard is running.

FYI, my 9.8 Nissan four stroke has some sort of "rectifier/regulator" function that will limit over voltage charging. I dont know how it works. but I have observed it working. The smaller outboard will often just have a rectifier so no over voltage limiting.

 

[Crazy Dave Condon]

Charging system for Tohatsu, Nissan and Mercury are the same manufacturer but your 9.8 produces more charging than a 5.