Small outboard motor battery charging coil

[quixotix]

I have a small (5HP) outboard motor with a battery charging coil.  I'd like to get more electrical power out of it, especially at lower RPM, to charge a lead acid battery faster.  Please bear with me and my stupid questions as I'm a mere ME.

This is the coil and rectifier kit:

https://www.tohatsuoutboardparts.com/Electrical-Kits.html

The first thing to note is that the rectifier kit is just a single rectifier -- not a full bridge rectifier.  Secondly, there is no voltage regulator or any other electronics except for the rectifier.  There is a 10 amp in-line fuse.

I have easy access to both wires from the coil, so I was thinking of changing to a full bridge rectifier.  This should roughly double the power output (for a given voltage), right?

The obvious danger I see is the possibility of overheating the coil.  Are there other problems I'm missing?

Thanks for your thoughts.


If anyone wants the details, here's what I know:

Coil wire diameter - 1 mm

Number of turns - probably 200, that's 50 per layer with probably 4 layers (but possibly 3 or 5 layers)

There are 4, rather long, magnets glued inside the flywheel, I don't know the arrangement of the poles.

The maximum RPM of the flywheel i s around 5000.

[tautech]

They're only using 1/2 wave rectification because of the higher frequency sinewave compared to mains and batteries don't really care too much if the power is particularly clean or not.
Full wave rectification may increase the voltage available to the battery and also the current drawn so you'd be wise to add a regulator to control the charge voltage to the battery and thereby also limit the charge current.

How it will all pan out will be an interesting exercise.

I used a cheap series regulator after a series 1/2 wave diode for charging a rider mower battery and it works just fine.
https://www.aliexpress.com/item/Voltage-Regulator-For-BRIGGS-STRATTON-691188-491546-793360-794360/32823611150.html?spm=a2g0s.9042311.0.0.a08d4c4dWFfB0d

[Circlotron]

It may be possible to put a large capacitor across the charger output and then feed this to a switch mode buck regulator. The capacitor will charge to a voltage rather high than normal battery voltage and the buck regulator will reduce the voltage to what is required while increasing the available current by the same factor as its voltage reduction.

To gild the lily somewhat further, you could put across the charge output a reversed biased shunt diode, then series iron cored choke of several tens of millihenries, then the capacitor. This will spread the current over rather more of the charging coil output waveform and reduce the peak current, likely enabling you to draw more power from the coil. Buck regulator after this, same as before.

[soldar]

I have easy access to both wires from the coil, so I was thinking of changing to a full bridge rectifier.  This should roughly double the power output (for a given voltage), right?

Correct.

The obvious danger I see is the possibility of overheating the coil.  Are there other problems I'm missing?

I would guess it should be adequately cooled if it is in a motor. That would not be my major concern. I would like to measure what kind of current it is putting into the battery now and make sure increasing it is safe for the battery. The bridge will add a one diode voltage drop but I do not think that would be too much problem. Again, I would like to see some voltage and current measurements before doing anything.

[quixotix]

Thanks for the responses.

I do plan to measure the output, but I need to get the full bridge rectifier first.  And rig up something to load it at different levels (I don't have a lot of electrical test equipment, but I figure I can rig up a combination of incandescent lights to vary the load).

In use, I will be connecting it to an approximately 75 amp-hour, 12V, wet cell lead acid battery (a group 24 size battery in the USA).  Even if fully charged, I think this can dissipate a few tens of watts of power without damage.

Tohatsu (the outboard motor manufacture) calls it a 60W alternator.  It's not clear if that's with or without the 1/2 wave rectifier.  It also doesn't specify the engine RPM, or the output voltage -- which will have a big impact since there is no regulator.  My guess is that it won't be able to put out enough to harm the battery, especially since I almost never run the motor above 75% of max RPM.

[tautech]

Thanks for the responses.

I do plan to measure the output, but I need to get the full bridge rectifier first.  And rig up something to load it at different levels (I don't have a lot of electrical test equipment, but I figure I can rig up a combination of incandescent lights to vary the load).

In use, I will be connecting it to an approximately 75 amp-hour, 12V, wet cell lead acid battery (a group 24 size battery in the USA).  Even if fully charged, I think this can dissipate a few tens of watts of power without damage.

Tohatsu (the outboard motor manufacture) calls it a 60W alternator.  It's not clear if that's with or without the 1/2 wave rectifier.  It also doesn't specify the engine RPM, or the output voltage -- which will have a big impact since there is no regulator.  My guess is that it won't be able to put out enough to harm the battery, especially since I almost never run the motor above 75% of max RPM.

Thanks for the further info.
If the charge voltage is uncontrolled the risk is gassing the battery and boiling it dry.....but that won't happen in short order.
So a 55W headlight bulb would be a good quick test load, it should draw ~5A and while doing so check the voltage across it.
Ideally you want 13.8 - 14.2V to charge an automotive battery.