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magneto to charge a battery

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bobdog

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Hello, I have a motorized bicycle that has a magneto on it to run the spark. the magneto has three wires coming out of the engine. it does not have a battery. two of the wires go to the ignition module/coil witch goes to the spark plug. the third wire does not go anywhere but is there to supply an optional 6v incandesant bulb. you do have to ground the bulb to the engine though.

i checked the output and it is 6 to seven volts on the third wire. i tried to hook up a 6v 55w halogen bulb to the wire but it killed to engine. i then bought two 6v 4.5ah batteries and hooked them to the bulb and grounded the whole thing to the bike frame. the bulb lights up bright and pulls 8.5 amps. i also hooked to 6v wire in line with that hoping it would run off of the battery and charge the battery i put two switches in line, one to the posative on the bulb and the other to the ground on the battery so that i could charge the battery when the light was not on. when i switch on either with the engine will not run because i am grounding out the coil.


i am an automotive mechanic and not an electrian but my friend told me i need an isolator to cut the load on the coil. but i dont know what kind or where to put it. any thoughts. is it possible to charge the battery during the day and run the bulb at night? thanks
 

tcmtech

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You may just need to put a diode inline with the third wire going to the batteries.
My old three wheeler was like that too. It had an AC output for the lights. By rectifying it I could get 12 volts DC to charge a battery.
Thats how the more expensive models did it that had the electric start option.
 

Hero999

Banned
The engine stalled because the lamp is too larger load for it to drive, you need a smaller lamp.

The engine stalled when you connected the battery to the alternator because it outputs AC and the battery acted like a short circuit.

You need a rectifier to charge the battery.

A bridge rectifier would do the job.

You still need a smaller lamp, even with the battery connected, because the engine won't be able to charge the battery fast enough. The battery will be drained by the lamp faster than it is being charged by the engine.
 

crutschow

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A bridge rectifier would do the job.
You can not ground the battery or bulbs if you use a bridge rectifier.

If you need to ground them, then you can only use a single rectifier at the magneto output. That, however, won't give you as much charge current.

You may also have to connect a small resistor in series with the rectifier to limit the charge current so it won't kill the engine.
 

tcmtech

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If your battery voltage is up it wont be a problem with the single diode setup.
Being your only pulling off the one half of the waveform the other half is still available to power the igniton.
Try it and find out! What the worst that can happen, it wont start?
 

bobdog

New Member
Okay thanks for the speedy replies. So I didn't expect the magneto to be able to charge as much as the halogen draws so that is why I put the two 4.5 amp batteries so I could get almost an hour use out of the bulb at night since most of my journeys are only 10 to 20 minutes. So I'm hoping it will get it's charging in during the day.

So if I put a single diode facing out of the third wire on the engine and into the battery it should charge and not ground it out? I'll give it a try but what voltage and amperage diode do I need? Would it be a six volt what ever amp
the engine puts out?

Also how do I hook the bulb to the batteries which are
Parrallel and hook the engine to the batteries. Do I need a diode on the bulb how do I keep the engine from trying to power the bulb?

Thanks again everyone
 

Nigel Goodwin

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Just to clarify this - you don't have a 'magneto' - you have a simple permanent magnet alternator, which is likely to be woefully underpowered.

There are usually two wires out of them, a low power winding that feeds a half wave rectifier used to charge a small battery, for indicators and brake light. The far higher power winding feeds AC directly to the headlamp bulb, which dims as you slow down.
 
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crutschow

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Just to clarify this - you don't have a 'magneto' - you have a simple permanent magnet alternator, which is likely to be woefully underpowered.
He stated two wires go to the ignition module/coil. Seems like that makes it a magneto, since there's no battery.

I had an old Triumph motor cycle that had two positions on the ignition switch. The clockwise position was the normal position with a charged battery. The counter-clockwise position allowed the ignition to operate as a magneto (but with poor engine performance) with a dead battery.
 

Nigel Goodwin

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He stated two wires go to the ignition module/coil. Seems like that makes it a magneto, since there's no battery.
Separate coil, separate ignition module - not what I would consider a magneto?.

But in any case, we're talking about the low voltage wires coming from the alternator.

I had an old Triumph motor cycle that had two positions on the ignition switch. The clockwise position was the normal position with a charged battery. The counter-clockwise position allowed the ignition to operate as a magneto (but with poor engine performance) with a dead battery.
Not a magneto, as they generate the spark directly, all the Triumph is doing is removing all load, and feeding the ignition coil directly from the alternator/rectifier.

BTW, what Triumph did you have? - strangely enough on the way back from work today I saw a guy sat on an old Triumph Speedtwin.
 

tcmtech

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Often the term magneto is just a relative term used to discribe any non battery powered ignition system that uses a magnet to generate the power for the ignition.
Some generate the ignitoin spark directly at a magneto ignition coil while others split the primary side into two different and seperate parts. A generator or source coil produces a constant alternating current that is phased to the engine rotation and a second step up coil is used to make the actual spark. The actual timing points or firing circuit is between the two.
Its still using the magneto principle but is just relocating the high voltage part to a different location.
 

crutschow

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Separate coil, separate ignition module - not what I would consider a magneto?.

But in any case, we're talking about the low voltage wires coming from the alternator.

Not a magneto, as they generate the spark directly, all the Triumph is doing is removing all load, and feeding the ignition coil directly from the alternator/rectifier.

BTW, what Triumph did you have? - strangely enough on the way back from work today I saw a guy sat on an old Triumph Speedtwin.
I'm using a more generic definition of a magneto, similar to tcmtech. To me, if it doesn't use a battery then it's operating in a magneto mode.

I've had two Triumphs: a 200cc Cub back from '60 to '62, and a '64 650 TR6 from about '66 to '72. Obviously I go back a ways. Right now I have an '83 650 Honda Nighthawk with 110,000 miles on it (every one mine). Still drive it 17 miles one-way to work every day (weather permitting).
 

Nigel Goodwin

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I'm using a more generic definition of a magneto, similar to tcmtech. To me, if it doesn't use a battery then it's operating in a magneto mode.
I have lots of experience from back when bikes had magnetos, so I tend to be more correct about it.

I've had two Triumphs: a 200cc Cub back from '60 to '62, and a '64 650 TR6 from about '66 to '72. Obviously I go back a ways. Right now I have an '83 650 Honda Nighthawk with 110,000 miles on it (every one mine). Still drive it 17 miles one-way to work every day (weather permitting).
I used to have Tiger Cub at one time as well.

I don't have a bike these days :(
 

Hero999

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I wouldn't recommend using a halfwave rectifier, use a fullwave and keep both sides of the output isolated from the chassis.
 

crutschow

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I wouldn't recommend using a halfwave rectifier, use a fullwave and keep both sides of the output isolated from the chassis.
I agree. It provides more current and avoids any possible dc saturation problems with the generator.
 

Nigel Goodwin

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I wouldn't recommend using a halfwave rectifier, use a fullwave and keep both sides of the output isolated from the chassis.
Such alternators are usually grounded inside the alternator, and you don't have easy access to the separate ground wires. As this usually includes the winding that energises the ignition system, you can't do it without splitting the wires.

They are also, obviously, designed to use a halfwave rectifier.
 

crutschow

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Originally Posted by Hero999
I wouldn't recommend using a halfwave rectifier, use a fullwave and keep both sides of the output isolated from the chassis.

Such alternators are usually grounded inside the alternator, and you don't have easy access to the separate ground wires. As this usually includes the winding that energises the ignition system, you can't do it without splitting the wires.
Don't quite understand your comment Nigel. You can still use a bridge rectifier, even if one side of the alternator output is grounded. But you just can't ground either output of the bridge rectifier, thus the battery and bulb need to be isolated.
 

Nigel Goodwin

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Don't quite understand your comment Nigel. You can still use a bridge rectifier, even if one side of the alternator output is grounded. But you just can't ground either output of the bridge rectifier, thus the battery and bulb need to be isolated.
That would be a VERY clumsy and potentially troublesome 'bodge' to do :D
 

tcmtech

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Do a little research into the older motorcyles and ATV's that had 6 volt battery systems on them. I bet you find most of them used a single diode for the charging circuit! Also pulling off the tiny magneto alternator every half cycle wil not likely bother the actual ignition system performance.

With a little looking I would not be the least bit surprized to find that the OP's "put put" has a big zener diode some place to regulate the peak voltage going to the head light and that zener actualy clips the little generators output voltage to limit the peak voltage going to the lights.

Many manufacturers did this for decades on basic motor bike stuff!

I think you are all over complicating this!
:rolleyes:
I am surprized that this thread is into page two of how to charge a battery and no one has brought up a "super ultra multistage microprossor monitored battery charger" system.

Are you getting lazy? :confused: :D
:rolleyes:
 

crutschow

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That would be a VERY clumsy and potentially troublesome 'bodge' to do :D
At the risk of appearing obtuse, I don't see the isolation as a big problem. Certainly it's easy to isolate the battery. And the halogen headlights I've seen already have their connections isolated from the chassis. What am I missing?
 

Nigel Goodwin

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At the risk of appearing obtuse, I don't see the isolation as a big problem. Certainly it's easy to isolate the battery. And the halogen headlights I've seen already have their connections isolated from the chassis. What am I missing?
You mean apart from all the wiring usually uses the chassis as the ground return?, and you would have to completely rewire and make extra sure that there are no chassis connections.
 
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