MTFCA Forum

I found this device in my 1909 Roadster and I do not know how it works. Mag cable comes from magnetoposg.
Could someone please explain?
Regards
Bernd

Could be a battery charger.

TXGOAT2 wrote: ↑

Thu Nov 13, 2025 6:32 pm

Could be a battery charger.

Diode indicates a rectifier.
Unit to change AC to charge a DC battery.

It appears to me that power from the magneto goes to the diode, thru the bulbs and is available as DC at the terminal marked 'Batt' partially hidden under the bulb. Magneto powered battery charger.

Half wave rectifier with(?) 2 12 volt bulbs in parallel to serve as a thermo-resitive regulator. The4 device allows limited battery charging from the Ford magneto. The magneto can deliver up to about 30 volts alternating current, depending on the engine speed, which is not suitable for charging a battery. The solid state rectifier allows only DC current flow, and the bulbs serve to keep voltage delivered to the battery within a tolerable range. The rectifier also prevents the battery from discharging through the magneto winding when the engine is running very slow or stopped.

I would think you would have to know how to use this. Would wrong connection to the magneto cause the magnets to be discharged accidentally?

I think the diode will always prevent battery current from reaching the magneto, unless it short circuits. The bulbs would prevent full battery current from reaching the magneto, which might prevent demagnetization. (?) I suppose a cutout or second diode could be inserted in the circuit for extra protection. A full wave rectifier would provide more charging current at low engine speeds, but it might load the magneto to the point of interfering with ignition operation at low engine speeds.

I use the more common 1-bulb 2-filament #1157 bulb magneto chargers on my two Ts.

The bulb serves two purposes: a current limiter (maxes out at 1.8 amps), and also as a safety/fuse in event the diode fails ‘closed’.

Per Ron Patterson (IIRC), a failed-closed diode will allow battery current to back flow into the mag and demagnetize it. However, with this setup, the bulb will blow almost instantly if the diode fails and thus protect the mag.

The 1.8 amps output sounds skimpy but is often enough… a hand-crank car that only uses the battery for starting spark would be totally fine. An electric start car that starts quickly would be fine also. Use of this charger on a battery-ignition car might drain the battery slowly.

I can break (open) the charge circuit on demand on my speedster. Road tests indicate a little reduction in spark when the engine is operating at high speed and the charger is connected, around 45-50+ mph in O/D.

Extended use of lights will also drain the battery, although short periods of use shouldn’t be a problem. My cars both have LED head- and brake lights, and incandescent tails. Both are driven after dark, occasionally at length. Never run out of battery yet.

Hook up the mag terminal to the mag post and battery terminal to — you guessed it — the battery.

Lots of posts of the form discussing these units.

To sum up, they are safe, inexpensive, and they work well within their limits.

Hallo Bernhard ;
I made nearly the same parts to charge the Battery of our 1912 .
And that works fine !! Greeting from Holland
Toon

Toon,
That's a pretty clever mounting. No wires, and an under-the-hood utility light!

Thanks for the information.
I did not know that battery charging could be done that easy - including protection of mag and battery. Great idea.
So I will use that again. Maybe in such a nice manner, as Toon did it.
But I installed 12 Volt indicators, head lights, brake light and a hazard warning light system with a 12 Volt battery.
Am I right, that need to exchange the current 6 Volt bulbs to 12 Volt bulbs and which watt performance should I take then?
Regards
Bernhard

I think it will charge a 12 volt battery or a 6 volt. The bulb voltage is not as important as how much current it will pass, with 12 volt bulbs having about twice the internal resistance as 6 volt bulbs of the same wattage or candlepower. You might need more engine RPM to charge a 12 volt battery, but a good magneto will exceed 14 volts at fairly low engine speed. If you attach a battery with a low charge level to the device and run the engine at about 1,000 RPM, the bulb will burn brightly. As the battery approaches full charge, the bulb will glow more dimly at the same engine speed. High wattage bulbs will pass more current than low wattage ones.

John Regan used to sell a neat little unit. It does a good job of keeping a battery charged as long as you don't run your lights all the time. It keeps the Odyssey battery in my '17 charged all season. If the diode shorts the bulb blows and protects the magneto. I had a diode short and it worked as advertised.

I've been considering making a few of these for our own use since they're apparently not available to be purchased anymore? Before I reinvent the wheel, has anyone ever made a PCB file available publicly to get these made? The John Regan version is such a neat solution.

Doesn’t Jeff make them along with the voltage regulator?