Warning: connecting the battery directly to the mag post will demagnetize the magnets and possibly damage the mag coil. It is really important to orient the diode in the proper direction when connecting the charger to the magneto/battery. A diode is a valve that lets electric current flow in only one direction. Diodes are rated by the amount of current they can carry and by the amount of reverse voltage they can stop. Generally when a diode fails it allows current to flow in both directions. A diode makes the magneto battery charger possible for two reasons: it blocks current flow from the battery to the magneto (which would demagnetize the magnets), and, it sends the positive half of the magneto’s alternating current to the battery. The magneto battery charger has three parts: a diode, an incandescent bulb and a bulb socket. The bulb acts as a ballast resistor and a fuse. It limits that amount of current allowed to pass from the magneto to the battery. If all the current went to the battery there would be none left to energize the coils. When the engine is idling the bulb hardly lights up. As the rpms increase the “mag” puts out more power causing the filament to heat up and glow. The hot filament resists current flow so the current takes an easier route through the coils. If the diode fails the bulb acts as a buffer limiting the current that could discharge the magnets. If the bulb glows when the engine is not running the diode has failed. Remove the bulb until you can replace the diode. Too much power and the bulb will act like a fuse and burn out, again protecting the magneto. I used a 1N5402 diode which is rated at 3 amps and 200V peak repetitive reverse voltage. I purchased several of these diodes years ago after reading an article by John Reagan. I haven’t heard or read anywhere of a failure if this diode. A strong magneto puts out about 28VAC. The potential charging voltage is: mag output, minus the battery, minus the bulb. For a 6V battery an 1156, 12.8V bulb will do. Our camper pickup is 12V so I used an 1129 bulb rated at 6.4V . If you do the math, the excess voltage over the sum of the battery and bulb voltage is about the same with each battery bulb combination. To build the charger I soldered the “battery” lead from the diode to the side of the bulb socket, encased the diode in heat shrink tubing and sealed the ends with some liquid electrical tape. I then connected the magneto lead from the diode to the terminal block screw where the magneto wire attaches. The wire from the bulb socket was then attached to the main power feed from the battery at the terminal block. If the bulb lights up the connection is backward. Use the diagram above as a guide. If you prefer to leave the engineering to an expert you can get a charger from Lang’s or from the manufacturer, John Reagan, at Fun Projects.
July 31: Peter and I tested the mag using an analog voltmeter and an 1156 – 12v bulb. At high rpm it showed that mag was putting out 30v, which is probably why the 1129 bulb only lasts a few minutes at road speed (65-75 kmph). I will try an 1156 – 12v bulb which should survive but may not leave must voltage to charge the battery.
August 20: Recently I drove about 200 miles, most at high speed and much of that on battery because I forgot to switch to mag. The 1156 bulb still burns brightly at speed. After three days of camping and touring with lots of starts, pulling a trailer with LED tail, signal and brake lights, and electric brakes, the battery had nearly a full charge when we returned home. The electric brakes were employed a lot in the mountains. Perhaps the 1156 bulb will provide some charging at touring speed.