Trouble with a Magneto battery charger

[Bruce Compton]

I followed the age-old schematic posted here several times and built a magneto battery charger for my '14 roadster using a 10A10, 1000 volt, 10 amp diode and a 1156 bulb in series. The initial system worked well for a day or so and then overnight I had a totally dead battery (small 12volt AGM). Apparently the diode shorted and allowed the battery voltage to drain through the mag connection. Thinking that somehow the diode got overloaded, I built another system and used two diodes in parallel (to double the current carrying capability). This set-up worked fine as well but drained the battery again (the diodes were actually hot). When functioning properly, the bulb glows dim at low RPM and brightens up as the engine RPM increases. With the engine off the bulb is off also (no glow). Yes, I'm installing the diodes with the bar towards the + side of the battery. Can't understand why the diodes work so well and then short out. Am I using an improper diode and if so where can I get one that will survive?

[cslandry]

That diode should work fine. The original kit came with a 6 amp diode. I burned that up pretty quickly in my car, but I think it was partly because I mounted it with heat shrink tubing around it, which acted as an insulator and caused it to overheat. I replaced it with a 10 amp diode and make a little heat sink to mount the diode. I don't know if that's what made the difference, but it's run a few thousand miles like that with no problem. 12V battery.

[Steve Jelf]

Check your wiring arrangement. Be sure all your low voltage wires are more than ¼" from any plug wire. If they are too close, high voltage can jump the gap and cause all sorts of mischief. Since John Regan warned me about this I go for a separation of at least ½".

[Bruce Compton]

Thanks Steve, but my charger lines are nowhere near the high voltage wires. I think I bought a bad batch of 10 amp diodes so I've ordered some 25 amp ones that should survive.