To start with, I have a '93 Yamaha 9.9 four-stroke, pull-start, tiller control, manual tilt, extra-long shaft outboard on a sailboat. The model number on the nameplate is not valid, after I compared it to the model/feature comparison for those motors...and the guy who sold it to me rebuilds motors from pieces and parts, so it's really a '49, '50, '51, '52, if you know what I mean. I wanted to see if I could set up the charging circuit to provide an extra battery charging option if needed. It's only supposed to supply 6 Amps, and of course that would be at a healthy RPM, so I'm not going into this expecting much. But I checked the magneto and the wiring harnesses in the motor, which has the CDI box, and it turned out that it had the wiring for a charging circuit in-place and wired to the magneto. So I bought a used rectifier off the "e" website online...came from a marina or boat repair place down on the Gulf Coast somewhere... I figured they pulled it off a motor that HAD been working, and they were just parting it out. I installed it according to manuals and parts lists I found online (boats-dot-net is a super-great resource for parts diagrams and such). Put the ears on the motor and ran it in the garage on the stand. With my voltmeter, I was getting voltage out of the red wire when checked to ground on the motor chassis. Voltage increased as I throttled up, so I gave it a thumbs-up and called it a success. Due to life getting in the way, I didn't get to check the motor out for almost two years - this past weekend. (I would run it periodically to make sure things worked, oil circulated, etc.) I had finally put the motor on the boat and was going through making sure things worked. I ran a wire from the red output wire on the rectifier to a common positive connection so it would feed the positive battery circuit, and ran a wire from the common battery ground connection to the motor chassis. Then I cranked up the engine to check the voltage output and see if there was any current going to my already-charged-by-a-solar-panel house battery (~12.7+V). To make the long story short, it seems that the power back-fed somehow and before I could even get my voltmeter on the terminals, it burnt the wires to the rectifier up. Toasted. Roasted. Fried, crispy-like. I quickly shut everything off, but it was connected that way for several minutes, and it melted wire connectors and the little wire holder, and even part of the cover. I also noticed during the event that the battery voltage dropped to 10.5V (which is why I say it was backfeeding from the battery through those wires to ground). Once disconnected, the battery voltage went right back up to 12.7V. So I'm wondering if a bad diode in the rectifier would have allowed the power to flow directly to ground? Was I wrong to connect the battery ground to the motor chassis? What else could have caused that much power to drain through those wires? Is there something else about a rectifier that I should know that I don't? (Not rocket science, I didn't think...) The magneto can't put that much power out... Any ideas?