Rotax B box question

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Well-Known Member
Jul 15, 2009
Fisher County, Tx. USA
I agree it is an interesting theory. Find you an orphan Rotax CDI unit, and let's give it a try! Seriously!
Test for AC, or DC, current, before you do any surgery. This can be accomplished with a hand-held DVOM with an amp clamp.


Well-Known Member
Jun 25, 2019
Might need to bump up the voltage up to 36 volts, add a converter to convert the DC to AC, a bypass switch between the Rotax voltage source and battery and then even the most anemic pull would fire up the engine...
As stated The CDI power does not come from the lighting coil. The Diagrams for the dual CDI ignition is confusing because it calls the CDI charging coil "LC"(Some sort of European nomenclature?)
LC= Ignition charging coil LA = Accessory power coil

From the rather simplistic ignition diagram in the manual you can see the Diodes in the CDI box that change the AC power from the coils to DC. Seems you could put correct polarity DC power in without needing to convert to AC

My concern with the battery power idea is current flow. Simply hooking up a battery to the CDI wires (green and white) seems like it would be a dead short to ground.

The Charging coils have a 230-280 ohm resistance that will limit the current they put out.
Using a battery - I would think you need to add some sort of resistor, properly sized for wattage depending on what voltage battery is being used.

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Well-Known Member
Apr 28, 2010
Memphis, TN
Shower of sparks ignition, creates the up and down AC voltage that is required for the regular coil to spark. It just feeds the AC to the coil but is not timed like the regular ignition circuit. Essentially a low voltage AC generator and that AC is feed to the low voltage side of the coil. The normal ignition is inhibited.