The most common reason for an erratic charging voltage is that the stator lead wire insulation is heat damaged allowing them to short together, especially after the engine heat further reduces the electrical insulating capability of the wires. Ducati has supplied seriously under-sized wires - given that they have to carry 30-plus amps continuously. Replacing the wires between the engine casing and the regulator with a heavier gauge cures the problem - if done soon enough. If not, the voltage surges from the wires being shorted together end up damaging the solid state components in the regulator/rectifier and you’ll soon end up having to replace it as well.
So first the wires from the stator to the regulator need to be replaced. At first glance it may appear that the high current being carried by the wire had just overheated a few inches of insulation next to the high-resistance bullet connectors. But when you remove the common sheath that the wires run in back to the left engine cover, you likely find that overheating (from the current, not the engine) has softened the insulation to a degree where you can damage it with your fingernail. A multimeter will say it's continuity is OK but at the higher AC output levels at higher rpms the insulation can’t prevent the wires from shorting together. Remember, the wires are held in physical contact along the entire sheath length.
You don’t really need to remove the engine cover. Inside the engine the wires don't touch each other. The approach that I took was to replace the damaged wiring (with a larger gauge to reduce electrical heating) up to where it exits the side cover. I pushed a piece of shrink wrap tubing over each of the wires and into where they pass through the seal in the cover and left the internal portion of wiring alone since they are separated inside the engine. Again, the main problem is where they touch each other along their length where they run in a common sheath.
I also took the oportunity to relocate the regulator itself to the licence plate holder to get it away from the engine and exhaust heat.
You can read the full explanation here:
http://ducatisportingclub.com/contro...d.php?tid=2042