Voltage dropping in flight

I have been having a similar problem. So far I changed the regulator and found one of the connectors was not tight into the regulator.  That got it to run at 13.5v or so which is not enough for an earth X.

Thanks. Regulator is also called the Ducati, correct?

A nominally charged and rested battery will settle out around 12.6 volts.
Immediately after starting, you should see at least 13 volts.
After a reasonable period of time, it should raise to the lower 14-volt range.
If you ever see less than 12 volts with the engine running, the battery is Dis-Charging and the Alternator has No Output.

Check the wiring first.
Pull the connector off the regulator and inspect the connections closely.

Re-install the regulator connector.
With the MASTER Switch ON, you should see 12 volts DC on the R, B+, And C contacts.
With the engine running, you should  see 20 to 60 volts AC on the G contacts.  ( G= Gelb = Yellow)
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The Rotax regulator is reportedly manufactured and supplied by Ducati.

Wilco, thank you!

If the regulator is faulty consider B & C Specialty Products. They make a plug and play replacement regulator with over voltage protection for lithium batteries.

https://bandc.com/product/avc1-advanced-voltage-controller-14v-homebuilt/#installation-kit