Originally Posted by BackPacker
The DA-40 I fly presumably has the same FADECs with the same problems. What's the time required for a relay (or two relays in series) to switch? Because on the DA-40 you've got the ECU backup battery, but it requires at least one relay to switch (maybe even two, I do not have the POH to hand at this moment) to activate after a main bus failure.
I've tried switching off the electric master on a DA-40 TDI, while the engine was running (on the ground!), and it continued without a beat. The ECU alternate power relay certainly switched quickly enough on that aircraft.