LeeP, Flyingstone is right,
When you press and hold the ECU Test button Both ECU A and B captions will illuminate. After a couple of seconds ECU swaps to channel B, raises the Engine speed from 900 to 1100 Rpm and cycles the prop once, Engine speed will then drop back down to idle, ECU will swap to channel A and repeat the process.
During the test, a battery tester, installed in the instrument panel, measures several parameters of the ECU Backup Battery. This will be indicated with a red flashing LED installed on the left hand side of the instrument panel.
If the capacity of the ECU Backup Battery is found to be less than 70% of its rated capacity then the 'ECU BACKUP UNSAFE'- light will be illuminated. It’s sometimes easy to confuse the flashing with a problem.
ECU backup battery will trickle charge from the ECU bus. the AFM is a little vague regarding departing with the light on, it only mentions IFR Flight, but the VFR Aircraft do not have these battery's fitted.
From previous experience ECU battery tester sometimes gets a little paranoid especially if it’s cold, ECU test button is released before the test has finished or the ECU test has been performed several times before takeoff.
Soay, ECU Internal battery is separate to the ECU backup battery. ECU backup battery is located under the rear seat Starboard side. ECU internal battery’s are soldered onto each channel in the ECU.
AFM is freely available on Diamond's Website, Checklist can be found here
Diamond Aircraft :: Checklisten