With the device in the cradle, turned on and powered by the bike's battery, when you turn the ignition off you should get an instant message that say something like: 'External power lost, turning off in X seconds, stay on YES or NO'. The 'X' seconds count down from about 30 to zero.
If you touch NO, the device should power down at once.
If you do nothing, the device should power down when the seconds expire.
If you click YES, the device should stay on, running from its own internal battery.
Somewhat oddly, on my 1600, the message repeats again about four minutes later. This means that the external power has been lost a second time. If I then say YES, then the device stays on for hours on its own battery; I can take it out of the cradle and it stays on. If I click NO, or just leave it to count down, the device switches off, all power having been lost.
I guess that when I first touched YES, the device did not switch to its internal battery; instead it ran on the residual four minutes of power provided by the bike's Canbus system. When that too shut down after a few minutes, the device detected that external power had been lost again, reactivating the prompt.
This sort of makes sense. I guess the device will always default to external power (however brief the supply) if it can, thereby saving the battery.