I think the reason that the Garmin Edge 800 is not charging is on purpose.
You should plug in the external power, when the Garmin Edge 800 is fully charged. Then the device pulls the power from the external power supply till this is discharged.
Then it uses its own internal power.
I might be wrong, but thats what I've read. If somebody has the extra knowledge, how to proceed with an external power supply at the Edge 800 I'm very interested in knowing! Thank you!