when using Garmin's genuine car adapter
(5VDC 1A) the device can be used
to navigate whilst charging.
using a different one or a DIY (same specifications as above) -
the device turns into PC connection mode and remains there
(data wires green & white not connected).
I suppose Garmin's car power supply is either
using the data cable somehow too or the output is
different (cannot measure because the case is glued).