Just let battery run down with the USB XM weather antenna connected.

I did this to see how much battery life I would get with the screen set
at half brightness (5+ hours), but when I reconnected external power,
the unit begin an endless series of re-boots, never getting past the "I
agree" lawyer acknowledgement screen. If I disconnect the XM antenna,
the unit will boot up and run normally, but if I plug the XM back in,
the unit shuts down.

By fiddling around with the unit for a while, I discovered that by
starting the unit in simulator mode, then plugging in the XM antenna,
then putting the unit back in normal mode I can get the whole mess up
and running as it is supposed to.

I installed the latest firmware rev (2.40) but that didn't help, so I've
e-mailed Garmin tech support for help. Stay tuned for their response.

--
Dan
C172RG at BFM