Charging the Tom Tom - unit shuts off with car...

Tom Tom 720 new user - connected to home dock for 2 hours (not sure if the unit should be on or off???)

Took Tom Tom out for a test naviagate = worked for about 90 minutes - very cool, then battery icon came on the screen, unit shut down. So I plugged in the car charger to the cig lighter and that worked, but it kept shutting down every 3 minutes and asking me if I wanted to connect to the computer when I turned it back on. I'm taking a 5-hour trip tomorrow and I do not know if my Tom Tom is charging, how much battery life is left, and if I have to keep tapping the blessed screen every 3 minutes. HELP!