When the battery runs low, the Portable will display a warning message.  You can click the scroll wheel to dismiss the message and the Portable will continue running.  If you ignore the message, then the Portable will power off.

The Portable provides up to three warning messages.  Once the third message is displayed, you can click OK, but the Portable will only be able to run for another minute or two.  You must charge the battery.  If you ignore the third message, then later when you charge the Portable, the Portable will go through the restart process, which can take up to 30 seconds.

IMPORTANT: If you let the battery run out completely and then plug the Portable in to charge, the screen will be blank for up to two minutes.  However, after those two minutes, the screen will turn on and the Portable will go through the restart process.