Short answer: Nothing! KEEP can last up to 6 hours on battery power :)
Longer answer: We suggest that you keep your KEEP connected to a power source whenever possible, to avoid the possibility of it turning off due to battery loss!
But if you need to unplug it for a while or if there's ever a power outage, have no fear! The battery should keep it up and running until you can plug it in again.
Our suggestion would be to put it on sleep mode by pressing the back button to help conserve its energy, though :)