In case you haven't been watching the news ( we don't while in Europe), there are enhanced security measures being put in place for flights to the US. Travelers may (will?) be required to power up their electronic devices at the screening. If they are out of juice and can't be turned on, you cannot take them on board, and you may be subjected to additional screening.
http://www.cnn.com/2014/07/06/us/tsa-security-measures/index.html