If you are traveling to the U.S. from international airports, then you will need to make sure that your smartphone or tablet is charged, and your device can be turned on, this is due to new rules introduced by the TSA.
The TSA will block people from traveling to the U.S. if their smartphone or tablet is not charged, you device now must be charged, and be able to be turned on and checked.
The new rules are apparently aimed at stopping an al Qaeda terror plot to blow up an airplane. And this will apply to all electronic devices.
The TSA are concerned that a bomb may be disguised as an electronic device, so devices which cannot be turned on will not be allowed on the airplane.
The check will take place at international airports of your are flying to America, and we presume the major European airports like Heathrow and more will be complying with these new rules.
So if you are traveling to the U.S., make sure that all of your gadgets, tablets, smartphones and laptops are charged, otherwise you may end up having to leave them at the airport.
Source The Verge