So if the device has to be charged, then that means you dare not use your Kindle, phone, or whatever while delayed awaiting departure in case the battery runs down too far.
Confusion abounds. Even the BBC manages to contradict itself within the space of a single news item:
A new rule states that if a "device doesn't switch on, you won't be allowed to bring it on to the aircraft"
but
the BBC understands that passengers flying to the US will be advised to remove relevant chargers from their hold luggage at check-in, so that they can top up carried-on gadgets if necessary. Alternatively, they can opt to place any powered-down kit in their stowed luggage.
BBC News - Heathrow and Manchester issue must-be-charged gadget advice