This is for all the people who have to fly and need their toys while in flight. US DOT has declared the rules for carry on batteries, notably lithium ones. Why? They could explode. Although unlikely, there are no chances and a no exception policy has arrived.
The new rules state that spare batteries cannot be packed inside checked luggage, but spare batteries can be carried on board in carry-on baggage. Those brave enough to check baggage with electronic devices inside can leave installed batteries in the devices.
The U.S. Department of Transportation does not specify how many batteries are acceptable for travel. The Department states passengers can carry spare batteries for electronic devices and that the lithium content in all batteries must weigh less than 25 grams.