Flying with lithium-ion ( Li-ion) batteries requires precautions — the main one being that loose batteries cannot be packed in checked luggage. Here are safety considerations governed by federal regulations, airline rules and common sense.
Carry-on or checked?
FAA regulations allow “personal” devices with Li-ions installed to be packed in carry-on or checked bags — if the device is “protected against accidental activation.” If the light doesn’t have a secure switch lock, you might be able to tape the switch in place, or cover the batteries’ terminals with tape before installing them.
Protected from short circuit
For loose Li-ion batteries taken in your carry-on, you must take precautions to make sure their terminals can’t be shorted by contact with other contents such as keys, coins or other batteries. Pack them in their original packaging, individual plastic bags, or cover terminals with
heavy tape (duct tape works great), and make sure they can’t shift around.
How many, how big?
The FAA doesn't limit the number of “consumersize” Li-ion batteries in your carry-on. Consumer-size is defined as a rating of less than 100 watt hours ( Wh). The biggest battery in our test was rated at 57 Wh; the common 18650 Li-ion battery is 12.5 Wh. ( The FAA allows Li-ions up to 160 Wh in carry-ons — if your airline approves.)
What is my battery’s rating?
Newer Li-ions should be marked with a Wh rating. For older ones that list voltage ( V) and ampere hours (Ah), multiply volts by amp hours for watt hours.
The final word
The rules are written with common consumer electronics (not dive gear) in mind; the final decision rests with the airport security officer.