I fly internationally very often and keep them in my laptop bag.
While flying you may be required to carry any LiIon batteries with you. You may be asked to open your carry-on so that the lights can be inspected. You will be required to take the light out of your pocket along with your keys, change, belt, shoes, jacket, etc.
There is a review of how LiIon batteries on airplanes pending. You may be barred from carrying any spares (flashlight, laptop, etc.) on the flight.