I flew to Europe back in January and took a couple 18650 lights, and a AA light and hd no issues. I brought spare batteries as well, but ensured they were in a Storacell type battery case. Further, i only brought batteries that were reclaimed from laptop batteries (ie little cost to me) in case i was questioned. No problem on the outgoing flight, but on the flight coming back, they went for my bag. Turns out they were wanting to look at the kindle i had left in the bag, but when i mentioned the flashlight, they wanted me to turn it on. If i hadn't said anything, they wouldn't have bothered with the light.