We just boarded United in Cancun. We were required to put all lithium batteries, both spares AND those installed in devices, in carry-on. So all my lights for photography in caves had to be moved to carry-on. At security my carry-ons rang every bell they had, and they swabbed and looked at everything. No issue with the numbers, and they did not open the lights....just looked at the front if each to see the leds and lens. This is NOT what we expected from studying the Mexican rules beforehand.