if you own an older tk15 it will not go into turbo mode with a 18650 battery . i discovered this after about a year of ownership . i contacted fenix store and they said it was the battery . they gave me a rma when i explained i was using brand new 18650's and they worked in all my other lights . after examining the light they new it was not the battery . they shipped me out a newer model of the tk15 as well as a 18650 battery . i want to know if anyone else has experienced this problem with a older tk15 and if so how was your experience dealing with customer service . judy and zane responded quickly to my issues and more then went out of there way to make me happy . they also sent me a 15% off coupon to apply towards a future purchase .