I have an 11, leave BT enabled (and in use) all the time, don't see big battery declines. Also didn't see it on the previous XS I had. But again, I keep the screen brightness at about 1/3 of possible, use WiFi for calls because my cell signal in my house is rubbish. When a cell phone is seeking a tower to work with, it uses a tremendous amount of power in that search. My provider, Verizon, supports VOIP over WiFi, so by using that, I stopped the constant searching for a tower with at least one bar (there aren't any).