I finally figured out the cause of my huge battery drain overnight. When looking at my iPhone 6 Plus the LTE & 4G keep switching back and forth every 15-30 seconds. If I turn LTE off 4G stays on and everything is fine, as soon as I turn on LTE the switching starts again. Also I have full bars on both so it's not a low signal issue. I called AT&T about this and she told me she worked with another customer that has this same issue. They advised him to go to the Apple Store. Anyone know what might be causing this?