As previously stated in this thread, my Band 2's battery life became was really bad (about 14 hours) since I upgraded my 640 XL to the Redstone builds on Insider about 2 months ago. I wasn't sure of the correlation because that is about the same time that I had to get a warranty replacement because of a loose connection in the wrist strap where it would suddenly die with a full battery. At first, I thought the refurbished replacement unit had a bad battery until I received a second replacement (new in box from a retail store) and had the same problem with short battery life.
After spending the last week before the official Anniversary Update release with a brand new Lumia 950 on the 10586.x builds, I saw my Band 2's battery life return to about 30 hours. That is about what I got with watch mode (rotate on), using sleep mode at night, and tracking a 30 minute to hour run workout with GPS.
I installed the Anniversary Update on August 9, and currently running 14393.67. I haven't had any battery issue with the Band since the official update was released. When on the Insider program, I would have about 40% battery left after a 9 hour work day. Since the official Anniversary Update, I've had about 72% battery after work with the same usage.