There are TWO separate RFID transmitters in a MB.
One is passive (the one that requires that your TOUCH the Mickey head on the MB to the Mickey head on the reader)
for park entry, FP+ use and "pay via MB."
The other RFID unit (battery-powered) TRANSMITS its code for quite a distance (comparatively.) That high-powered RFID is what enables the MB to trigger the on-ride photos to your
MDE account, as well as other little "extras" (in-use now and/or "planned") for guest interaction. It also is the factor that allows Disney to "track" where you and your MB are (and have been) at any given time at the parks (and maybe a few other locations, too.)
The internal batteries in the MB are speced to last between 2 and 3 years.
We don't know how long that the batteries will actually last under real-world conditions yet.