My first post was off a bit (I sold mine a couple months ago). The spare and the power battery are both in the large turret that turns the sight on and adjusts the brightness and color. The spare is in the very top. If you unscrew the turret and see a compartment with a battery and battery contacts then the spare is inside the piece you removed. If no battery contacts then you are looking at the spare and you need to unscrew the next section down to get to the power battery.
The lid for the spare compartment is only about as thick as a dime.