To change or install the batteries in a Honeywell/Ademco 5890 or 5890PI motion detector, follow the procedure below:
1) Looking at the sensor, the lens should be on top and the LED light on the bottom, this is how the sensor will be installed as well.
2) On the bottom of the sensor in the very middle is a small notch. Take a small flathead screwdriver insert it in and twist it, you will notice the cover pop open. You can then remove the cover and have direct access to the battery compartment.
3) These sensors require 2 CR123A batteries. When you remove the cover you will see the battery compartments on the left hand side of the sensor. When you install them make sure to observe polarity.
4) Place the cover back on putting the top of the cover in place first then make sure the bottom snaps into place tightly.