Both the sensor and monitor use a single CR2032 coin cell battery, which is available at your local grocery, home improvement, electronics stores, and many online stores.
You can easily open the devices by inserting a narrow object such as a screw driver or butter knife into each side and popping open. Below is a video showing replacement of the sensor battery. Also below are links showing diagrams of each unit with arrows showing where you must pop open the device.
The XL sensor uses a single CR2450 coin cell battery, available at your local grocery, home improvement, electronics stores, and many online retailers.
The XL sensor has a sealed battery door, that rotates to open.