The Micro Door/Window sensor requires (1) - 'CR2032' 3VDC Lithium coin-cell battery. You can purchase this battery at stores like Walmart, Superstore and Best Buy. Liberty recommends using one of the following:
*Note: Before changing the Batteries on any of your devices, please ensure that your security system has been disarmed.
For a demonstration on how to replace the batteries please watch the following video: