The batteries that came with the product should last about 6-8 months. You will get a notification when they are running low and need to be changed. See below for what that looks like (battery level red with an exclamation point).

WHICH BATTERIES

Use 2 x AA batteries. We recommend lithium batteries.
Alkaline batteries last 6-8 months while lithium batteries last 1-2 years. 

Rechargeable batteries work, too.

HOW TO REPLACE

To replace, slide the front of the sensor upwards (away from the sensor dome) and pull the front cover off. Pull the batteries out and replace, ensuring the batteries are fresh and in the proper orientation. Be careful not to pull the back cover of the sensor off the wall.

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