1. They use small coin cells
Most contact sensors take a CR1632 or a CR2032, motion sensors often take a CR2450, and leak detectors usually take a CR2032. The code is printed inside the cover.
2. They last a year or two
A door sensor wakes only when the door moves, so its battery lasts a long time. A motion sensor wakes constantly and uses its battery much faster, often in a few months.
3. The pairing usually survives
Changing the battery doesn’t unpair the sensor in most systems. Open the app, check the sensor still reports, and only re-pair it if it shows as offline.
4. Watch for the warning
A sensor can keep working after the app starts warning about the battery, and it can also stop reporting altogether once the cell runs out. Change it when the app asks.
5. Take a photo of the contacts
Some covers have a metal tab that has to sit under the battery to make contact. A photo before you close it saves guessing when the sensor refuses to wake.