AirTags can play a sound on your iPhone to help you find them, but they're also designed to work without a phone. These small, circular trackers operate on 3V CR2032 coin cell batteries. You can purchase these cheap batteries from many stores or online retailers, including this Energizer Lithium pack of two for less than $4 on Amazon.
The batteries last about a year, though the exact lifespan can vary depending on how often you use your AirTag and the condition it's kept in. Apple says the battery should last for at least a year, but you may want to keep spare batteries on hand in case yours run out.
You'll only need to replace the batteries if you receive a notification in your Find My app that says it's time to do so. This warning will appear only once, so it's easy to miss if you're not looking out for it.
AirTags don't rely on Bluetooth to transmit their location, but the extensive Apple device network that allows iPhones and other devices to locate the tags. However, the tag's battery must still have power for it to communicate with Apple's servers. If the battery dies, the AirTag will no longer be able to contact your iPhone for its location, and you'll be left searching for your keys or bag.
Fortunately, swapping out an AirTag's battery is simple and only requires both hands. First, remove the tag from any attachments such as keyring holders or mounts. Next, press down and twist the shiny stainless steel battery cover counterclockwise until it stops rotating. This should reveal the battery slot, and you can now pull out the old battery and replace it with a new one.