airtag using up internal battery quickly

The Apple AirTag's battery should last up to one year with normal use.

If it doesn't, and you have already replaced the battery with another "known good" one, the the issue may be one or more of the following:

• The replacement battery has a bitterant coating.
• The tag's Bluetooth transmitter is "working harder" than normal. This could happen if you place the tag in "Lost Mode."
• The tag is operating in a harsh environment at either temperature extremes.
• The tag itself, is defective.