You cannot charge an Apple AirTag
AirTag uses a replaceable CR2032 coin cell battery
Open the AirTag by pressing down on the stainless steel back and twisting counterclockwise
Remove the back cover and old battery
Insert a new CR2032 battery with the positive side facing up
Reattach the back cover and twist clockwise until it locks
Use a CR2032 battery without a bitterant coating if possible, as some coated batteries may not work properly