I have an AirTag purchased a while back. I used it to “track” my son in case something happened. Recently I needed to replace the battery as I received warning messages from the Apple devices and hence I found out how to replace the battery in your AirTag.

I know it is silly but I never knew there was a battery inside the AirTag. I know right… so blur… :p
You will receive this message when the battery starts to run out.

There are also instruction give in iPhone/iPad on How to replace the battery in your AirTag.

How to replace the battery in your AirTag
Place the AirTag with the weak battery on a flat area.
Press down on the polished stainless steel battery cover of your AirTag and rotate anticlockwise (right to left) until the cover stops rotating. I find that using two fingers is the easiest to do so.

You will then be able to remove the cover and can see the exposed battery.
It is a CR2032 Duracell…. atas battery…

Luckily I have some around :p. From Japan :p

Insert a new CR2032 lithium 3V coin battery (available at Shopee) with the positive side facing up. You’ll hear a sound indicating that the battery has been connected.

Replace the cover, making sure that the three tabs on the cover align with the three slots on the AirTag. Rotate the cover clockwise until it stops.
Simple 🙂