Well I didn’t get a notification. Only noticed the battery was low when I removed it and tried to add it to another persons Apple ID before their trip - threw an error that it couldn’t be added because battery is too low. Thankfully I had a spare CR2032 lying around. Even then I had to insert and remove/reseat the new battery 3-4x before it would register the new battery level and let me add the AirTag. The whole process is as seamless as a cheap 3rd party accessory you’d pickup from Aliexpress.