The page at https://www.faa.gov/hazmat/packsafe/resources/airline-passengers-batteries (scroll down to “Batteries Allowed in Airline Passenger Baggage in the US”) shows “Most A, AA, AAA, C, D, or button cell batteries” as being allowed in checked baggage. However, I don’t doubt that some zealous enforcer would be unable to read and understand this. [See correction by @Shamino below.] (Personally, I would rather use a stock AirTag in travel bags.)
It seems shortsighted of the designer not to include a space to store the back in the device.
I’d say a potential problem with using the manufacturer recommended AA lithium batteries is an airport screener will see the word “lithium” on the batteries (note that it is lithium ION batteries that are restricted for air travel, not non-rechargable lithium iron disulfide batteries) and assume the Elevation device contains lithium ion batteries.
If you get a Lithium metal AA battery, then the text on the “Lithium Metal Batteries” row applies, which is different:
“Must be protected from damage and short circuit. Spare batteries can be carried in carry-on only. 2 grams or less lithium per battery.”
So you should be able to install lithium metal batteries in an AirTag battery extender and put the device in checked bags, but if you are transporting any spare batteries, they have to be in your carry-on bags. But this paragraph also applies to any other lithium metal cells, including the CR2032 cell that AirTags (and quite a lot of other things) also use.
The main reason for my statement is that other countries may have other rules, and those rules sometimes change without notice. So you should (especially if you’re not a frequent traveler) check the rules for the time and place of your flight. But with alkaline batteries, it is highly unlikely that there will ever be an applicable restriction. (But now you have to deal with the possibility of leakage.)
But all that having been said, I recommend checking the rules as a matter of course, because some countries and/or airlines have rules that make absolutely no sense, including many that have nothing to do with batteries.
For example, the last time I flew on Alitalia, they had a rule that only permits laptops that don’t have optical drives, but banned units with CD/DVD drives (and portable CD players). I have never been able to figure out how this could possibly be a safety issue, even in the wildest tin-foil-hat conspiracy theory. I know that there’s a lot of paranoia over using electronics in general, but how could adding an optical drive to a laptop change it from “safe” to “dangerous”? And, needless to say, I’ve never seen this rule on any other airline.
And something travelers to/from/within the European Union may be interested in from late 2022:
(it appears AirTags fell under European regulations for devices with much more powerful batteries and the regulatory agencies were planning to review AirTags’ classification…plus CR2032 batteries are not lithium ion)
My guess is that the rule is a reaction to the fear that a slot and/or optical drive bay could provide enough space for explosives. And as those of us who fly know, once one of these “movie plot threat” rules is put into place, nobody at an airline, airport, or government agency wants to be the one to rescind the rule in case something ever happens, no matter how unlikely (see: taking off shoes at security checkpoints).
Then again, the two recent Delta stowaway incidents don’t inspire much confidence in airport and airline security practices.