It's airline-dependant. According to the airline I am flying with, I can have an in-equipment Lithium-Ion battery of up to 160Wh as checked baggage. Spares must be in carry on luggage. The Di2 battery is well below that amount of power.

Disconnecting the battery is a right royal pain, as it requires bottom bracket removal. I don't really fancy doing that level of work in my hotel room! 😐