Traveling with lithium powered massager

Here is the site of the TSA recommendations for lithium batteries. You might want to read through this, and see what it says about your device. https://www.tsa.gov/travel/security-screening/whatcanibring/all?combine=batteries&page=1

Thank you, I did read the TSA information. I also looked up information on each individual airline we're flying on. They all want to have lithium battery operated appliances in the cabin. What I've not been able to determine is how large the battery is for their specifications. They use a different measurement for lithium batteries.

I researched this topic for Lufthansa a while ago - here is some info:

For "medical devices" in carry-on [closest relevant category for your massager]:

Power restriction for lithium batteries: max. 160 Wh or 8 g LC per device

Here is the link:

https://www.lufthansa.com/gr/en/prepare-for-your-trip/baggage/electronic-devices-and-batteries#:~:text=Permitted%20in%20or%20as%20carry%2Don%20baggage%20in,Wh%20or%200.3%20g%20LC%20is%20prohibited.

Obviously this may vary by airline - try to query on "[airline] lithium batteries"

Since you are not allowed to carry lithium batteries in checked luggage, carry-on is your only option.

Any idea how 2500 mAh would translate to Wh? That's what I'm struggling with... Ah, but I see markcw's response contained a nifty formula: watt hours (Wh) = milliampere hours (mAh) x voltage (V) / 1,000. None of the airlines we're flying had this. Thank you all for your help, and especially you, markcw, for that formula. I think that may be my answer.

and then flying on to Stockholm

Don't fly from Copenhagen to Stockholm, take the train.