Hector Martin

I believe the underlying check-in rule is "does this have exposed contacts on it anywhere?" and even then if you cover them appropriately it's OK. I vaguely recall reading about it somewhere in ICAO regs.

ANA made me take out two little portable USB mobile batteries from my check-in sometime this year (they pulled my suitcase out to the gate and called me to go through and take them out!). I was confused because to me they aren't "batteries" per se. Protected & only USB A exposed.

where was this? i’ve been travelling for a year and a half with 300Wh of li-ion (including li-ion stand-alone batteries, albeit circuit protected) in my carry-on. i get some really amazing looks during xray but haven’t been questioned yet

The trick is you can take them carry on but not checked. I believe part of the motivation also is that you can put out a fire in the cabin more easily than the hold. The rules are a bit all over the place though.

LiOn batteries are prohibited because there's no guarantee that the hold will always be heated, and batteries can spontaneously combust due to cracking at very low (-20C) temperatures, and cause a baggage fire.

The driver for this, if I recall correctly, was an Air France flight where this happened. Nobody hurt, thankfully, but it damaged the aircraft and forced an emergency landing. So the FAA banned LiPos in the hold and everyone else followed suit.