Smatree
Spare lithium cells are the part of a camera kit with rules attached. We are not an authority on any particular airline, and the specifics change — but the handling habits below are the ones that keep cells safe in a bag regardless of who is carrying it.
Cells travel with you, not under the plane
Loose lithium batteries belong in hand luggage. That is near-universal, it is the reason our charging cases are sized to go in a small bag, and it is worth planning for before you pack the hold bag.
Contacts covered, always
The actual hazard is a short across the terminals — a key, a coin, another cell the wrong way round. A slot that holds one cell with its contacts facing nothing is the simplest possible fix, and it is why our battery cases are built as slots rather than as a box.
If you are using a plain pouch
Tape over the contacts, or keep each cell in the plastic sleeve it shipped in. It looks fussy and it is the difference between a warm pocket and a fire.
Charge before, not during
Arrive with a full set. Charging in transit means a dock, a cable and a socket you may not get, and some carriers are unhappy about charging in the cabin at all. An overnight dock the evening before removes the whole question.
Cold mornings
Lithium cells lose a large part of their useful capacity near freezing. On a cold shoot, keep the spares in an inside pocket and swap them in warm; a cell that reads flat at dawn is often half full once it has been against your body for ten minutes.
Storing a kit you are not using
Cells keep best part-charged and cool, not full and not empty. If the camera is going in a cupboard for the winter, run the batteries down to roughly half, put them in their slots and leave them somewhere that does not get warm.