I too have had issues with JAL. They require you to have registered your plan to use a CPAP with their Special Services department at least 24 hours prior to travel, and they will inspect your hardware at every point of check-in to make sure your equipment is as registered and is approved. In one instance all of the approvals were not in order and the flight crew stopped me from using my CPAP in flight. They were also very picky about the capacity of my backup battery.

In general the rules change often and vary from airline to airline, so it is essential to check with the operating carrier prior to travel. I always travel with the FAA approval document that my CPAP manufacturer provided; in some countries where they don't have regulations having a government document helps (even if from another government).