We're heading to the Grenadines in early March for advanced sailing school. My CPAP only draws about 7 watts dry (70 watts with humidifier), and runs off a 12v power brick, so it should be easy to power (under 5Ah per night). Note that others could have very different power draw due to different models of CPAP and different pressure settings.
However, I've developed a lot of experience powering CPAP off of 12v batteries, and things are sometimes a little more complicated than expected. I have had problems powering off a 12v adapter on 3 different boats (my old 25' boat with group 24 battery, my friend's boat with large, brand new 4D banks, and a charter boat) using two different model of CPAP device. In each case, it appeared that the voltage dropped too much during the initial surge of startup current to get the blower running. Once started up, it was fine, but it sometimes needed a boost by starting up the motor alternator or other unusual means to get it going. It seems that the problem does not have anything to do with the battery capacity, but instead with the long wire run from battery to outlet causing the voltage drop.
I've written elsewhere about using a jump starter on my own boat, which results in a much shorter wire run that leads to flawless operation. But that device will be very inconvenient for air travel due to large bulk and unnecessary air compressor that's built into it. So I started hunting around for an economical solution. Face it, portable batteries are not "rocket science" and so-called "medical grade" batteries are basically a scam. And the jump starter that I have been using is basically just a SLA battery with nice case (and worthless compressor) around it.
I found a workable solution inside one of my UPSs in my house. It has a 12v-12Ah SLA battery inside, which I happened to replace about a year ago (so I know it's still in good shape). It has F2 connectors on it, so I took a Battery Tender harness that I have lying around, clipped off the ring terminals, and spliced on the appropriate connectors. In testing it, it drops down to about 70% charge after one night of use, and was successfully recharged by my car's alternator by attaching a double-headed 12v pigtail for a couple hours. This simulates how I would charge from the sailboat's battery bank, although without the motor running it may take longer to charge.
So here are the components of my system - I had all of them lying around the house or on the boat:
I'll continue to test out these items this winter. But I wanted to show that you can put together something that is perfectly workable for under $50. I've attached a picture that shows the setup.
I could search for a Lithium battery of similar capacity which would be lighter weight, but they're getting a bad reputation for air travel and according to FAA would need special permission of the airline for the needed 101-160 watt hour capacity. Plus, all it takes is one airline incident between now and my departure to change everything immediately.