Follow-up for anyone still following this thread or finding it in a search
I have a Philips Dreamstation 2 CPAP which runs off a 12 volt power supply judging from the specs of the power brick that comes with the machine (hard to find actual electrical specs on the Philips web site). Apparently there are some other brands or models of CPAP that run off 15 volts which makes finding a backup battery more problematic. We have humid air here in Western Washington and I sleep with the windows open so I only run the machine dry on the #10 setting. I don't use the built-in humidifier which I suspect pulls more power. It takes more energy to warm and evaporate water than to run a small electric fan.
Here is what I bought off Amazon this week:
Philips 12 volt cigarette lighter power cord:
https://www.amazon.com/gp/product/B07N45FSNY/ This is a big 12 volt cord that is fused (they provided extra fuses) and has some sort of circuitry built in, probably to regulate voltage. There are cheaper options, I went with the Philips branded one just to be safe in case there are voltage regulation issues required for reliable use with my CPAP.
Anker 521 Portable Power Station (256 Wh) backup battery
https://www.amazon.com/gp/product/B09FF46FQ9/ This is a pretty big backup battery that is about the size of a small car battery.
It is too big to legally take on airplanes even as checked baggage. So not something you would take backpacking or adventure traveling. But it is a solid backup battery for power outages for laptops, cell phones, etc. in the event of a home emergency which is probably 90% of how I will use it. I have a car camping trip planned with my daughter which will probably be the only time I ever use it with the CPAP unless we have an extended overnight power outage at home. Normally when I travel I stay in hotels with power.
Test results:
I ran the CPAP for approximately 6 hours last night while sleeping. The backup battery performed flawlessly. During that time the digital battery meter on the Power Station dropped from 100% to 87% so assuming the battery meter is anywhere close to accurate, I used 13% of the battery during one night. Doing the math, that means I used about 33 Watt hours of power over the night or that the CPAP draws 5.5 watts in the power setting that I use it. Or that this battery will supply me with backup power for about 7 nights. So I can probably get by for one night with a much smaller backup battery in the 50 Wh range. Perhaps something like this 65 Wh battery recommended on the Wilderness thread.
https://www.amazon.com/gp/product/B08JV788NL/ But I would need to find a different cable that runs off USB-C, such as this:
https://www.amazon.com/gp/product/B08NS491XW/
So looks like success. I paid about 1/2 of what Philips charges for their branded backup battery for a generic one that is nearly 3x larger. And there are clearly much cheaper options if one only wants one or two nights of power for a CPAP. The key seems to be having a 12 volt machine. Users who have 15 volt machines seem to need to be more creative to get backup batteries to supply a steady 15 volts of power, especially through USB outputs.
NOTE: I ran the PowerStation in the normal mode, not the power saving mode. This backup battery has a power saving mode that detects when current drops when you are charging a cell phone or laptop and shuts down. You DO NOT want to power a CPAP in that mode for fear it will cut off current to your CPAP. The literature says do NOT use a CPAP in power saving mode. There is a slider switch on the battery to turn on power saving mode.
