I don't have a battery pack, but I do plug my system into a emergency power block for a computer. Works pretty well, but I am also interested in responses to your question.
I have a Resmed Series M CPAP machine, with a pressure of 10. I took some sailing lessons in the spring and we did a full weekend on the boat. I tried out a couple of battery packs that said they'd last 14-16 hours. Maybe I got a bad set but they did not work but a couple of hours. So I went and bought a deep cycle marine battery (yeah, it was really heavy) and carted that onboard.
I don't remember all the numbers for the AmpHours and such, but the deep cycle marine battery went two full nights and had room to spare. It was heavy, probably 75 pounds or so, but the smaller battery packs just did not work.
I also had to purchase the clips that went to the deep cycle battery posts with the cigarette lighter adapter for my machine.
No complaints but for the heaviness of the battery. The deep cycle battery was also a lot less expensive than the CPAP small battery, like $245 to $120.