I have a Medistrom CPAP battery, which is quite small but I find it only works for one night and maybe half of a second night before you have to charge it up. As noted above, the humidifier and air tube heater in your machine suck up a lot of power and need to be disabled when running off a battery. I also have one of those larger portable battery backup units (a Yeti 400) in my rig, and it will run my CPAP for a couple of nights easily. Between the Medistrom and the Yeti, I can boondock for a few nights without worrying about being without my CPAP.
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