I bought a 12 volt tractor battery from Costco plus a automatic recharger from Harbor Freight and the total cost was under $75. My ResMed CPAP runs on 12 volt anyway, so I wired up a plug to just plug my machine in. I use this at home in the event the power ever goes out at while I'm asleep. Based on the power needs of the CPAP, the battery should last several nights before needing charging.
In our coach, we have 4 house batteries and an inverter/converter supplying 120 volt power 24/7, so I haven't bothered to use my battery backup in the coach. When we've dry camped, the batteries would easily supply power for several days without needing a recharge. I don't use a humidifier.
