I pulled up an old email trying to convince a couple of CPAP users to camp with me in the backcountry. I could not sway them, but here's the result of my research from 6 months ago:
Battery features from best to least important are
watt-hours (Wh for short), if the CPAP plug fits (most seem to be DC), and weight. The watt-hours measurement is a universal measurement that tells us how much battery power is available. I compared CPAP-specific batteries (very light 1-2 pounds) with back-up batteries on Amazon that sell for more (less?) than half price.
Many backpackers already bring their C-PAP machines with them and there are some affordable and lightweight options. I started with CPAP.com. Two (2) samples of many batteries they sell are shown below as 1a. and 1b. Two (2) other comparable batteries from Amazon show a significant price difference from the "medical-cpap-batteries" and are fairly lightweight (5.6 or 3.6 pounds).
Option 1a: BPS Freedom V2 Travel CPAP Battery + Machine Cable Kits has 99.9 watt-hours of power capacity and weighs 1.7 pounds. This V2 battery lasts from 1-3 nights; up to six with a second battery. Each one is $349.99.
BPS Freedom V2 (99.9 Wh) Runtimes for Popular CPAP Machines
| Machine | Single Battery | Dual Battery |
| AirSense, AirCurve, or AirStart | 1-3 Nights | 3-5 Nights |
| AirMini | 1-3 Nights | 3-6 Nights |
| DreamStation Series | 1-2 Nights | 2-4 Nights |
| S9 | 1-3 Nights | 3-5 Nights |
Option 1b: EXP 48 Battery (153.6 Wh) = "Four full nights of CPAP use"
12-24V/2A Outlets. $379.99.
Option 2a: Amazon's AimTom Battery (155 Wh); 110V AC, DC 12V, and USB Outlets, 3.6 Pounds; $129.99. This is an "Amazon's Choice" item.
Option 2b: FlashFish Portable Power Station (222 Wh) 110V AC, DC 9-12V, 5.6 pounds.
Option 3: This likely won't work for a CPAP or yours for that matter, but my backup battery for my iPhone and hearing aids has 100.5 Wh. It only has USB outlets. This cost me $45.99, charges my phone almost seven times, weighs 0.78 pounds.
I have a 28W solar charger and it has three ports for charging other devices. It works great in direct sun, but cloudy weather makes it ineffective. When in the backcountry, I don't get a signal, so I turn it on Airplane mode and it lasts for three days.