• ResMed provides a battery for this unit, though it's fairly expensive and not very high-powered. (8 amp-hours, so it would only last 3.5 hours)
• Somebody made their own battery.

There are battery-powered CPAP machines on the market:

• Transcend (18V, max 1.67A)

General bits of advice:

• "Most batteries with enough 'oomph' for a CPAP are about the size of a car battery." [2]
• Is a normal car battery capable of running a CPAP machine all night, and still have enough charge to start the car?
  • No: [1] [2]
  • Yes: [1] [2]
  • Note that car batteries (lead-acid) will operate at greatly reduced capacity in cold weather.
• Even if you could carry the weight, it's still impossible to hike with a normal (lead-acid) car battery, because they're vented / unsealed.
• A number of people used a deep-cycle marine battery [1] [2]
• There are several ways to reduce the power that your CPAP draws:
  • Disable the heated hose.
  • Disable the humidifier.
  • Reduce the pressure setting. (obviously not a great option, but if you can get by with a little less pressure, it may be enough to make a marginal battery work fine)