Connecting a CPAP machine to a battery?

Using it in a van, your most economical method would probably be to add a deep cycle battery of at least 85 and preferably 105 amps, on a battery isolator circuit. You could install the battery under your bed in the back and keep it accessable for removal for outside charging and your vans alternator would recharge whenever you are driving, and the isolator would keep the CPAP from killing your starting battery.

A deep cycle marine Battery would run you less than $100 and the isolators are about $50.
Happy motorin