An extremely common question that pops up on the CPAP Facebook and Email groups is:
What battery can I use when I take my CPAP camping/RVing/boondocking/overlanding?
I encountered the same question about 4 years ago when I started therapy, being an avid camper and Cub Scout leader. How on earth do I take this big machine with me to my tent?
There are a number of solar generators and other products using lightweight batteries and an inverter style setup that people lean towards. While convenient, this may not be the ideal solution.
For reference, an CPAP machine typically uses a 120 VAC power adapter when issued to a sleep apnea patient. This converts the power down to the 12-24 VDC that the machine uses (each machine is different on the input voltage).
The inherent problem with using an inverter is conversion loss and inefficiency. We start with a 12 volt DC battery source, step up to 120 volt AC inverter output with a 70-80% efficiency rating, then step back down to 15-24 volt DC input of the CPAP machine. Combine this was the use of a humidifier and heated hose additional current draw and you find yourself barely making it through a night before the battery is depleted.
Resmed provides a handy chart for battery use for their DC-DC converter, the preferred method of off grid powering:
In my instance, I’m an average IPAP pressure of 16 running an AirSense 10 Autoset. The chart Battery Size (AH) assumes an 8 hour night with 50% safety margin. The “All above with SlimLine tubing” box applies to any hose in the heated/climate control set to OFF and humidifier set to OFF. I’ve found in the past couple years of camping almost monthly, the humidity outdoors is optimal already and the hose temp has never rained out outdoors with the heater turned off. This is camping from 90 degrees F in an awful midsummer heat wave night to below 20 degrees F in late season mountain camping.
Based on my settings, I have a current draw of 1.23 Amps or their spec’d battery of 15 amp hour per night (1.23 A x 8 hours x 1.5 safety margin). I generally sleep a minimum of 2 nights for monthly and 6 nights for summer scout camping.
If I chose to do the same configuration but turn on humidity (which really wouldn’t run much considering how humid it is typically outdoors) and the heated hose, the chart indicates a current draw of 4.21 Amps or their spec’d battery of 51 amp hour per night. WOW! A typical lead acid car battery is 60 amp hour – running this off of your main vehicle battery may result in not being able to start the engine in the morning. OUCH!
This is for DC to DC powering – the optimal method. Need a cable for your machine? I use CPAP.com – here is a link to all of their DC cables. No referral bonus for me – just pointing you in the direction of one of the many options to purchase from.
Resmed doesn’t recommend an inverter for the Autoset 10 so let’s look at S9 Autoset to compare DC converter vs AC inverter current consumption:
Our baseline for no climate control on a DC to DC converter is 0.9 A or an estimated 11 Ah battery per night. For a week of camping, we’re still in the reasonable deep cycle battery size, maybe a top off with a solar panel.
Turn on humidity and heated hose (which we already mentioned above is a no go for camping anyways) and on a DC adapter we’re consuming 5.58 A and a full deep cycle battery every night. OUCH. If we run that same configuration through an inverter, we have an additional 8% penalty pushing us to over 73 Ah of battery each night. For comparison – on DC with climate off, 5 nights is still less than a single night of AC with climate on. Let that set in for a minute.
I use a Group 31 AGM battery for car camping. They’re available in deep cycle flavors pretty much anywhere car and marine batteries are sold. They’re typically rated for 82-100 Ah which is more than enough for the style of camping I do right now. I’ll run my CPAP for 8-10 hours a night (I sleep so well in the mountains) and charge my phone/run some LED lights in the tent and have a few other random uses for the single battery. I use a standard AGM charger when I get home which takes overnight to recharge for the next trip On longer camp outs, I’ll throw up a small 15 watt solar panel to get some minimal charging during the sun. Solar charging is a bit of a thing when tent camping as we usually don’t set up our tents where they’re always in the sun. You could always add a longer power cable but that’s just going to increase loss and weight resulting in less current actually making it to your battery.