How to Power a CPAP Machine Off-Grid

A CPAP battery sized by guesswork can run flat in the middle of a trip, because the heated humidifier and the pressure setting change the draw: ResMed's battery guide prints 0.93 A at 12 V for an AirSense 10 with SlimLine tubing at 10 cmH2O with no humidifier, and 3.66 A with the humidifier at 8, which is 3.66 / 0.93 = 3.9 times as much. The fix is to start from the current your machine's maker prints for your setting and work forward from there.

Our CPAP battery runtime calculator works out runtime from that printed draw and your battery, and rounds the hours and nights down. This guide covers the math behind it and walks you through building an off-grid CPAP power system, whether you are camping, traveling in an RV, or preparing for power outages at home. Confirm your own machine's draw at your pressure with its maker's battery guide before you rely on any figure here.

A Real Camping Trip: Where the Numbers Go Wrong

Consider a three-night backcountry camping trip. For this example, take a ResMed AirSense 10 with SlimLine tubing at 10 cmH2O, with the heated humidifier at 8. ResMed's battery guide, linked below, prints a draw of 3.66 A at 12 V DC for that machine and setting, which is 3.66 x 12 = 43.9 W. The guide prints a current for each of its machines and settings, so take your own from there.

Over an 8-hour night that is 43.9 x 8 = 351.2 Wh, and three nights come to 43.9 x 8 x 3 = 1,053.6 Wh. At 43.9 W the calculator's ResMed 8-hour sizing gives a 44 Ah battery for each night, the battery size ResMed's guide prints for that row, so 44 x 3 = 132 Ah for three nights. Now take a single 12 V 100Ah lead-acid (AGM) battery and, as this example's assumption, a 50% depth of discharge limit. Our calculator gives 13 hours 40 minutes at 43.9 W, or 1.7 nights: one full night and part of the second, then the battery is done.

Now run the same example with a 12 V 100Ah LiFePO4 battery at 80% DoD, the calculator's default. At the 12.8 V nominal voltage the calculator uses for LiFePO4 it delivers 100 x 12.8 x 0.8 = 1,024 Wh, which runs 43.9 W for 23 hours 19 minutes, or 2.9 nights: two full nights, not three. A 200Ah LiFePO4 at the same depth delivers 200 x 12.8 x 0.8 = 2,048 Wh, which runs 46 hours 39 minutes, or 5.8 nights, so all three nights are covered with room to spare. In this example the battery chemistry decides whether the trip works. For the full breakdown of why LiFePO4 gives more usable capacity than lead-acid, see our LiFePO4 vs lead-acid runtime comparison.

CPAP Power Draw by Machine and Setting

CPAP power draw depends on the machine, the pressure, the humidifier and the tubing. ResMed's battery guide prints the current each of its machines draws at 12 V DC and a battery size for one 8-hour night, by pressure, humidifier and tubing, so read your own machine's row there. The rows below are copied from that guide (document 198103/7, 2018-08): the draw and the battery size are ResMed's prints, and the watts are this guide's arithmetic, ResMed's current multiplied by 12 V.

Of the battery-size column, ResMed's guide says: "All battery sizes are based on 8-hour usage and include a 50% safety margin." The last two rows are the S9 AutoSet at the same pressure through two power paths: ResMed prints 0.55 A in its DC-to-DC converter table and 1.02 A in its DC-to-AC inverter table (with EPR at 0). Since 1.02 / 0.55 = 1.85, the inverter path takes about 85 percent more current at these two printed figures.

In ResMed's rows for the AirSense 10 at 10 cmH2O, the draw is 0.93 A (11.2 W) with no humidifier and 3.66 A (43.9 W) with the humidifier at 8, so without the humidifier the machine draws about a quarter as much (11.2 / 43.9 = 0.26). Whether you can lower or switch off humidification for a trip is a question for your provider and your machine's manual: ask before you change a setting.

Choosing the Right Battery Chemistry for CPAP

LiFePO4 (lithium iron phosphate) batteries are the chemistry this guide's examples use, for the three reasons below: usable depth, weight and voltage.

A LiFePO4 battery can be discharged much deeper than a lead-acid battery of the same rating. This guide's examples assume 80 percent depth of discharge for LiFePO4 and 50 percent for lead-acid. On those assumptions a 50Ah LiFePO4 gives 50 x 0.8 = 40Ah usable and a 50Ah lead-acid gives 50 x 0.5 = 25Ah. To get 40Ah usable from lead-acid you need 40 / 0.5 = 80Ah rated.

Weight matters if you travel with your equipment. For the same usable energy a lead-acid battery has to be larger, 80Ah rated against 50Ah in the example above, so compare the weights your batteries' makers print before you plan to carry one. For car camping a heavy battery is a nuisance; for airline travel, check a battery that large against the FAA's limits below before you plan to take it.

LiFePO4 holds its voltage flatter than lead-acid as it discharges; lead-acid voltage sags as the battery empties. ResMed's battery guide prints that its DC-to-DC converters "shutdown automatically when the voltage drops below a certain threshold and protects the battery from damage by not allowing a full discharge"; how your own machine and its power path behave as the battery voltage falls is in your machine's manual.

Purpose-built CPAP batteries vs. general-purpose batteries

Several manufacturers sell batteries made for CPAP use, such as the Medistrom Pilot-24 Lite. Medistrom prints 95 Wh for the Pilot-24 Lite, says it uses lithium ion cells, and lists the machines it is compatible with. Assuming the full 95 Wh reaches the machine, it runs 95 / 11.2 = 8.5 hours at the 11.2 W from ResMed's 0.93 A for an AirSense 10 at 10 cmH2O with no humidifier, about one 8-hour night, and 95 / 43.9 = 2.2 hours at 43.9 W with the humidifier at 8.

A general-purpose 12V LiFePO4 battery (50Ah or 100Ah) with a DC-DC adapter for your CPAP model stores far more energy: a 50Ah LiFePO4 at 12.8 V nominal holds 50 x 12.8 = 640 Wh, against 95 Wh for the Pilot-24 Lite. The trade-off is energy against size and fit: a general-purpose battery stores more but needs the correct adapter cable for your machine, while a purpose-built pack is made to connect to the machines its maker lists. For airline travel, set each one's watt-hours against the FAA's printed limits below.

Setting Up Your Off-Grid CPAP System

Once you know your power draw from the table above, follow these steps to build a system that works.

1. Calculate your total energy requirement. Count nights away from a charging source. Multiply by nightly energy use. Then add a margin. For example, assume a 3-night trip with the AirSense 10 at 10 cmH2O and the humidifier at 8 at the 43.9 W from ResMed's printed 3.66 A, and a margin factor of 1.5 (an assumption for illustration, not a measured figure): 43.9 x 8 x 3 x 1.5 = 1,580 Wh, and 1,580 / 12 = 131.7Ah at 12V. The CPAP battery runtime calculator's ResMed 8-hour sizing gives 44Ah a night at 43.9 W, so 44 x 3 = 132Ah for three nights.
2. Choose your battery. For the example above, take a 12V 150Ah LiFePO4 battery: its 150Ah rating is above the 132Ah from the ResMed sizing. At the calculator's 80 percent depth of discharge it delivers 150 x 12.8 x 0.8 = 1,536 Wh, which runs 34 hours 59 minutes at 43.9 W, or 4.3 nights. That is 1,536 - 1,053.6 = 482.4 Wh more than the three nights draw, but short of the figure with the 1.5 margin, unrounded 43.9 x 8 x 3 x 1.5 = 1,580.4 Wh, by 1,580.4 - 1,536 = 44.4 Wh; the 200Ah battery above, at 2,048 Wh, is 2,048 - 1,580.4 = 467.6 Wh over it. If weight is a constraint, two smaller batteries let you bring one for short trips and both for longer ones. Our LiFePO4 runtime calculator takes the battery capacity, system voltage, load, depth of discharge and system efficiency, and returns the runtime and the usable energy at that depth.
3. Get the right power adapter. Your machine's manual says whether its maker sells a DC-to-DC converter for it and which inverter type, if any, it accepts: follow that. ResMed's battery guide, for its own devices, says: "Whenever possible, we strongly recommend the use of a DC-to-DC converter to power your ResMed device." For an S9 AutoSet at 10 cmH2O it prints 0.55 A through the converter and 1.02 A through an inverter (with EPR at 0).
4. Test the complete system at home. Your machine's manual says how to connect it to a battery, and your provider can say whether to try a night on battery at home before a trip. If you do, record the battery's state of charge before and after: our battery runtime guide explains the formula for turning that into a runtime.
5. Plan recharging for multi-day trips. A portable solar panel's daily output depends on its rating, your sun hours and the season, so work it out for your site rather than trusting a rule of thumb. At ResMed's printed draws for an AirSense 10 at 10 cmH2O (0.93 A and 3.66 A at 12 V), one 8-hour night uses 11.2 x 8 = 89.6 Wh with the humidifier off, or 43.9 x 8 = 351.2 Wh with it at 8. In an RV, a DC-DC charger connected to the vehicle alternator can add energy while you drive: for example, a charger rated 30A at 12V adds at most 30 x 12 = 360 Wh per hour of driving. Our solar charge time calculator takes your battery capacity, panel wattage, peak sun hours, system voltage, chemistry, depth of discharge, system losses and charge efficiency, and returns the charge time in days, the daily solar energy and the energy to replenish.

Flying with CPAP Batteries: FAA Rules

The FAA's passenger battery chart, "Batteries Allowed in Airline Passenger Baggage in the US", prints: "Lithium-ion batteries are allowed in your carry on based on watt hours (Wh). Batteries 0-100 Wh are allowed on passenger aircraft, 101-160 Wh require air carrier approval, and batteries exceeding 160 Wh are forbidden." The FAA's lithium battery page carries its current guidance: read it, and ask your airline, before you fly.

Medistrom prints 95 Wh for its Pilot-24 Lite, a figure below the 100 Wh at the top of the chart's first band. A general-purpose 12V 50Ah LiFePO4 battery stores 50 x 12.8 = 640 Wh, which is 640 / 160 = 4 times the 160 Wh above which the chart prints that batteries are forbidden.

Whether your CPAP machine travels in the cabin and how it counts against your carry-on allowance are questions for your airline: ask it before you fly. For a spare battery or power bank (its examples include "external batteries") the FAA's chart marks carry-on yes and checked no, and prints: "Must be carry-on only and protected from damage and short circuit.."

International travel considerations

Mains voltage and outlet shapes differ by country: check the input range printed on the label of your power supply before you plug in abroad, and bring the right plug adapter. Your machine's manual and your converter's manual say which supply each accepts abroad. On a long trip away from outlets, a small foldable solar panel can recharge the battery.

Temperature, Altitude, and Other Variables

Cold temperatures hit you from both sides. Cold air is drier, and the humidifier is the largest addition in ResMed's rows above: at 10 cmH2O ResMed prints 0.93 A (11.2 W) for an AirSense 10 with no humidifier and 3.66 A (43.9 W) with it at 8. Agree any humidifier setting with your provider. Cold also reduces the capacity a battery can deliver; your battery's maker prints how much for its own cells. Size your battery for the coldest night you expect, not a mild one.

Altitude and pressure. Whether your machine's pressure changes at altitude is in its manual and is a question for your provider. ResMed's guide prints a higher current at a higher pressure: for the AirSense 10 with SlimLine tubing and no humidifier, 0.93 A at 10 cmH2O and 1.66 A at 20 cmH2O. If your machine's pressure can rise, read the row for the highest pressure it may reach.

Battery aging reduces capacity. Every battery loses capacity as it ages and cycles, and the rate depends on the cells, the depth of each discharge and the temperature. Check your battery maker's printed cycle-life figure, and plan an older battery at less than its label.

Have a backup plan. A power failure in the backcountry leaves your machine without power until you can recharge. Carry a backup battery, and ask your provider what to do on a night your machine cannot run. Equipment failure at 2 AM in a remote campsite is the wrong time to discover you have no Plan B.

Troubleshooting Common Off-Grid CPAP Issues

Even with proper planning, off-grid CPAP setups can hit problems. The five below each say where to look first.

CPAP shuts off mid-night with battery showing charge remaining. A falling battery voltage is one thing to check. ResMed's battery guide prints that its DC-to-DC converters "shutdown automatically when the voltage drops below a certain threshold". Your machine's manual and your converter's or inverter's manual give their input voltage ranges and say what each does at a low battery voltage: set any low-voltage cutoff from those figures, and ask your provider what to do on a night your machine stops.

Inverter buzzing or humming audibly. ResMed's battery guide says "modified sine wave converters alternate voltage in steps which can damage sensitive electronics." Check your machine's manual for the inverter type its maker accepts, or drop the inverter and use the DC-to-DC converter your maker names for your CPAP model, if it sells one. In ResMed's rows the DC route also draws less: 0.55 A through the converter against 1.02 A through an inverter (EPR at 0) for an S9 AutoSet at 10 cmH2O, and 1.02 / 0.55 = 1.85, about 85 percent more through the inverter.

Heated humidifier drains the battery faster than expected. In ResMed's rows for the AirSense 10 with SlimLine tubing at 10 cmH2O, the humidifier at 8 adds 3.66 - 0.93 = 2.73 A, or 2.73 x 12 = 32.8 W, which is more than the 11.2 W the machine draws without it.

Ask your provider before you change your humidity setting or add a heat moisture exchanger (HME) filter. On the power side, you can plan a midday top-up from solar or vehicle charging; our battery charge time calculator takes the battery capacity, the charge current, the current depth of discharge and the chemistry, and returns the charge time with its bulk and absorption stages.

Battery will not charge from the solar panel during the day. Check three things in order: the solar charge controller is set to the correct battery chemistry (LiFePO4 requires a different charge profile than lead-acid), the panel is getting direct sunlight (even partial shade on one cell reduces output dramatically due to string wiring), and the cable gauge between panel and controller is thick enough for the distance. A wire too thin for its length loses enough voltage to stop the charging.

Our wire distance calculator takes the current, the one-way distance, the system voltage and the maximum voltage drop, and returns a recommended wire gauge with its voltage drop.

Condensation buildup in the hose on cold mornings. Your machine's manual and your provider are where to ask about condensation in the hose. Off-grid, a heated hose costs power. For an AirSense 10 at 20 cmH2O with the humidifier at 8, ResMed prints 4.59 A with SlimLine tubing and 5.61 A with ClimateLine Air tubing at 30°C, so the heated tube adds 5.61 - 4.59 = 1.02 A, or 1.02 x 12 = 12.2 W, all night. Ask your provider and check your machine's manual on its tubing before you cover, wrap or reroute the hose.

The key is doing the math before you leave, starting from the draw your machine's maker prints for your pressure. For a phone on the same trip, the portable power bank runtime calculator takes a power bank's capacity, its cell voltage, the device's power draw and the conversion efficiency, and returns the runtime, the usable energy and the number of Galaxy S24 charges.