How to Run a CPAP on Battery: Camping, RV and Flights

Step 1: Find your real watts, not your label's watts

The single number you need is what the machine draws on a meter while it is running at your pressure, with the humidifier and tube heater set the way you sleep. ResMed's own bench figures are the best published reference: 7 W for an AirSense 10 with nothing heated, 22 W with the humidifier on, and 34 W with a ClimateLineAir heated tube added. Bi-level machines measured 41 to 44 W.

Ignore the 90 W printed on the power brick and the 104 W maximum on the rating plate — those are ceilings, and sizing a battery from them is how people end up buying a 2,000 Wh station for a machine that draws 34. If you want the full explanation of the difference, how many watts a CPAP uses walks through the label, the brick and the meter.

Step 2: Choose the path between the battery and the machine

This is the decision with the biggest effect per dollar, and it takes one look at the back of your machine. If it has a round DC barrel input — and almost every CPAP does, because that is where the wall brick plugs in — you can feed it 12 V directly and lose nothing.

Three practical rules fall out of that table:

• Buy the DC cable first. A vehicle adapter or a 12 V barrel lead for your specific machine costs a fraction of the capacity an inverter wastes, and it is the only way a 96 Wh bank covers a heated machine for anything approaching a night.
• Use a converter only when the voltages disagree. Some machines want 24 V, and a 12-to-24 V step-up is cheaper and better than an inverter — it costs 18% instead of 35%.
• Use the inverter, or a power station's AC outlet, when you have no choice. That is the shared-battery case: an RV house bank or a station also running a fridge. The overhead is fixed, so the bigger the machine, the less it matters — 6 W idle on a 10 W travel unit is devastating, and on a 55 W bi-level is noise.

One safety note worth stating plainly: use the cable rated for your machine's amperage, and do not splice a barrel plug onto thin wire. A CPAP draws only a few amps, but a badly made lead in a tent, unattended, all night, is not a risk worth taking.

Step 3: Size the pack from nights, not from a feeling

The usable share is the part shop floors do not mention: 90% for LiFePO4, 85% for a lithium power bank or station, and 50% for the AGM battery already in the van. Lead-acid is limited to half its capacity not because it cannot be drained further but because doing so shortens its life sharply — so a "1,200 Wh" AGM bank is 600 Wh of CPAP. For the night-by-night figures at every common pack size, use the CPAP battery size chart, or run your own numbers through the CPAP battery calculator, which also does the FAA check.

Buy the smallest pack that covers the nights you will actually be off grid, plus one night of margin. Margin is not hedging — it is the difference between a machine that stops at 4 a.m. and one that does not, and it costs far less than the sleep clinic appointment that follows.

Step 4: Plan the recharge before the first night

For a trip longer than the pack's nights, the charger decides what you buy. A realistic daily solar harvest is panel watts × peak sun hours × 0.85:

Panel rows assume four peak sun hours and a charge round-trip efficiency of 85%; a cloudy week roughly halves them, and a winter day at 45 degrees latitude is closer to two peak hours.

Four peak sun hours is the assumption behind the panel rows, and it is a fair one for a clear mid-latitude day; a week of cloud halves it, which is why a solar CPAP kit wants a pack that holds two nights even if the panel covers one per day. If you are running a generator instead, the fuel runtime calculator says how long a tank lasts — but a generator that runs all night to avoid a battery is a poor trade for a 34 W load.

Charge LiFePO4 above freezing. That is not a preference: below about 0 C the chemistry will not accept a charge, and forcing current into a frozen cell damages it. A pack that has slept in a cold van needs an hour indoors before it goes on the panel or the charger.

Step 5: Clear the airport before the taxi to the airport

Airlines treat a CPAP as medical equipment and batteries as hazmat, and the two rules are not the same rule.

Three things follow. The machine itself, with its battery installed or detached and protected, can go in either bag, but a loose lithium spare is cabin-only — a battery in checked luggage is the one mistake that will stop you at the gate. Second, the 96 Wh and 99 Wh CPAP banks on the market are sized to that 100 Wh line on purpose; a 300 Wh station is not, and needs the airline's agreement up to 160 Wh and cannot travel at all beyond it. Third, plan 150% of the flight. A five-hour trip wants 7.5 hours of runtime, which at 10 W is 75 Wh usable and 88 Wh of nameplate — comfortably inside a 96 Wh bank for a travel unit, and not inside it for a heated-tube setup. Ground the machine on the plane's own power where a seat outlet is available and keep the battery as the reserve.

At security, take the CPAP out of its bag the way you would a laptop and tell the officer it is medical equipment; a spare battery in a separate pouch gets inspected faster than one buried in a toiletry kit. Carry the machine's paperwork if you have it — the same letter some CPAP users bring for in-flight use covers a battery over 100 Wh.

Step 6: Run it once on the kitchen floor

The last step costs twenty minutes and is the one everybody skips. Charge the pack fully, wire it up exactly as you will use it, set the humidifier and tube heat to the positions you sleep in, and run the machine for an hour with the lights off. Then read the pack's state of charge, not the machine's screen.

A 1,000 Wh lithium pack on a 34 W machine should have lost about 4% in that hour. If it lost 12%, something in the path is burning energy you have not counted — usually the inverter's standby, a humidifier on maximum, or a cold room. Extrapolate what you measured, not what the box promised, and put the answer into the calculator's custom watts box so the rest of the plan is built on your machine rather than on a spec sheet.

When it fails anyway

Notice that two of the six rows are answered by turning something down rather than buying something bigger. A heated tube at 30 C adds about 12 W to the draw and 96 Wh to the night, which is the difference between a 500 Wh pack covering two and a half nights and covering one and a half. Amps to watts helps if you are reading a cable or fuse rating, and power station runtime covers the shared-bank case where the CPAP is one load among several.

Two pages finish the set: the watt figures and where they come from, and the size chart that turns them into nights for every pack from 96 to 3,600 Wh. The energy tools hub has the rest.