CPAP Battery Runtime Calculator

The single biggest factor in CPAP battery life is whether you run the heated humidifier and heated tube, and ResMed publishes the numbers for its own machines. An AirSense 10 with SlimLine tubing and no humidifier draws 0.93 A at 12 V at a pressure of 10 cmH₂O. Turn the humidifier to 8 and the same machine at the same pressure draws 3.66 A. Multiply by 12 V and that is 11.2 W against 43.9 W — close to four times the draw, not the doubling most guides quote.

On a 12 V 100 Ah LiFePO4 pack discharged to 80%, that gap is 91.4 hours against 23.3 hours: eleven nights of therapy or three. If you are camping and battery life matters more than comfort, switching off heated humidification is the single most effective change you can make. Measure your own draw with a watt meter in the mode you plan to use, or read the current your maker prints for your pressure.

ResMed publishes a battery guide with the current each of its machines draws at 12 V, by pressure and humidifier setting, and the deep-cycle battery size it recommends for one night. Every wattage below is that printed current multiplied by 12 V.

The battery-size column is ResMed's own, and the guide states the basis plainly: All battery sizes are based on 8-hour usage and include a 50% safety margin. That works out as current at 12 V × 8 hours × 1.5, which is why the calculator above prints ResMed's figure beside your own result. Ours rounds up to the next whole amp-hour, which reproduces ResMed's own column on 25 of its 30 printed rows, reads 1 Ah above it on four, and 1 Ah below it on one: the S9 on a converter at 20 cmH₂O, where ResMed prints 1.00 A and a 13 Ah battery. ResMed worked its column from unrounded currents while printing the amps to two decimals, which is why that row does not come back exactly, and it is one more reason to check the number against your own machine's guide. The last two rows are the same machine at the same pressure through ResMed's two published paths: its DC-to-DC converter needs 0.55 A, a DC-to-AC inverter needs 1.02 A. An inverter costs 85% more current here, not the 10-15% a general efficiency figure suggests.

For machines other makers build, use the label. Philips' DreamStation 2 manual publishes only an AC input envelope (100-240 VAC, 2.0-1.0 A) and no wattage, so there is nothing to copy — read the rating plate on your own machine or its power supply and enter that. ResMed's AirSense 11 user guide does publish a machine-level figure, Typical power consumption 56.1W, with a peak of 73.2 W; that is an AC-side figure for the whole unit, which is why it sits above the 12 V draws in the table. Pressure and mask leak both move the number, so measure before you buy.

Lithium batteries for CPAP machines are permitted on aircraft, but the FAA sets strict size limits. Batteries rated under 100 watt-hours (Wh) can go in carry-on luggage with no restrictions. Most dedicated CPAP batteries like the Medistrom Pilot-24 Lite, which its maker rates at 95Wh, are built to sit under this threshold.

Batteries between 100Wh and 160Wh require airline approval before boarding. You must contact the airline in advance, and you are limited to two spare batteries in this range. Batteries over 160Wh are prohibited on passenger aircraft entirely.

To calculate your battery's watt-hour rating: multiply voltage by amp-hours. Chemistry matters here, because a "12V" LiFePO4 is really 12.8V nominal: a 20Ah pack is 256Wh, well over the carry-on limit, and even an 8Ah pack is 102Wh — over it. Drop to 7.5Ah and you are at 96Wh, just under. The same 8Ah in a 12.0V lead-acid pack is 96Wh and would pass, which is exactly why you check the watt-hour figure printed on the battery rather than working it out from the label voltage. If you fly frequently with your CPAP, choose a purpose-built CPAP battery that stays under 100Wh. For longer trips, bring two sub-100Wh batteries rather than one large one. Our camper battery sizing guide can help you plan how many batteries to bring on a camping trip.