Guide
Can a car battery run a CPAP?
On DC, a 12 V starting battery can cover one night of CPAP without humidity only as a planning example. It is not a deep-cycle battery. Philips' DreamStation 2 instructions, and the DreamStation BiPAP manual, say not to run the device from a vehicle battery while the engine is running, because damage to the device may occur, and to use only a Philips Respironics DC cord (DreamStation 2, DreamStation BiPAP).
Medical safety notice: Will It Run? gives general power estimates only, not medical advice. Power draw varies by model, settings, pressure, humidifier and altitude. Before relying on any battery, power station, inverter or generator for a medical device, confirm approved power sources with the device manufacturer and your doctor or equipment supplier, and test your setup before an emergency. Never change prescribed settings to extend battery life. If your therapy is life-sustaining, keep the backup your supplier provides, register with your utility's medical-needs program, and have an evacuation plan. Run generators outdoors only, at least 20 ft from windows and doors, because of carbon monoxide risk. In an emergency, call 911.
The math
Usable Wh = 12 V × amp-hours × the share you choose to drain. Amp-hours are capacity. Cold-cranking amps are not: East Penn describes a CCA rating as cranking over a 30-second period, which is a start, not a night of therapy (East Penn). The same maker's marine guide says thinner plates of a starting battery cannot handle constant deep cycling and will fail in short order (East Penn marine battery guide). There is no single published discharge floor that fits every starting battery, so the half-drain figure below is a planning example, not a limit from your battery's label.
Example only: half of a 50 Ah, 12 V battery is 12 × 50 × 0.5 = 300 Wh. On DC, with no inverter, that is about five 8-hour nights for a 7 W AirSense 11 with no humidifier (300 ÷ 7 ÷ 8), and under one night at the 55 W heated-tube figure (300 ÷ 55 ÷ 8). If the machine instead runs through an AC inverter, apply this site's AC estimate, nameplate Wh × 0.85 ÷ (watts + about 15 W): 300 × 0.85 ÷ (7 + 15) is about 11.6 hours, a little more than one night, not five. The 0.85 and the 15 W are planning allowances. BLUETTI counts about 15 W of self-consumption in its AC180 runtime formula. They are not a measurement of a cigarette-lighter inverter, and a larger inverter can idle higher. Read your battery maker's own discharge guidance before you use the example.
| Machine and setting | Average draw | 8-hour night | Nights on a 1,024 Wh station, AC | On DC, best case | Source |
|---|---|---|---|---|---|
| ResMed AirSense 11 AutoSet, no humidifier | 7 W | 56 Wh | 4.9 | 15.5 | ResMed |
| ResMed AirSense 10 AutoSet, no humidifier | 7 W | 56 Wh | 4.9 | 15.5 | ResMed |
| ResMed AirSense 11 with HumidAir 11 humidifier, no heated tube | 19 W | 152 Wh | 3.2 | 5.7 | ResMed |
| ResMed AirSense 11 with HumidAir 11 and ClimateLineAir 11 | 55 W | 440 Wh | 1.6 | 2.0 | ResMed |
AC nights = 1,024 Wh × 0.85 usable ÷ ((watts + about 15 W inverter draw) × 8 hr). DC nights = 1,024 Wh × 0.85 ÷ (watts × 8 hr), a best case with the maker's DC converter (ResMed Air11, Air10 or AirMini) in a 12 V car port; — means DC isn't modeled for that setting (ResMed's AirSense 10 guide says humidity off on an external battery, and Philips publishes no DC figure). ResMed figures are lab measurements from ResMed's published power table; the Philips row is the 80 W power-supply ceiling, a worst case. The 15 W is the inverter's own draw: BLUETTI counts about 15 W of self-consumption in its AC180 runtime formula, and an owner test measured about 17.5 W on a DELTA 3 Plus; larger stations can idle higher. Method, DC converter limits and picks: CPAP power station guide.
A power station is safer: it protects itself from over-draining and has the right outlets.
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Questions
Can I run my CPAP from my car battery?
Briefly, on DC, with the device maker's cord and the engine off. Philips' DreamStation instructions say not to run the device from a vehicle battery while the engine is running, because damage to the device may occur, and to use only their DC cord. A starting battery's thinner plates are not meant for continual deep cycling. The five-night figure on this page is a DC example; through an inverter that same example is about 11.6 hours.