Can you run a CPAP from a car battery or cigarette lighter?

Can you run a CPAP from a car battery?

Yes. A typical 60Ah car battery holds about 768Wh, but only half is usable if the car still has to start — about 384Wh, or roughly 45 hours of dry CPAP therapy on DC. With the humidifier on, the same half-battery gives about 9 hours.

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Medistrom Pilot-24 Lite
Purpose-built alternative
At 95Wh this covers a single dry night (about 9.1 hours) without touching the car battery, and being under 100Wh it is the one capacity class that travels without airline approval.
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Power-draw figures for the machines below are provisional estimates, not yet our own meter readings. See how we test.

The capacity is generous and the catch is not the CPAP — it is that a car battery which runs flat leaves you with a machine, a dead phone and a car that will not start.

How much is actually usable

A starter battery is built for short, enormous bursts, not deep discharge. Taking one below about 50% both risks the start and measurably shortens its life. So the honest figure is half the nameplate.

Dry CPAP runtime from a starter battery, on DC
Battery	Nominal	Usable (50%)	Dry	Humidified
45Ah	576Wh	288Wh	34.2 h	6.8 h
60Ah	768Wh	384Wh	45.6 h	9.1 h
75Ah	960Wh	480Wh	57.0 h	11.4 h
100Ah	1,280Wh	640Wh	76.0 h	15.2 h

12.8V nominal, 95% DC efficiency, 8W dry and 40W humidified. A deep-cycle leisure battery can use far more of its capacity than the 50% shown here. Draw figures are provisional pending our own meter testing — see How We Test.

The socket is the real constraint

Most 12V accessory sockets are fused at 10A to 15A — roughly 120W to 180W. A CPAP on DC needs about 8W, so the socket is never the limit for the machine itself. It becomes the limit when you put an inverter in the chain and run the machine on AC, or when you try to charge a large power station through it.

Many cars cut the socket with the ignition off. That is the single most common reason this setup fails overnight. Check yours before you depend on it — some have one always-live socket and one switched.

Use DC, not an inverter

If your machine takes a 12V or 24V DC cable, use it. Running a car's 12V through an inverter to AC and then through the CPAP's own brick back to DC wastes about 12% for nothing. Check the voltage: ResMed AirSense units are 24V and need a converter from a 12V source, while the Philips DreamStation 2 is 12V natively.

If you do use an inverter, it should be pure sine wave — see whether a CPAP needs pure sine wave.

Idling the engine is a poor trade

Running the engine to top the battery up works, but a CPAP draws about 8W and an idling engine burns something on the order of half a litre of fuel an hour. It is loud, it is expensive per watt-hour, and it must never be done in an enclosed space — exhaust in a garage or near a tent is a carbon-monoxide risk, not a comfort issue.

A power station charged from the car during the day is almost always the better arrangement.

Will a CPAP drain my car battery overnight?: At about 8W on DC, eight hours costs roughly 64Wh — under 10% of a 60Ah battery. Humidified, at about 40W, it is roughly 320Wh, or around 40% of usable capacity if you hold to the 50% floor.
Can I plug a CPAP into a cigarette lighter socket?: Yes, with the correct DC cable for your machine's voltage. The socket's 10-15A fuse is far above a CPAP's needs. Confirm whether your socket stays live with the ignition off.
Is a deep-cycle battery better than a starter battery?: For this, yes. Deep-cycle and LiFePO4 leisure batteries are designed to be discharged deeply and repeatedly, so far more of the nameplate capacity is usable and cycle life is much longer.

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