Model 3

Camping with a CPAP Machine

I was wondering if anyone had done this and what kind of inverter (I think that's the correct device) to get to plug in to the 12V in the center console.
I'm new at all this so be gentle. ;)
At 8 hours in Camping mode, anyone know what the drain on the battery would be? I'm thinking of taking a 1000 mile trip and would rather not sleep in a hotel if it can be avoided.


  • How much power does your CPAP machine draw? There are 12V to 120V inverters available but the thing that you want to be careful of is that you don't overload the 12V plug and that you don't drain the battery.
  • Well, just checked the one upstairs. It says [email protected], which is 90W. Normally, when one sees numbers like that, it usually means the typical power draw is a bit less than that, but let's stick with the above.
    Next: You'll have a 12V to 120 VAC inverter. Let's assume that that's 85% efficient, probably not too far off, so power draw from the 12V circuit would be 106W, about 8.8 A at 12V.
    Assume that the car's in Camp Mode and is using its internal high-voltage to 12V inverter to supply the power. Things like this are over 90% efficient, but let's use 90% for fun. 106W/0.9 = 117.8W.
    Now, say you use this for 8 hours. That would be 942 W-hr.
    On a M3 running at 65 mph and getting 250 W-hr/mile, that would be a reduction in range of 3.77 miles.
    Not really significant.
  • I checked with the manufacturer of my CPAP and it draws 80w per hour.
    So, I think you are saying that this is not going to be a problem. Yes?
  • You also need to know how many amps of power can be provided by the plug.
  • > @Sparkylulu said:
    > I checked with the manufacturer of my CPAP and it draws 80w per hour.
    > So, I think you are saying that this is not going to be a problem. Yes?

    80w/hr is not a thing.
  • OP probably means 80W. In which case get a 120W inverter for your car and should be fine.
  • I doubt that the 12V plug in the car can supply 80W. Does anyone know what the 12V plug is fused at?
  • Aren't we have regular outlet under rear ac controls?
  • "The power socket is suitable for accessories requiring up to 12A continuous draw (16A peak)"

    Per page 24 of the manual. So will supply up to 144W continuous.
  • I've gone road tripping, sleeping in my model 3 with my CPAP. It was in winter and the big drain on my battery was due to heating my car overnight. [I forget the numbers, but the CPAP used a negligible amount of juice.] For an inverter, I used . I have an AirSense 10 CPAP.
  • Sorry I have mistaken it for my SUV power outlet location. )
  • 80W CPAP machine would use about 800Wh over 10 hours. In otherwords, less than 4 miles of range.
  • @Sparkylulu: Yes, it's not a problem. As I pointed out and others have chimed in, this won't be a problem.
    From the top: The 12V socket in the car, according to @andy_conner_e: is good for 12A. Power (electrical style) is Volts * Amps, so the socket can supply 12*12 = 144 Watts. Obviously, your 80W CPAP machine isn't going to strain the system.
    My analysis above was taking into account the inefficiencies going from one voltage, or kind of voltage to another. Your CPAP machine runs on 120 VAC; at 80W, the current (not that it matters for this) is 80W/120V = 0.666 A.
    However, you need to get from 12V to 120V. You walk into an electronics store or, better, a truck stop, and they sell inverters that accept 12V and convert it to 120 VAC. One end has what looks like a cigarette lighter-style 12V connector, which you plug into the car in the general area of the arm rest; the other end has one or two 120 VAC wall-socket connectors that you plug your CPAP machine into.
    In engineering, Nothing Is Perfect, there are always losses. I'm guessing that the inverter is about 85% efficient; so, if the CPAP eats 80W, the 12V socket will be seeing 80W/0.85 = 94W. The extra 14 W is heat, which is why these little inverters run warm.
    In addition, somewhere in the car there's _another_ inverter, that takes the traction battery voltage, somewhere up near 600V or so (I think) and down-converts it to 12V for the car's regular lead-acid batter, the lights, and the computers and all. And your CPAP. That inverter is probably around 95% efficient or more. Picking a lower efficiency for fudge-factor's sake, 94W/0.9 = 104W of power taken from the battery itself while the CPAP machine is running.
    Now, energy. In Meter-Kilogram-Seconds, Power is in Watts and is the rate of usage of energy, which is in Joules. If one uses 1 Joule per second, that's a rate of 1 W.
    So, for example, take a 60W lightbulb in your house. That's 60 Watts; that implies that it's using 60 Joules/second. Say you run it for one minute then turn the lightbulb off? 60 Joules/second * 60 second = 3600 Joules of energy used.
    Now, the fun/idiot part: Power companies deal in energy. If you want a coal fired plant to make yea many Joules of energy, then you pay for and throw into the furnance such-and-such weight of coal. So, one would think that when the electric bill shows up at your house, they'd charge you for the Joules of energy you used.
    Naw. The numbers are too big. So, they did it another way: They charge you so-many-cents per Kilowatt-Hour. That is, if you had a load of 1 kW for One Hour, they'd charge you, say, $0.15.
    For the record: 1000W = 1000J/s. 1000J/s * 3600 s/hr = 3.6MJ. So, 1 kW-hr is 3,600,000 Joules.
    So, Teslas get connected to city power and the city power guys charge people based upon kW-hrs. So, rather than play around Joules, they play around with Watt-hours, one Watt-Hour being 3600 Joules.
    So, back to you. Your CPAP is using 104W when it's running. If you run it for a good night's sleep for eight hours, the energy usage is
    104W * 8 Hrs = 832 W-hrs.
    So, when the car is motoring down the road it uses roughly 250 W-hr per mile. (It's actually less at, say, 30 mph than, say, at 70 mph, but close enough.)
    Compare this to your CPAP 8-hour energy usage. How much of a reduction in range, back of the envelope style, have you got?
    832W-hr/(250 W-hr/Mile) = 3.3 miles.
    As I said the first time, not a significant amount of range. Sleep tight!
  • Tronguy you have so much patience. thank you for your time and knowledge.
  • Most of the 80W of your CPAP is likely the heater, so you may very well be using a lot less depending on your settings. Also, your CPAP may work with a direct 12V cable plugged into the unit (manufacturer or third party), bypassing the power brick. That would also reduce double conversion inefficiencies.
  • I care for a fair number of patients on cpap. A lot of them camp up here in Alaska, from what I hear Goal Zero yeti 1500 (1500 Wh) is the device of choice for weekend trips, it covers 2-3 nights of use before recharge. That is in line with Tronguy’s estimate above. Not sure how the BMS works drawing power when the car is off. I would probably go the safe route and use a Yeti? But I tend to err to the side of caution and don’t use a CPAP.
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