Inverter vs. Hooking directly to 12V

Are you sure the 1.8A is at 110v? Some quick Googling shows that most CPAP draw in the 10s of watts, which would make sense for the 1.8A to be at 12v.

 

It shouldn't. The limitation is with the design of the outlet itself. Some products stretch it, but not be enough to ensure reliable operation for OP's application.

Woah! This is what we were trying to verify earlier. 1.8 amps at 110-120V means that it exceeds the power limits of the 12V socket.

My searching for CPAP machines found a wide range, some consistent with that current at 110-120V, and some others that are friendly to 12V systems.

 

Hold the Presses: (if you know what that means you are over 50 and might need a CPAP)

1.8 amps at 120vac does not mean 200w will be used when using a direct 12vdc adapter made by the CPAP manufacturer. It reflects the MAX current output capacity of the power brick in question which varies considerably with load. BUT considering the OP was suggesting the use a direct dc to dc adapter (converter), the dc amp requirements are WAY less than 18 or even a properly calculated 16 (120x1.8=216/13.5 =16).

Maximum draw on the most popular Resmed S9 with optional H5i humidifier when using their adapter? Less than 7 amps at 12v nominal. Given the humidifier is optional, the CPAP machine itself can be as low as 1 amp at 12v nominal. This level of current is low enough where a Harbor Freight Jump Starter and Power Supply would probably run it without the car being on at all.

In any case, your results may vary. See attached for the Resmed specs (specifically p4). The S9 Series is the most common with the H5i being a breathing tube humidifier option. The obvious solution is to try it and measure the current. You might get a free meter with the Jump Start if you bring a coupon.

 

OK, so if you are going to do this, you need to go directly to the battery.....with a 20 amp fuse probably......since 200 watts at 12 volts is about 17 amps. And an appropriate gauge of wire to handle that load too.

 

If you drive at night a lot, I'd be careful how you sleep in the car.
It can be a dangerous habit to get into.
(YMMV)
That having been said, I've slept in the cargo bay in a secure location, but I always make sure that I'm laying down rather than just reclining the pilot seat for a quick battle nap.

 

Yes. In fact the car will move when shifted into drive.

 

Acc-mode just drains your battery; ready-mode doesn't, but as mentioned, it will move if someone bumps the shifter-knob.
Use the parking brake.

 

... while the brake pedal is pressed?

 

The car's on when you do this, right? How often/long does the engine run?

 

nice.

 

Can you stretch right out? In a regular Prius, you can but only if you're under maybe 5'-6".

 

With the parking brake engaged, no worries at all.