Friday, 8 June 2012

Self congratulation post #2 - CPAP power supply.

A uses a CPAP machine (Constant Positive Air Pressure machine) when she sleeps. This type of machine helps people treat sleep apnoea. Use of any kind of medical equipment generally ties the user to a power outlet.

A's machine is one of the very popular Philips Respironics System One machines. The unit is supplied with a 12 Volt transformer which attaches to a 240 Volt outlet.

Previous camping trips have worked with the use of a long extension cable reaching in to our tent. While successful, this approach is inconvenient and has severely restricted our choice.

Respironics do have a battery pack for the System One, for which CPAP Australia charge their discount price of $520. But they advise to not use the humidifier heater unit, and state that the battery life will vary depending on the pressure being delivered by your specific machine. To me at least, I don't see the point in spending that much money on a battery pack to then not be able to use the device it is meant to be powering. A tells me that using the CPAP without the humidifier/heater results in her nose bleeding.

The battery pack isn't the only thing which is bloody expensive. The power cable to attach the CPAP machine to a standard 12V cigarette-lighter outlet is $60. You got that right: $60 smackeroos!

Combined, that's a serious amount of money for a very unsatisfactory solution. I can imagine that a large number of people feel tethered to a power point, and less able to enjoy an active life.

This situation was hard to believe: there is no commercially available solution for the users of CPAP machines to go camping without compromising on their CPAP therapy.

The Waeco battery pack with charger cable and the
Respironics CPAP with my home made 12V supply cable.
So, after a few simple tests using my trusty multimeter, I determined that the machine draws 2.2 Amps, and uses 12 Volts. After some searching, I found the perfect equipment: Waeco, who make car fridges have a battery pack to use when you are not using your car, thus converting your car fridge into a camping fridge. They call this the Waeco CoolPower RAPS 36, and I bought one from a local Queanbeyan firm Caravans Plus for $301.50. I also soldered together a plug and a cigarette lighter plug pre-fitted to a curly cable which I bought from from Jaycar and Autopro respectively. I could have saved myself a soldering job and just bought the cable pre-made. The pre-made cable is a mere $4.95 from Jaycar. I figured that at 2.2 Amps, the RAPS 36 (being rated at 36 Amp Hours) could supply the CPAP for a theoretical 16.3 hours: 36/2.2 = 16.3636. 

These off the shelf components totalled $306.45, which compared very favourably to the "official" solution which would have cost $580.
In fairness to the Respironics people, I don't know if their equipment is specifically approved by the Therapeutic Goods Administration (the Australian equivalent of USAs Food and Drug Administration). The Respironics battery pack is promoted as being permitted for use on airlines, but as far as I can tell, so is the absorbent glass mat technology used in the Waeco battery pack. The official Respironics cable also comes with shielding - but shielding from what I don't know. I did note that the image depicts a ferrite bead filter fitted at either end of the cable. I can't see why this is important, but should the need become apparent, I shall be spending another $3.95 to fit some.

The only substantive differences between the two battery packs are weight: the Respironics weighs in at 2 kilograms and the Waeco is a whopping 13.6 kilograms, and the other difference is capacity: the Respironics is about 8 Amp hours and won't last the night with the humidifier in use while the Waeco has more than four times the capacity at 36 Amp hours.

Before leaving for the camping holiday, A tested the unit overnight and it performed flawlessly taking the Waeco unit to half of its charged capacity. In theory this means that this solution could be used for two nights running without the need to charge the battery, but we haven't tested that. The battery pack comes with a car charging cable which charges the unit as you drive, and two terminals at the top may be used to attach an ordinary car battery charger for charging from the grid. The battery pack also has a socket enabling it to be joined to a second identical unit, and I am confident that such a setup could supply power for at least three nights.


  1. I wonder what A's pressure is.

  2. A's pressure is 11cm H2O. Sorry for the delay in getting back to you.

  3. Well done! Thanks for that.

  4. I followed this advice...the Waeco power supply was only $280 on Ebay and I picked up the JayCar bits for a few dollars. On a full charge it powered my CPAP machine for 8 days (7 hours per day)....great advice...thank you

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  6. Could one connect two CPAP machines to this system? Both me and my wife use CPAPs and I wonder if we could get a full night out of one charge.

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