Discussion in 'General Electronics Chat' started by Germain, May 21, 2013.

  1. Germain

    Thread Starter New Member

    May 21, 2013
    I have a CPAP machine for sleep apnea and I'm trying to find a solution (not to expensive) to have a backup for 6 to 8 hours in case of a power failure.
    My machine draws 200 Watts.
    The fact that UPC for this would be way to expensive, someone suggested a 12 volts Sealed Lead acid battery (but did not tell me how many amps I should get), a 300 watts inverter and a DPDT relay. Anyone knows if this would work, if so, can you help me choosing these equipments. how many amps my battery should have, is a 300 watts inverter enough, what kind of DPDT I should get and how do I plug this all together to make it work.Any other suggestion would be appreciated.
    Thank you.
  2. MrChips


    Oct 2, 2009
    I would have suggested a UPS.

    If you just want to use an inverter, 300W should work. You have to make sure the inverter can power the CPAP. Presumably the CPAP converts AC line voltage to DC in order to drive a DC motor. I don't know. Check into it.

    If it draws 200W, that's about 1.6A @ 120V or 16A @12V. That seems to be a lot. Are you certain about the 200W?

    Batteries I have looked at advertized for CPAP are typically 12V 7Ah.

    Is there a huge concern if the mains power goes off while you are in bed? Can you not wake up and manually switch over to backup power?

    (I am looking into being prescribed for a CPAP as well.)
  3. Rbeckett


    Sep 3, 2010
    This is very close to violating the TOS since it is a life support issue. A Battery back--up system that is commercially available is the safest way to pursue this quest. Any stop gap or home built contraption may endanger you and your health. I would hate to think that a 3 cent chip failed while you were asleep and you suffocated. Please consider carefully what exactly you need done and see if your health coverage wont fund a good portion or all of it if you can prove that it is a medical need. Sorry, I too am disabled, but I don't trust non life support rated components and systems to be safe enough to rely on in my particular situation.

  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    First things first, isn´t sleep apnea that condition where you could stop breathing for a longer time and die? And your´re looking for a cheap backup? :confused: I say if someone´s life depends on it, don´t look for cheap solutions, but look for solutions that don´t cost much but add the most to reliability.

    On a more technical note, a standard UPS for example rated 500W when loaded with 200W will last for something between 5 to 10 minutes, depending on the age and condition of the battery. If you want to power that device during say a 8 hours long blackout, you will need a much larger battery that that, but anyway the average and not maximal consumption of the device will dictate how large battery capacity you need.
    A more typical solution for long blackouts is a combination of a UPS and diesel generator, where the UPS provides power for the short time until the backup generator starts.
    The soultion off course needs a way to start up the generator automatically, but there sure will be a way to arrange that.
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    All I know about CPAP will fit in a thimble. I was prescribed one once and tried it for 30 minutes, didn't like it (that's an understatement), and gave it up.

    That being said, I thought that CPAP was to prevent sleep apnea, and as such was only worn during sleep time. So, if the power goes off and the CPAP stops working, and the person wakes up, where is the threat to life?

    Maybe there is a possibility that the person wouldn't wake up if the CPAP stopped, and would then be in a life threatening situation? At any rate, if a UPS connected to a CPAP failed, wouldn't the person be at the same risk as if there was no UPS at all?
  6. MrChips


    Oct 2, 2009
    CPAP is not a life support system. CPAP is commonly prescribed for persons who suffer from obstructive sleep apnea, a condition where the airway becomes partially or fully obstructed and awakens the person. This results in disturbed sleep.

    Without CPAP the person would continue to experience sleep apnea and would be awaken when it occurs.
  7. timescope


    Dec 14, 2011
    I have no experience with continuous positive airway pressure machines and can only advise on battery sizing. Using a relay instead of a UPS would depend on how sensitive the machine is to brief supply interruptions. If it uses a switched mode power supply, a relay should work.
    200 watts @ 12v is 16.7 Amps. For 8 hours, 16.7 x 8 = 133 Amp-Hours.
    Deep cycle batteries should not be discharged more than 50% so the battery capacity you would need is 133 AH x 2 = 266 AH.

    The choice of inverter power rating would depend on the type of inverter. The cheap modified sine wave inverters have limited surge handling ability so a larger unit may be more reliable, 500 watts to 1000 watts. My pure sine wave inverter is 800 watts and I use it with my 750 watt blower.

    The DPDT relay coil should be rated for your mains voltage 120v or 240v.
    Connect the coil and normally open contacts to the mains Live and Neutral wires.
    Connect the inverter output to the normally closed contacts.
    Connect the machine to the common contacts.
    Use a 5 Amp to 10 Amp relay.

    If you need a diagram, let me know.

  8. crutschow


    Mar 14, 2008
    Certainly if waking up was a problem, you could add an alarm the sounds when the power goes off.
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    When my Cpap stops working I have always woke up. The fight for air has a way of doing that to you. :)
    MrChips likes this.
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    My Doctor, who specializes in sleep disorders and treats me for sleep apnea, has oft stated the only way you can injure someone with a CPAP machine it to sneak up behind them and bash them in the head with it. has complete battery backup solutions. one night $250, multinight $335. Do check if it plays well with your machine.

    Many machines run off 12VDC with an adapter inbetween that runs off most live voltages world wide.

    I once had my machine die on me in the Atlantic Ocean 2nd night of a 7 day cruise. I was banished from the stateroom for sleeping but had a decent time out on the balcony. 3AM saw the most amazing sight of a crescent Moon under a brilliant Venus. (Saw the same arrangement 2 weeks later and had to stop and start to understand why I could see it again.)

    I was a little tired few days after we got home as it took a while to hook back up with a replacement machine.
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
    CPAP machines can take 200 watts - they are more than simply the fan because the water tank is heated to humidify the air so you don't dry out and, on some machines, heat the supply hose so you don't have condensate dripping everywhere. The heaters propably take about 150 to 180 watts and the fan might take about 20 to 50 watts (depending on the pressure you run). If you can figure out how to run just the fan, you can run a much smaller battery.
  12. Papabravo


    Feb 24, 2006
    I have used deep cycle marine batteries from Wallmart to provide that sort of power level for those kind of times.

    The deep cycle battery is designed for continuous discharge as opposed to a typical automotive battery which is designed to run an engine starter for a short period at very high current.
  13. ramancini8


    Jul 18, 2012
    I used a CPAP machine for more than 10 years, and I continue to use one. I live in a high lighting strike area and hurricane area, thus we suffer many power outages during the year. I used to wake up when the machine powered down during an outage, and I stayed awake or tried to sleep in a chair until power came back---very uncomfortable, but not dangerous. I purchased a UPS for about $50.00 about 7 years ago, and the problems are gone. I suggest that a UPS will work well for you. During long power outages, weeks, I recharge the UPS from my gas powered generator with no problems.
  14. bertus


    Apr 5, 2008