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  • Ups switch

    • Hi , having had 2 power cuts over the last week considering an uninterrupted power supply ( UPS) for my Dreammaker auto with humidifier and heated tube . I have a spare car battery so looking for the ups switchover mechanism . Has anybody gone down this route . The transformer is 12v with 7amp rating

  • #2
    Good Morning

    Thank you for your post.

    I would not suggest using a car battery with your CPAP machine in case it damages your machine and invalidates the manufacturers warranty.

    I would look for a CPAP battery that is compatible with your CPAP machine. I would normally suggest using a Tracer 12V Compact CPAP Battery Pack with your machine

    https://www.cpap.co.uk/tracer-compac...tery-pack.html - along with the relevant 12v DC cable to connect it - https://www.cpap.co.uk/respironics-12v-dc-cables.html

    However at present we do not have stock of the battery.

    Kelly

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    • #3
      Hi Kelly thanks for info , whilst the backup power source is going to be important it’s the UPS switch that needs to be sourced first

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      • #4
        Personally, I’d probably just buy the batteries Kelly mentioned as it’s less to worry about an you should get a guarantee, but here goes.

        DISCLAIMER: This is a general background as a starting point so you can figure out what direction to take your research. It is not an endorsement and if you do this, you assume all of the risk.

        If your machine has an official 12v cigarette lighter lead, and the documentation say it’s okay to plug it into a caravan or camper 12v supply, a car battery would obviously be fine.

        Just because it has an official lead, however, is not guarantee it’s safe. Cigarette ports average at 12v, but can fluctuate between 6v-14v so out-of-the-box working on any CPAP machine is NOT guaranteed. For example, since the sockets are common in cars, starting a car will drop it down the voltage down to 6v, and with an engine running it will peak to around 14v.

        It would be highly unlikely you’d have these conditions while using CPAP, so the engineers may assume tolerances could be much smaller like 9-13volts. Or worse, the lead may only exist as it is because off the shelf parts are cheap. So the engineers may assume the power source is always regulated, potentially making your device very sensitive to over-voltage. Either way, you can’t assume the machine will work well nor can you be sure it is safe.

        If your CPAP manufacturer sells a 12v lead, I’d email them and ask if it’s fine to run on the caravan’s 12v.

        A car battery also isn’t great as ideally you’d use a deep-cycle battery. Assuming it’s a standard lead-acid, you’re not suppose to discharge it below 50% capacity or you will seriously shorten the life of it. 7 amp at 8 hours is 56Ah. That means your battery would need to at least be 110Ah for the worst-case use, and that may only work for a few months before the battery is junk. (if it’s just a backup, though, it would probably last a decent time)

        Also, as batteries discharge the voltage starts to drop too. Your machine could turn off is it drops too low even if there’s plenty of energy still in the battery. If this is the case, you would need a DC-DC converter.

        I don’t think I’d ever bother with an UPS circuit. I’d probably opt for a caravan BMS and have a trickle charge. It’s much simpler.

        Seriously though, if i were to do this, I would need more information from my CPAP manufacturer OR I would need to take an oscilloscope to the original PSU and check the output of my cigarette socket to try to minimise any unexpected problems.

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        • #5
          I forgot to mention it, but you should definitely have a fuse inline to stop over-current fault conditions. An automotive fuse rated at 7 amp would work fine.

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          • #6
            Thanks for fuse advice , just ordered an ups switch which will be linked to spare 12v battery , https://www.ebay.co.uk/itm/193574022553

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            • #7
              I would be very careful when buying chinese components as a lot of listings usually lie about specs. For example, that listing states 12v at max 10amps. That works out to 120w, but the listing says 100w. The numbers don’t add up, so be careful. It’s probably just a glorified voltage sensing relay. (which isn’t bad. but would need to support the correct amp/voltages) If the true amp/watt is lower than advertised, expect the item to get hot and probably melt if you run it at 12v 10A. Hopefully it will be fine.

              Also, you wouldn’t be able to fully charge the battery from a standard CPAP power supply. 12v will charge a 12v battery, but to get it fully charged you’d need something like 14v. Normal PSUs usually only put out 12 volts.
              Last edited by Reno; 21 June 2021, 17:44. Reason: Fixed typo

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