Bi-PAP battery question - why is this cable SO expensive?

tonycog wrote:The text on the CPAP.com site claims that it is 12V. The machine says it is 24V. That is not straight. The original question I asked was about the ridiculous price for this specific cord, when other cords for Respironics machines are about $25.

The difference is that some of the older REMstar BiPAP units, such as the BiPAP AutoSV really do run on 24vdc, not just 12vdc. So, this unit must step the voltage upto 24v from 12v. The image shows the "box" that is inline to convert from one voltage to the next. That is why the unit costs $160 instead of $25. I spent the day trying to decipher exactly the same thing.

Hope that helps.

tonycog wrote:For the record, I just looked at the underside of my Bi-PAP.

If you machine is labeled 24v I don't think this cable is for your machine.

I found this at your link -

This is a 6 foot long cable that connects a DC-compatible CPAP machine to any 12 Volt battery source. One end attaches to the machine's 12 volt jack and the other plugs into a socket similar to a car's cigarette lighter outlet. The 12v jack that connects to the machine will fit several of our most popular CPAPs.

Maybe you want to call them to ask?

btw - my bipap uses the $25 dc cable.

Well if it is 24 VDC input then It need to step up the voltage from 12 VDC to 24 VDC and this is why it is more expensive than the regular cord.

lessismore wrote:That is way over priced, I can help I am an electrical engineer by trade, and I went to radio shack for an 12 vdc to plug to power my cpap machine, yes the 12 vdc does change under load, and rise when the engine is running, but I do not beleive they are regulating the voltage, do you have a picture of the cpap machine where you plug into to power from a battery.

Except for the fact the the machine in question requires 24V DC to run, not 12V. When I still did electrical engineering by profession that was quite a significant difference.

tonycog wrote:Thanks. That helps. But I checked the specs for the Bi-PAP AutoSV and it says this:

https://www.cpap.com/cpap-machine/bipap ... -cpap.html

"12VDC (when operated with the external DC power adapter accessory)"

But they still call for the expensive cord for it, just like the Bi-PAP you referenced. So I'm still puzzled that the cord they want me to buy is 6X more expensive.

Thanks,
Tony

As your quote says, it can be used with 12V DC when operated with the external DC power adapter. It requires 24V DC, and the adapter converts 12V DC to 24V DC, which is why it costs more than a cable that connects a 12V battery to a 12V CPAP.

Yes thanks 12 VDC is not 24 VDC that has been true for some time now.

I would not invert a battery DC voltage to an AC voltage to solve the problem, the efficiency to convert AC to DC is not very good. If the cable converts 12 VDC to 24 VDC , which can be done with 90% efficiency. Anyway depending on many nights you need, we can figure out many hours for the battery you select. I like this SEARS modules 18 AH since it is all enclosed, plug and play, but you will need a much larger battery. If this unit takes 24 VDC at 3AMP max, wow that is a lot of power, anyway you will need a 12 VDC battery of 40AH to last 8 hours, wow, that is pretty good size battery, it will last less when converting DC to AC due to the loss from efficency. The 3 AMP rating is max, the typical current will most likely be less. So I do not know why your unit needs so much power, my REMSTAR AUTO SERIES M will last 20 hours on a 18AH Battery, so that is drawing less than 1 amp at 12 VDC, your machine is stating you need 6 AMPS at 12 VDC or 3 AMP at 24 VDC, wow.

The advanced BiPAP machines use more power because they are doing more things to create the desired therapy. I seem to remember somewhere that they use a 'door' to create the bilevel pressure as opposed to modulating the motor speed. Or, perhaps, they do both.

You can find power converters designed for use with older laptop computers that will do the 12 to 24 volt conversion for a lot less than what Respironics is charging. I had to get a similar adapter for my older astronomical telescope, which requires 18 volts. That same converter could be switched for anywhere from 16 to 24 volts and cost about $50. It is also a lot more efficient than the OEM power converter. It is also regulated.

Another way to do this is use two 12 volt batteries in series to get 24 volts. For those machines, both batteries should be about 30 AH for a guaranteed night of therapy at foll power draw. Chances are very good the typical user will get two or more nights use out of the same batteries.

The standard disclaimer applies. Although I am sure enough that either of these schemes will work, use at your own risk.

lessismore wrote:the efficiency to convert AC to DC is not very good.

It is 90% (and often higher) efficient with today's off the shelf technology!

OK I seen DC to AC rated to 90%, but this at a certain load, if you read the fine print the efficiency will be dependent on the load. I have seen DC to DC converters 90% at 1 AMP load but 75% at say 100ma, so when you select a convertor make sure it is efficient at the current load that your CPAP will be drawing.

I promised myself not to post to this thread anymore.......