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Old 11-13-2019, 05:43 PM   #1
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Efficiency - for my CPAP: DC to DC converter (12v to 24v) vs. 1000W inverter

I have a ResMed AirCurve 10 CPAP machine. Currently only have a 120v cord w/ brick. As far as I've read on the bottom, it's a 24v machine.

I'm wondering if a DC to DC converter like this would be more efficient than my existing 1000W inverter:
https://www.amazon.com/ResMed-AirSen...dp/B07LGBMZ6Z/

I have the 1000W inverter already wired up and it automatically makes all of my outlets hot. But, I'm wondering if there's efficiency to be saved in installing a 12v outlet and plugging the machine into that instead?
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Old 11-13-2019, 07:38 PM   #2
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It should definitely be more efficient. It's how I would do it....
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Old 11-13-2019, 07:49 PM   #3
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It should definitely be more efficient. It's how I would do it....
Someone on Facebook mentioned that it's absolutely more efficient going DC to DC vs. DC to AC (inverter) to DC (the brick on the CPAP cord).

Made sense that I'm double converting it.

Now to get off my bum and actually wire up an outlet. If I do that, I'll also add some 12v USB chargers. We have them in the outlets but again, efficiency says it'd be better to go without the inverter.
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Old 11-13-2019, 08:21 PM   #4
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This should give you all the info you need.

https://www.resmed.com/us/dam/docume...de_glo_eng.pdf
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Old 11-13-2019, 08:24 PM   #5
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Inverters loose 10-20% in the conversion. Depends on the quality of the device.

Then the second conversion ac to dc again looses.

I would assume going from 12 volts to 24 there would be a lot less power loss.
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Old 11-13-2019, 08:45 PM   #6
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This should give you all the info you need.

https://www.resmed.com/us/dam/docume...de_glo_eng.pdf
That document is awesome- doesn't have my specific machine, but gives me useful information none-the-less. Opening it twice in separate windows, I was able to compare the same device on DC-to-DC vs. DC-to-AC and see the difference.
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Old 11-13-2019, 08:59 PM   #7
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I didn't even notice it was you when I replied...

Anyway, yeah, inverter has losses even running nothing. We even have tylt wireless chargers on our nightstands and their "wall warts" put out 12v, so I cut off the end and wired into some 12v lines I ran. I also got a 12 to 7.5v DC to DC converter for the baby monitor in my daughter's bunk (we were burning through AAA batteries at an astonishing rate). Anything that can be 12v, should be.

I'm not saying I haven't run the inverter overnight to charge my phone, but......
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Old 11-13-2019, 09:02 PM   #8
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Inverters loose 10-20% in the conversion. Depends on the quality of the device.



Then the second conversion ac to dc again looses.



I would assume going from 12 volts to 24 there would be a lot less power loss.
To be fair, the AC to DC part is pretty dang efficient with switching (everything these days) supplies. Not like the old transformers that spewed out heat.
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Old 11-13-2019, 09:12 PM   #9
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Why not this at 1/3 the price: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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Old 11-13-2019, 09:53 PM   #10
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How long you think a jumper box could run a cpap . Itís 35 miles of dirt road to hunting lease. So I donít take 5th wheel just sleep in bed of truck. CPAP would have been nice or when we dry boondocks wonít run down trl battery
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Old 11-13-2019, 10:06 PM   #11
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I like RBDtx's suggestion. Moose, I never tried a jump box. Depends on how many amp hours it provides. Before I got my RV, I would hook up a 12v receptacle with alligator clamps to a marine battery I bought from Tractor Supply, and plug my 12v cord from Phillips CPAP into it.

Now I still use the 12v cord, but installed a DC outlet next to my bed so I don't have to use the inverter. They do sell 12v to 24v step up converters on Amazon pretty cheap.
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Old 11-13-2019, 10:12 PM   #12
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Quote:
Originally Posted by ependydad View Post
That document is awesome- doesn't have my specific machine, but gives me useful information none-the-less. Opening it twice in separate windows, I was able to compare the same device on DC-to-DC vs. DC-to-AC and see the difference.


You might visit here also.
This guy has done extensive hands on testing WRT CPAP and battery usage.

http://www.cpaptalk.com/viewtopic.ph...30596#p1130596
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Old 11-13-2019, 11:06 PM   #13
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This DC-DC unit is the way to go. Easy to add a 12v DC outlet near the bed if you don't currently have one.

Strangely enough my Resmed manual says to use an inverter and the stock AC cord, but I agree, that is the least efficient way and when boondocking, every watt matters.
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Old 11-13-2019, 11:27 PM   #14
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Strangely enough my Resmed manual says to use an inverter and the stock AC cord, but I agree, that is the least efficient way and when boondocking, every watt matters.
So as not to confuse the general public. The majority of whom are as dumb as a box of rocks.
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Old 11-14-2019, 12:09 AM   #15
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FWIW, I've found the lighting circuits to have large (12ga) wire and large enough (15A) fuses. I've just tapped into existing circuits for everything I've done. I've added half a dozen 12v outlet pairs. They run fans, chargers, a "stick" computer, etc. Haven't blown a fuse yet. I'd have no worries about a CPAP outlet running off a lighting circuit.
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Old 11-14-2019, 12:18 AM   #16
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We both have Resmed S9 machines. I ordered two dc to dc convertors for our machines. Ran dedicated 14 gauge cable directly to main DC distribution and fused each at 15 amps. the calculated full load at highest humidifier setting and creating back pressure was 12.5 amps at 12 vdc. Now this was worst case but providing reserve was key. I have two 6 volt Trojan batteries. One night we lost AC power at 11 pm. No need for Air Conditioning so i didn't bother with the generator. Ran both CPAP, Residential Fridge and water pump till the next day. Really worked well as a CPAP UPS not to mention surge protection and isolation from AC power spikes or surges on DC float.

Efficiency of an Inverter is maybe 60 % on the low side. Conversion loss AC to DC isn't that bad. DC to DC is the best way.
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Old 11-14-2019, 10:30 AM   #17
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Technically, a DC-to-DC converter that has a higher output voltage than its input voltage is an inverter internally.

An inverter as RV'ers use the term takes DC as the input and converts it to approximately 120 volts alternating current (AC) at 60 hertz (cycles per second) with either a sine wave or a variation thereof. The frequency is very closely controlled.

A DC to-DC converter that increases voltage is a device that very rapidly switches the 12 volts DC input on and off rapidly, converting it to alternating current. The switching frequency is far, far higher than 60 hertz.

That rapidly switched AC voltage is applied to a transformer, coil (inductor) or some other type of storage device to boost the AC voltage. That stored voltage is rectfied, filtered and regulated back to DC.

Why does it work this way? There is no way to increase DC voltage directly like a transformer does for AC voltage.

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Old 11-14-2019, 10:32 AM   #18
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So as not to confuse the general public. The majority of whom are as dumb as a box of rocks.
Did you perhaps mean "not trained or educated on the intricacies of electricity"?

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Old 11-14-2019, 02:42 PM   #19
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Hmm...

Quote:
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Why does it work this way? There is no way to increase DC voltage directly like a transformer does for AC voltage.

Ray
Well, not sure whether a switched-capacitor voltage doubler counts as AC or DC. There's definitely a frequency involved, but no alternating current.
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Old 11-14-2019, 04:32 PM   #20
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Well, not sure whether a switched-capacitor voltage doubler counts as AC or DC. There's definitely a frequency involved, but no alternating current.
Any frequency above zero is alternating and there has to be some kind of current, right? Just put a radio frequency interference monitor or a spectrum analyzer next to it and you'll see all kinds of RF (radio frequency) garbage generated.

That's why those devices have to have that FCC certification statement on the label, to prove they've been tested and their emissions are within regulatory limits. https://apps.fcc.gov/oetcf/kdb/forms...tch=P&id=20450

It's definitely not "alternating current" in the context of house power, though. I've had to track some of this rubbish down during my career when it caused harmful interference. Unfortunately some manufacturers just print the statement on the label without testing. Or don't even bother printing the statement and just sell it.

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