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Camping using Converter
#11
There is an insignificant inefficiency in stepping up DC voltage, but it is not anything like the inefficiency of a DC to AC inverter. A number I have seen is greater than 85% efficiency. The main disadvantage is the cost of the proprietary connector and the bulk of the converter rather than just using a simple connector cord. If I recall from an earlier discussion, the Resmed power supplies deliver a small current measured in Ohms to the center pin. Without that signal the machine will not operate on an unauthorized power supply. If all we had to so was step-up from 12-24 volts, that would be pretty easy.

I have a Philips Respironics BiPAP backup machine and use that on straight 12V power if I use a battery.
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#12
Code:
current measured in Ohms...

Current is measured in Amperes (with or without a multiplying prefix such as "milli")

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#13
(04-26-2017, 09:37 AM)justMongo Wrote:
Code:
current measured in Ohms...

Current is measured in Amperes (with or without a multiplying prefix such as "milli")

Yes, ohms measure electrical resistance
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#14
sorry, i am not a technican, but i wonder instead of having the 37297 Resmed dc/dc converter, why not to use a common converter/inverter car-battery 12v to 110VAC or in my case (european) 220VAC and plug in the normal power supply of your unit (airsense 10 autoset)? So you could have extra power sockets for other things also, or USB sockets?
This should be much cheaper and I can not imagine that the Resmed units recognize what is behind the power socket....
I will have this problem soon for travelling, so I am looking also for a solution...
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#15
(02-07-2018, 03:57 AM)christian4870 Wrote: sorry, i am not a technican, but i wonder instead of having the 37297 Resmed dc/dc converter, why not to use a common converter/inverter car-battery 12v to 110VAC or in my case (european) 220VAC and plug in the normal power supply of your unit (airsense 10 autoset)? So you could have extra power sockets for other things also, or USB sockets?
This should be much cheaper and I can not imagine that the Resmed units recognize what is behind the power socket....
I will have this problem soon for travelling, so I am looking also for a solution...

It can work, but it's exceptionally inefficient and greatly reduces the effective capacity the battery would otherwise yield.  That's why I personally advise to never use inverters.

Using an inverter in this situation takes nice steady 12 V DC electricity, burns up a bunch of it to turn it into 220 V AC electricity, then the standard Resmed converter plugged into the inverter has to turn that 220 V AC power back down into DC like you started with - for some, 12 V DC, for Resmed users (S-9 and later) 24 V DC.

Using the Resmed DC-DC converter directly, you don't have that power loss.  The conversion from 12 V DC to 24 V DC is only a very slight loss - negligible.

If you go to Resmed's Battery guide, it has sectionis for both the use of an inverter and their DC-DC converter.  You will see the current draw, and required battery size to run for a given amount of time is much greater if using the inverter. 

https://www.resmed.com/us/dam/documents/...lo_eng.pdf

OMMOHY
There.  I said it.

OMMOHY
Contrarian in Residence  
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#16
(02-07-2018, 06:54 AM)OMyMyOHellYes Wrote: If you go to Resmed's Battery guide, it has sectionis for both the use of an inverter and their DC-DC converter.  You will see the current draw, and required battery size to run for a given amount of time is much greater if using the inverter. 

https://www.resmed.com/us/dam/documents/...lo_eng.pdf
I understand what you mean, but I compared the Converter and the inverter for a Resmed S9 auto, because i did not find the S10 autoset to compare.
For the S9 i find at pressure of 16 mbar (i have 15) and no EPR, no humidifier, not heated (i do none of it) a value of 15 ah for 8 hours with the inverter and with the coverter 11 ah, which is about 30% less.
I think this means, if my battery has - lets say 20 AH (amp/h) I can easily charge every night, als long as i reload the battery every day? Is this correct, or do I make any mistake?
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#17
Don't know. If you look only at the Air 10 section for the DC-DC converter section of the battery guide, how does the current draw (amps) used by the S-9 compare to the Air 10? If they are reasonably close at your temp/humidity settings, then you should probably be able to estimate usage for the inverter.

For DC-DC with your settings with no heat, it shows the S-9 drawing 0.9 A and the Airsense 10 drawing 1.23 A so the Airsense 10 seems to draw about 35% more current at those settings. (15 Ah X 1.35 = 20 Ah). Those numbers include the 50% "safety factor" for battery degradation over time or imperfect charge condition when you start using it. For what it's worth, in my personal testing at my settings, I found the current draw in the Battery Guide is overstated for what the S9 and S8-II Autosets actually pulled. I think they have to be conservative in their presentation.

OMMOHY
There.  I said it.

OMMOHY
Contrarian in Residence  
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#18
Thanks. There is also a rule not to empty a accu/battery completly. Means, if i need 20AH for a night, which size do the battery has to be?
I think 20AH would be too less?
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#19
(02-07-2018, 08:41 AM)christian4870 Wrote: Thanks. There is also a rule not to empty a accu/battery completly. Means, if i need 20AH for a night, which size do the battery has to be?
I think 20AH would be too less?


You'd need a 35 - 40 AH battery minimum in that case.
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#20
I have two 6 volt deep cycle golf car batteries in my camper trailer with a capacity of 232 amp/hour. My Airsense 10 Auto uses about 5amps per hour with the heated tubing and humidifier switched off and running via the Resmed DC-DC converter, so allow for 40amps overnight. I have a camper that can connect to mains electricity as well, so if it on mains power the CPAP runs as it would at home. If camping away from mains power I have 400watts of solar to charge the batteries back up, and that is usually only for a few days at a time.

James
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