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Old 01-17-2018, 11:15 AM   #1
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CPAP / APAP 12v and 24v conversion ?????

Long-time camper but new to the CPAP. Still on a trial period and will have to purchase a machine soon as DW really appreciates the decreased noise in the bedroom. Will also need if for camping, frequently without shore power so I will need the battery capacity to run the CPAP for 1 to 3 nights before firing up the generator to recharge batteries.

I would prefer to not deal with the inefficiency of an inverter. Couple of models I am looking at one that will run on 12v DC, the other will run on 24v DC. The portable battery packs are ridiculously expensive with very limited run duration. An additional deep cycle battery or two strapped to the camper would be a much more cost effective solution.

I know the difference between hooking batteries up in parallel and series, but would it be possible to the same batteries for the normal 12v system on the camper and connected to a 24v outlet to use with the CPAP ? Could it be accomplished with a switch ? Would you connect the batteries 2X12v or 4X6v in series and then somehow reduce it to 12v for connection to TT with 24v running to an outlet in camper ?
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Old 01-17-2018, 11:36 AM   #2
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if you like the 24VDC system you can get a DC to DC BOOST converter (12>24VDC). Wire 2 in parallel and you will have 6 amps output. Any way you look at it the CPAP uses 2 amps without the humidifier and about 5 amps with the humidifier (I have one). Do the math.. 8 hours of sleep @ 5Amps (with humidifier... only way I can use mine)... 8Hrs * 5Amps = 40Ah's.

A standard size 24 battery is around 85Ah, but wait, you can only use 40Ah's of the 85 (battery 50% rule). So with 1 85Ah battery, you can use the CPAP for 1 night. And that is not running anything else. Add a second 85Ah battery and you will get 2 nights, without anything else (heat, lights,,....)

So, if you upgrade to a size 27 battery you will probably jump to 100Ah x 2 = 200Ah of which you can only use 100Ah. One size 27 still only gets you 1 night.

You want to go 3 days without charging, you will need to add a 3rd battery, but you may be running the generator all day on the 3rd day to get back to an 80% charge.

Another option is to get 2 separate size 27 batteries and wire your CPAP directly to them, but then you have 2 sets of batteries that would need to be charged separately on the 3rd day.

Why can't you charge them each day?

Or you could add 300 watts of SOLAR on the roof and that will care for your daily charging need, as my SOLAR does.

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Old 01-17-2018, 02:50 PM   #3
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DC/DC converter is the ticket there. For dry camping, I bought a DC cable from my medical device supplier that got it from the manufacturer. I went straight to the camper batteries, two 12V in parallel. One problem I had - when the heater would kick on, I got a low voltage alert on my CPAP (actually an ASV) and it shut off. I now have a dedicated 12V battery mounted up just for the CPAP and it works well.
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Old 01-17-2018, 05:03 PM   #4
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Quote:
Originally Posted by Mustang65 View Post
if you like the 24VDC system you can get a DC to DC BOOST converter (12>24VDC). Wire 2 in parallel and you will have 6 amps output. Any way you look at it the CPAP uses 2 amps without the humidifier and about 5 amps with the humidifier (I have one). Do the math.. 8 hours of sleep @ 5Amps (with humidifier... only way I can use mine)... 8Hrs * 5Amps = 40Ah's.

A standard size 24 battery is around 85Ah, but wait, you can only use 40Ah's of the 85 (battery 50% rule). So with 1 85Ah battery, you can use the CPAP for 1 night. And that is not running anything else. Add a second 85Ah battery and you will get 2 nights, without anything else (heat, lights,,....)

So, if you upgrade to a size 27 battery you will probably jump to 100Ah x 2 = 200Ah of which you can only use 100Ah. One size 27 still only gets you 1 night.

You want to go 3 days without charging, you will need to add a 3rd battery, but you may be running the generator all day on the 3rd day to get back to an 80% charge.

Another option is to get 2 separate size 27 batteries and wire your CPAP directly to them, but then you have 2 sets of batteries that would need to be charged separately on the 3rd day.

Why can't you charge them each day?

Or you could add 300 watts of SOLAR on the roof and that will care for your daily charging need, as my SOLAR does.

Don

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RVing with SOLAR
Thanks so much for the info on the DC Boost Converter to get to 24v. I am sure the info is available on the web somewhere but it would have taken a long time to find. My math is not strong so the AH calculations are helpful as well. I wonder whether a 24v system would be less prone to low voltage alerts mentioned by Tom51.

I currently have 2 X 12v Deep Cycle (I think Group 27) batteries that were 110Ah when new but they are going into their 8th season which is twice as long as I have had my BHBE. The batteries have been sufficient to run water pump, LED lights and even limited furnace fan night use for 3 or 4 days without hooking up generator, but I question how they would hold up to a CPAP every night.

I am looking into new batteries this spring and a couple of extra T105's would be less expensive than one 12v plus a CPAP battery. If my math is correct, if I had 4 X T105's @ 220Ah each I would have 440Ah at 12v of which 220Ah would be usable according to 50% Rule. Not going without humidifier unless I absolutely have to, so presuming the CPAP I get draws the same 5amps for 40Ah per night, I should be able to go 3 or 4 nights without the necessity of starting the generator. We do a lot of 4 to 5 trips during summer so I can either run the generator the 4th or 5th day or we are home plugged into shore power.

I would prefer to avoid having to charge up batteries every day to ensure the longevity of a new battery pack. A lot of the State Parks we go to have fixed hours for generators. Back when I just had a single battery, I hated having to arrange my day around the generator hours ie. have to be back by 4:00 pm to fire up generator for 3 hours or waiting to shut off generator at 11:00am before going kayaking to make sure batteries were charged up.

You solar install would be ideal, but I little too complex for my limited abilities. Our current needs do not make it practical enough to justify the time and expense but if we had as much sunshine here as you do in Florida I would definitely place it higher on the priority list. I am currently looking into portable solar for kayak camping, not for the CPAP, but that is for another thread.

Thanks again for the great info !
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Old 01-17-2018, 05:18 PM   #5
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Check with the mfr of the CPAP machine to see if they already have a 12VDC plug option for their machine. I know some mfr do have them... as an option of course.

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Old 01-17-2018, 05:39 PM   #6
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My Cpap machine came with a 12 volt adapter which I connected directly to the batteries along with a 400 watt inverter which I rarely use. I have 200 watts of solar on the roof connected to two 220 amp batteries. I have gone many days and never got the batteries to halfway. The only time I got close was in 17 degree weather for two days and no sun, the furnace uses lots of battery. Left there and driving in the sun the batteries came back to full off of solar and the truck.
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Old 01-17-2018, 05:50 PM   #7
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The two models my CPAP vendor is recommending are the Phillips and ResMed. The Phillips will run on 12v with a proprietary cable you can buy and plug into any cigarette lighter type 12v outlet. The ResMed apparently runs on 24v or you can get a proprietary 24v battery for about $400. I do not think the ResMed battery would have as long of a lifespan as $400 worth of deep cycle batteries that are properly maintained.

If anybody is interested, here is a link to the Technical Services Battery Guide from ResMed which has some good information on inverters, converters (more efficient than inverters) as well as the current draws for the various ResMed machines and estimated battery size for 8 hour use including a 50 % safety margin endorsed by battery experts, including those here on JOF.

https://www.resmed.com/us/dam/docume...de_glo_eng.pdf

I would imagine the current draws would be comparable for other brands similarly equipped. It is interesting to note the different draws depending on different pressure and humidifier settings.
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Old 01-17-2018, 07:15 PM   #8
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Here is a 12volt > 24Volt voltage booster and it is 8 amps. No need to purchase (2) 3 amp models.

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MISC - 12VDC - 24VDC - 8Amp.jpg  
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1500 Watt Ramsond PSI, 2 Trojan T145 Batteries (260Ah)
2 - AirSight Wireless IP Cameras (used as rear view cameras)
EnGenius WI-FI extender, D-Link wireless (n) modem
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Old 01-17-2018, 10:27 PM   #9
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https://www.resmed.com/epn/en/health...w-output-.html
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Old 01-18-2018, 05:16 PM   #10
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So, I just ran into this. Sounds like others have their fixes. I haven't fixed mine.

I am replying because I have the Resmed Airsense 10. I too have a generator to recharge the batteries. I bought the ridiculously expensive 24v to 12v adapter. I have 2 Costco 6V batteries in series for batteries.

My first trip out last month I was able to get through the night, but like others have said, late during the night when the heater would kick on (it was really cold) I would wake up because the CPAP would reboot. I can't be sure that it was the heater every time as I swear I caught it rebooting before the heater would come on. My batteries were full when we went to bed (as close as I could get them, but we did use lights for a while after turning off the generator).

My plan was to add another set of 6v batteries in parallel to get the capacity I need. 90% of the time we are plugged in at a campground so I surely don't want to do that if I am not going to fix my problem while boondocking.

Can others confirm I should be ok? I think by adding the second bank I should discharge at ~50% less right during the night?
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