Using a DC 24 volt CPAP while boondocking

[Sweenja1]

Looking for some help as to what I need to operate 2 - DC 24 volt CPAP machines while boondocking. Both machines currently use a 90 watt AC adapter to operate and draw 3.74 amp each at their peak. I currently have 2 - Interstate 29M deep cycle batteries run in parallel. What do I need to do to be able to use these when not connected to shore power. I currently do not have any DC outlets in my Bighorn. I plan on using the 2 - Honda 2000 inverter generators in parallel to recharge house batteries.

Any suggestions would be greatly appreciated.

 

[TandT]

Sweenja,
I have several friends who use cpaps and boondock at times.
I have an on-board Onan, but I dislike using fuel and making noise just to recharge my batteries. Obviously I do use it to run A/C or microwave etc. I have a solar set-up mounted on my roof just to recharge batteries.
Consider getting a solar set-up to recharge your batteries. Systems have become very affordable and after installation, they are silent and free. Typically if it is set up correctly, it will recharge your batteries in about 8 hours of daylight.

After shopping for three years, I finally settled on this place for my solar; RENOGY SOLAR

 

[Oregon_Camper]

Will this be a one time thing (bookdocking) or will you want a permanent solution?

Is price an issue?

 

[Sweenja1]

I'm looking for a permanent solution and one that is cost effective. Looking for something that we could boondock 2-3 days at a time before going back to shore power. 5 days boondocking would be the absolute max and I think that would even be rare.

 

[Jim.Allison]

I'm concerned about the 24 volt aspects, I would encourage you to check that voltage rating for that appliance.

I too have a CPAP that has about a 4 amp rating at 12v DC. which is .4 amps at 120v. Somehow I don't think your specs are adding up.

Could you please provide your brand and model number and I can answer your question fairly accurately. I run a single CPAP with heater and hardly use any of my battery capacity.

BTW, a small solar system will automatically care for your batteries, provide the power you need, eliminate the need to disturb others with your generator which is incapable of fully charging your batts, for a very small price.

 

[ICamel]

More questions for you Sweenja1:
What make and model of CPAP machines?
Do you use and need a humidifier?
Will your CPAP machines run directly off of DC power?
I presume the two batteries you mention are for powering the 12V trailer system?
I have two 12V batteries(besides the RV batteries) located under the bed which are dedicated to power a 12V to 120V inverter. This is not the most efficient method but for various personal reasons what works best for me. I can sleep at least 4 nights without having to recharge these batteries. The RV is parked next to our house and plugged into shore power when not out traveling. A smart charger on the two dedicated batteries keeps them ready for my next trip. If I need to recharge those batteries while camping I will run my Honda EU2000i generator during the day.

 

[RoadJunkie]

I am lucky because I have a 12v CPAP unit. But, I also have a 24 volt unit that I researched for camping. To use the same batteries for both house and CPAP you would have to somehow rewire your batteries, in series, and isolate them from the house system so that you don't apply 24 volts to the house. Too much trouble, clearly. So, you can purchase a couple of 12 volt CPAP machines or buy the separate battery units as sold by your CPAP manufacturer. Finally ,you can run the 24 volt units through an inverter.

Do do you have residential refer in your 2016 rig? If so, unplug refer at night and use that inverter.

 

[Jim.Allison]

If he has 2, 12v batts in parallel, then he has 12v only and if he is using common RV/Marine deep cycle/starting batts, he has about 210 amp hours or 105 available/usable amp hours.

At 3.74 amps at 12v he should have about 3, 8 hour nights of sleep time or more, perhaps 4 or 5 without recharge. Which sounds about right. Plunketts law states that the slower you discharge a battery the more amp hours it will provide. Consulting the battery data sheet can show the actual amp hours available when discharging at 3.74 or less.

One should realize that charging batts through your charger with a generator is a long process. The bulk charge is accomplished fairly quickly, but the finishing charge comes very slowly and is responsible for about 75% of the generator runtime. Much of the capacity of a batt is forced in in the later stages of the charge curve, a generator is running in order to pump in charge at 2 amps. This is a waste of fuel, and creates unnecessary disturbances. Running generators is considered bad form, and is becoming more and more unacceptable.

Solar is much cheaper and more efficient at charging batts. A small 300 to 400 watt solar charging system can be purchased for between $500 and $900.

I boondock and never use my generator UNLESS I'm using the air conditioning, for me its solar only, with generator back up.

As far as the 12v vs 24v CPAP, I think he has a converter that allows his CPAP to run on either 12 or 24v.

If you think about it, unless you are going to have a generator large enough to run your air conditioner, then what is the point, forget the generator and buy solar, about 400 to 500 watts of solar and about 450 amp hours of batts will provide you with everything you need.

 

[Bohemian]

The simplest thing might be to install a 12V to 24V converter just for the CPAP. That way the rest of the rig needs no change from the standard 12V system of batteries, converters, etc. Just one small simple, inexpensive converter.



http://www.amazon.com/Converter-Reg...41060475&sr=8-2&keywords=12V+to+24V+converter

 

[Sweenja1]

The CPAP machines are ResMed Mirage S9 Series. We do use the humidifier. The machines do not run directly off DC power, they have an AC cord attached. The label on the machine states it is 24 volt with 1.25 amp (AG) and 3.75 amp (Sys). The batteries I want to use are the house batteries which are wired in parallel. I have a total of 420 hours in Reserve Capacity between the two batteries.

 

[Jim.Allison]

The CPAP machines are ResMed Mirage S9 Series. We do use the humidifier. The machines do not run directly off DC power, they have an AC cord attached. The label on the machine states it is 24 volt with 1.25 amp (AG) and 3.75 amp (Sys). The batteries I want to use are the house batteries which are wired in parallel. I have a total of 420 hours in Reserve Capacity between the two batteries.

You need this to run off of a twelve volt system.

http://www.cpapsupplyusa.com/36970-ResMed-S9-DC-Converter.aspx

Install a cigarette lighter adapter plug near your bed, make sure you use a large enough gage wire so that you don't have power loss. With this device you can run off a 24 volt supply or 12v supply, heater or no heater. Power rating for appliances can be misleading, many times they label the appliance for way more than they actually consume. 3.75 amps is a small power drain, but your usage is more likely half of that, perhaps 1.5 amps. A pair of healthy interstate RV/Marine batts hooked in parallel to provide 210 amp hours at at the 20 hour rate. At your stated load they will last about 37 hours and leave you with 50% discharge (you should not discharge your batteries to less than 50% very often, you should never go below 20%) Your appliance probably uses much less than 3.75 amps, but lets say that it uses 2 amps. Peukert's law calculates your battery run time at 79 hours and 8 minutes and still maintain 50%.

The upshot is that you should get between 4 to 8 days from your system, depending on how accurate your advertised amp draw is.

Doing this correctly will prevent you from running a generator, and make it more feasible to run solar. If you want to know how the Peukert's law works, I can send a link.

Good luck

PS reading your post more closely, I see you have way more amp hours available and you have two machines. I will work on this a little and post the calculations again. But you need that charge cord adaptor that allows you to un off of 12v. See the link.

- - - Updated - - -

What kind of batteries do you have? I see where you say you have 420 hours of reserve capacity, but that is not enough information.

Tell me how many batteries and what kind they are, and I will tell you how much capacity it will take to run your appliances for 8 hours of sleep.

example, you have 420 amp hours, only 50% is usable (battery life diminishes after regular discharges beyond 50%) so you only have 210 available at the 20 hour rate. So you can see how this gets involved.

So if you could provide me with the number, brand and style of batteries you have in your rig, and I will get you a figure that you can work with. See the link in my previous post.

 

[Sweenja1]

I have 2 of the Interstate SRM-29 deep cycle batteries. The king bed slide has an electric motor that runs off the 12 volts system. Is it possible to tap into those wires to install the cigarette lighter type outlet?

 

[Jim.Allison]

I have 2 of the Interstate SRM-29 deep cycle batteries. The king bed slide has an electric motor that runs off the 12 volts system. Is it possible to tap into those wires to install the cigarette lighter type outlet?

I did that and I'm not satisfied with the wiring. I think a dedicated line with a dedicated blade fuse near the battery is better. I blew a fuse the other night on my CPAP DC outlet. I'm still not clear why. I cannot advise on that until I get to the bottom of my failure. I will say that I suspect that the cigarette lighter adaptor cord may not have been plugged in completely and blew the fuse as a result of resistance created.

Now if you are running two machines off of this pair of batteries, then this is the range you can expect;

Using Puckett's Law your batteries will run both machines considering 3.74 amp draw each for 15 hours and 47 minutes and still maintain 50% of the capacity of the batteries.

However it is not likely that your appliances use 3.74 amps each. It is more likely that they use about 2 amps each. Using Puckett's law you can determine that 2 machines at 2 amps each will use 4 amps. At that rate of draw your batteries will power your machines for 33 hours and 59 minutes and maintain 50% capacity.

This comports with what I have witnessed with my own CPAP.

Clear as mud right?

 

[Sweenja1]

Actually it is very helpful.

 

[ICamel]

Easiest option is the Resmed #36970 DC/DC Converter.
http://www.resmed.com/us/dam/docume...10-s9-dc-converter-90w_user-guide_glo_eng.pdf

The CPAP machines are ResMed Mirage S9 Series. We do use the humidifier. The machines do not run directly off DC power, they have an AC cord attached. The label on the machine states it is 24 volt with 1.25 amp (AG) and 3.75 amp (Sys). The batteries I want to use are the house batteries which are wired in parallel. I have a total of 420 hours in Reserve Capacity between the two batteries.