Cpap, 12 volt frig and furnace- run all overnight on trailer batteries?

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ronthet

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Ok, I have a question for anyone running these three things off trailer battery. Here the deal. I have two 81 AH interstate srm-24 deep cylcle battery on my trailer. I have an 80 watt Go Power Solar. Not much I know. What I am going to tell you is best case I believe.

I have a norcold 12 volt frig only, so it has to run off battery while traveling or boondocking. The youtube video Forest River put out with a rep from norcold states the frig will run for 40 hours on one standard coach battery (72AH).

I need to run two cpap machine without humidly and heated hose. My power supply says 2 amp on 1 machine and 1.5 on the other. I assume power will be less with those items turned off.

My furnace if needed I am not sure what it draws. Cannot find that in my documents and right now it's in the shop for warranty work 70 miles away.

So again two 81 AMP batteries, 80-watt solar panel, run two cpap, 12-volt frid, and furance overnight. Not for days but I just maybe want to stop one night in a rest area, walmart etc to save a little camping fee. It would be for only one night and back on the road.

Does anyone think I can run all that if I install two 12 volt outlest in the bedroom.

I know all about the power stations, but spending 500-600 dollars there, maybe better to spend the 500-600 dollars on batteries which I assume would be quite a bit more AMp Hours to get me through the night.
Thanks if you take time to read and respond
 

Ray-IN

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Assuming your 12V fridge runs constantly, it will draw approx. 1.8A/per hr., assuming that rep is correct.
3.5 A/per hr. for the CPAP's, added together is 5.3A/per hr. So, without running the furnace you will be OK.
The LP furnace however is a big draw appliance. Rather than re-invent the wheel, this website fully-explains amp-draw and the method of calculation. How Many Amps Does an RV Furnace Draw? - Survival Tech Shop

Bottom line, running the furnace all night requires more total battery amps than you have now.
 

Kirk

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Does anyone think I can run all that if I install two 12 volt outlest in the bedroom.
With something as important as most cpap machines are, I would think that you would want more than just our opinions. The power needed by an RV furnace is a very subjective thing since it all depends upon how often and how long the furnace runs and that is determined by the temperature it is set for, the temperatures outside, and the quality of insulation of the RV. To say more would be just a guess. While I do agree with Ray that it would probably be fine if you don't need the furnace but if you do my advice would be to stay where you have a 120V power supply available.
 

CharlesinGA

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For the cpap get a portable power station such as the Bluetti. They are just lithium batteries with a charger and inverter all packaged together. You could run the Cpap and an electric blanket (depending on the size of the unit you bought) and leave the trailer battery to the rest.

So you are saying you have a 12v compressor fridge?

Charles
 

Mark_K5LXP

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I need to run two cpap machine without humidly and heated hose. My power supply says 2 amp on 1 machine and 1.5 on the other. I assume power will be less with those items turned off.
Are these AC input or DC output specs? The difference in power is a factor of ten, so just making sure where these fit in the equation.

Mark B.
Albuquerque, NM
 

ronthet

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Below is what is on the input of each power supply
Ac inputs one is listed as 100-240V~50-60HZ 1.0-1.5A, 115V-400HZ 1.5A
the other is listed as 100-240V~2.0-1.0A 50/60 HZ
 

ronthet

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For the cpap get a portable power station such as the Bluetti. They are just lithium batteries with a charger and inverter all packaged together. You could run the Cpap and an electric blanket (depending on the size of the unit you bought) and leave the trailer battery to the rest.

So you are saying you have a 12v compressor fridge?

Charles
yes 12-volt norcold frig
 

Ray-IN

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Below is what is on the input of each power supply
Ac inputs one is listed as 100-240V~50-60HZ 1.0-1.5A, 115V-400HZ 1.5A
the other is listed as 100-240V~2.0-1.0A 50/60 HZ
That totally changes the picture! Assuming your CPAP machines have a built-in 12V power supply, my figures are correct; HOWEVER, if they are 120VAC only, the draw from a 12VDC source-through an inverter, will be 10X that amperage.
This leaves your one battery very questionable.
 

Pedro Dog

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something doesn't make sense to me, maybe a typo
Below is what is on the input of each power supply
Ac inputs one is listed as 100-240V~50-60HZ 1.0-1.5A, 115V-400HZ 1.5A
the other is listed as 100-240V~2.0-1.0A 50/60 HZ
The first CPAP power inputs look backwards. 100V @ 1A and 240V @ 1.5A seems backwards. The current for 240V should be less than for 100.

The second one makes sense
 

Pedro Dog

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So assuming that the first CPAP uses 100VAC @ 1A, then that is 100W at 100VAC. At 12VDC, its going to be 100/12=8.3A.

The second unit uses 100VAC @ 2A, that is 200W at 100V. At 12VDC, its going to 200/12=16.7A.

Every hour of 2 CPAP machines operation will pull 25A from the battery bank (300W/12V=25A).

You have 162Ah battery bank. Let's say you run the bank down to 20%. So you have about 130Ah available (162Ah x .8).

So if the CPA machines use the power that you show, then you have about 6.5 hours before the batteries are down to 20% SOC (162Ah/25A=6.48h)
 

Mark_K5LXP

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Just because the power supply is spec'd for a given input/output value doesn't mean that's what the device is drawing, so knowing what the CPAP itself is spec'd for (or you actually measure) is where the rubber meets the road. A few nights with the units plugged into a power measuring device (kill-a-watt, et al) will give you an excellent idea what the "wall to wheels" power draw is, and from there you can more accurately gauge run time on battery. Using the power supply *rating* I would offer is worst case and very well may be true but if it's not, you may falsely rate your storage as insufficient when it might be OK.

Mark B.
Albuquerque, NM
 

Pedro Dog

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When I designed electronic circuits for a living and designed power supplies for them the standard was always worst case needs. I even derated for temperature. In this case, I didn't include the inverter inneficiecies, lights, or the 12V fridge. My numbers are meant to provide the OP with a reality check. Of course nominal values are useful but for medical devices I suggest worst case is the way to go
 

Mark_K5LXP

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The "brick" for one of my laptops is marked 90W. Since I use it in the RV I measured the draw while at home over a few weeks just to see. It never went above 48W doing everything I ever do with it - worst case. So that's a 50% difference which is a lot when spec'ing anything running on battery. Not saying margin's a bad idea but it's useful to know under what conditions and how often worst case might happen.

Mark B.
Albuquerque, NM
 

Domo

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I purchased 12v cords (cigarette light plug) for my CPAP, since it's a 12 VDC device and simply installed a cigarette socket that I installed near the bed - avoided the use of the inverter (and power loss of an inverter creating 120 VAC and then the transformer stepping it back down to 12 VDC for the CPAP). That could be one way to stretch the use and simplify the system. Did the same thing with our TV and bought a 12 VDC TV (can't remember the brand now). On our pull-behind we only had the inverter for the coffee maker and charging phones/computer. Rest was all 12 VDC with gas as the alternative for the fridge (since 12 VDC was the least effecient for us).
 

Domo

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The "brick" for one of my laptops is marked 90W. Since I use it in the RV I measured the draw while at home over a few weeks just to see. It never went above 48W doing everything I ever do with it - worst case. So that's a 50% difference which is a lot when spec'ing anything running on battery. Not saying margin's a bad idea but it's useful to know under what conditions and how often worst case might happen.

Mark B.
Albuquerque, NM
If it's a 12 VDC brick output why not skip the brick and go direct (fused) to a battery and eliminate the brick space heater.
 

Gary RV_Wizard

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Just because the power supply is spec'd for a given input/output value doesn't mean that's what the device is drawing, so knowing what the CPAP itself is spec'd for (or you actually measure) is where the rubber meets the road.


Right, the rating placard of a power supply or appliance gives the peak usage and is not an estimate of average consumption.
As for the fridge and furnace, you need to have some idea of the duty cycle (percentage of time it actually runs) to be able to estimate power consumption. 50% or less is not out of the question.
 

Mark_K5LXP

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If it's a 12 VDC brick output why not skip the brick and go direct (fused) to a battery and eliminate the brick space heater.
20VDC. A "mobile adapter" might be incrementally more efficient than the AC inverter/brick but generally the inverter is on for other things anyway and the laptop goes along for the ride.

Mark B.
Albuquerque, NM
 

Pedro Dog

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The "brick" for one of my laptops is marked 90W. Since I use it in the RV I measured the draw while at home over a few weeks just to see. It never went above 48W doing everything I ever do with it - worst case. So that's a 50% difference which is a lot when spec'ing anything running on battery. Not saying margin's a bad idea but it's useful to know under what conditions and how often worst case might happen.

Mark B.
Albuquerque, NM

There is a difference between charging a battery for the laptop vs running a CPAP machine. The laptop draw will depend on the batteries BMS which may limit the current draw. A CPAP consists of a motor compressing air. Even If we used your 50% example, instead of the 25A I suggested, we use 12.5A.

This time I will add the 12V fridge draw and use Ray-IN's number of 1.8A, I will also add parasitic draw during the night of 0.2A. Adding it all up I get to 12.5+1.8+.2=14.5A.

If the OP has 130Ah available (he goes to bed with the batteries at 100% - he runs the generator before bedtime), then 130Ah/14.5A=8.9h. This is still marginal in my book, as he is now at 20% SOC. Better not run the furnace that night!
 

Mark_K5LXP

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Well, it was charging and running, wifi, bluetooth, hard drive access, video, whatever. The worst case equipment draw is the final answer, there's no telling just what drove the selection of that supply. No doubt the product engineers added their own margin to the supply, like in the case of my laptop I doubt they'd spec a 50W supply for a 48W laptop. Could be a case too of one supply fits all powering a variety of products, a 50W supply can run a 10W device. Knowing the actual draws can then lead one to coming up with a realistic usage and desired operating margins, vs estimations and arm waving which nets a conservative result but doesn't answer the question of what's really enough. Worst case really isn't if that case ends up never happening. In this specific example the real answer is likely somewhere north of the estimate but it'd be a guess by how much. Measure, and know for sure. This applies to everything - fridge, furnace, inverter overhead, water pump, TV, whatever. It's an academic exercise to go around the RV turning stuff on and off and noting what the AC or DC draw is. For something like a CPAP I'd run it over time like my laptop and get a peak/average. I acknowledge that most folks won't bother with this and then I guess you just build for 50% margin and you get what you get. Just making the point that none of this is guesswork, measure and know.

Mark B.
Albuquerque, NM
 
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