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Camping and CPAP
#1
My solution is not for backpackers, but works great for camping in a campground with no AC power. My popup camper has a heavy duty 12v battery, but I was afraid to run down using CPAP. I bought one of those converters (Resmed) that takes 12 volts and converts to 24, so I could just plug it in to my camper. I also got one of those 12V jumpstarter things from Walmart. It's a Stanley 500/1000 amp, for 49.95. Figured that I would return if not good.

Everyone says to avoid using the humidifier due to the amount of current required. I really need the humidification, so I left it on and it ran CPAP for three nights in a row, and still had SOME power left. The unit can be recharged via 12v or 110v. This way my camper battery is good to run everything else.
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#2
(08-10-2014, 10:43 AM)drgrimes Wrote: My solution is not for backpackers, but works great for camping in a campground with no AC power. My popup camper has a heavy duty 12v battery, but I was afraid to run down using CPAP. I bought one of those converters (Resmed) that takes 12 volts and converts to 24, so I could just plug it in to my camper. I also got one of those 12V jumpstarter things from Walmart. It's a Stanley 500/1000 amp, for 49.95. Figured that I would return if not good.

Everyone says to avoid using the humidifier due to the amount of current required. I really need the humidification, so I left it on and it ran CPAP for three nights in a row, and still had SOME power left. The unit can be recharged via 12v or 110v. This way my camper battery is good to run everything else.

http://www.apneaboard.com/wiki/index.php...tery_power

I've been using this for camping and power failures, and it's been working out nicely, and will run the machine and the humidifier for a least a couple of days and maybe more depending on settings and battery size.

Being 110v AC, there's no chance of trying your machine if you hook the cables up incorrectly, since it just uses your normal power supply.

Terry


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#3
Hi there, is there a reason no one uses a computer UPS for this purpose? I know of many units that are self enclosed, always charged and are available between a $100-$200 and should run a cpap unit for at least 30 hours on a charge (I would think) but I am just wondering.
If everyone thinks alike, then someone isn't thinking.
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#4
It's all about ampere hours. According to the label on my S9, it draws 1.25 amps as a stand alone and 3.75 with a humidifier. Those are maximum values and do not take into account the duty cycle of the humidifier or the pressure setting on any individual system. With that in mind, a 100 ampere hour source that delivered 24 volts d.c. would run the device for 80 hours (no humidifier attached) at its maximum rating before dropping down to a voltage insufficient to supply the needs. Normal acceptable voltage drop would mean you need at least 19.2 volts to keep the S9 going properly. That's a 20% drop from the rated voltage of 24. A 12 volt battery running a 12/24 converter would deliver 24 v. at a reduced ampere hours rating of a little less than 1/2 the 12 v. rating of course. Depends on the efficiency of the converter . Your 80 hours would shrink to just under 40 under that situation.

If you check the ampere hour rating of the power supply device you are thinking about using, you can use that rating to get an approximation of how long it will run your CPAP system using simple math. The fly in the ointment is in estimating the duty cycle and average pressure rating as it affects current draw.

I notice the 500 amp rated Stanley jump starter is rated at 19 ampere hours. I'd guess it would run an S9 that is set at a medium pressure (10 cm h2o with a humidifier operating at a duty cycle of 50% for a period of approximately 10 hours before it started to sag below an optimum voltage. I don't say it will quit at that point but I suspect it won't be too happy with things and will squawk pretty soon. It would help if you used it in multiple time frames and allowed it to recoup between sessions. That might get you two or three short nights usage.

If the power supply you want to use is not rated for deep cycling and you sap it down completely for a few times, it'll go south pretty quick.
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#5
(08-10-2014, 05:01 PM)Galactus Wrote: Hi there, is there a reason no one uses a computer UPS for this purpose? I know of many units that are self enclosed, always charged and are available between a $100-$200 and should run a cpap unit for at least 30 hours on a charge (I would think) but I am just wondering.

The small UPSs are only good for a few hours. If you had a really big UPS it would work just fine, but then you would have almost the same thing you get with a big battery and a cheap inverter, although it would have cost you about $1,000 8-)

If you look at your power supply, you'll see it's rated in watts. This is the maximum your machine can use, running at max pressure with the humidifier on "high", so it's a useful number to work with.

If you have a 60 watt power supply, and want 8 hours of sleep, you'll use 60*8 watt hours, or 480 watt hours.

A 100 amp hour, 12V deep cycle battery like the one listed in the wiki article has an energy capacity of roughly 100 * 12 watts, or 1200 watts. 1200 watts / 60 watts = 20 hours of operation.

These are rough numbers, since the battery discharge curve changes depending on the size of the load. A huge load like starting a car will actually give you less total energy from the battery, while a small load will give you a little more.

It also doesn't take into account the efficiency of the 12v -> 115v inverter, which isn't always listed by the manufacturer, however given average settings on your CPAP machine, you would probably get two full nights of usage out of the 100 amp hour battery and the listed inverter.

Terry

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#6
Surfer Dude i'm not sure if my very simple experiment results fell within the data you provided. I have a 12 to 24 converter that comes from resmed. It has a cigarette lighter plug on the power end and regular S9 connector on the other. Some time I seemed to remember that it's more efficient to convert 12 to 24 for the S9 rather than using an inverter to convert to 110, then back down to 24v. Is that right? Someone asked why not use a computer back up battery, but this might be the reason?

My pressure range is 5-15 but actual average pressure is around 8, and humidifier setting is on 3.

The Stanley jump starter has a crude set of lights to indicate battery condition, two red and one green for max charge. I sort of expected poor results in first night with the humidifier, but it still had two red lights so I went a second night. Still had two red lights the next morning, and s9 was running fine, with good AHI

After third night, it still seemed to be running fine, but had only one red light. The owner's manual says to recharge when it gets down to one red light. I was happy with three nights of approx. 8 hours of use each night.

An odd thing about the Stanley unit. It can be charged in 2-3 hours using 12v or charges overnight using 110v. I called the company and they verified that 110 uses more of a slow charge and switches to trickle when fully charged.


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#7
Yes, you are correct. The more conversion, the more losses you incur. That would seem to explain your situation since the ResMed is a 24 volt machine and a simple conversion from 12 volts to 24 is the most efficient way to get what you need. As you point out, using an inverter to get 120 volts a.c. from 12 v.d.c and then converting (involves a transformer and rectifier or better yet a solid state switching circuitry) it to 24 v.d.c. would be less efficient. Most losses are converted to heat so if you can come up with a method of reclaiming that heat to use for the humidifier, you'll have something to sell.

However, my crude calculations were for the maximum ratings of the devices and not meant to be an exact indication of how long a 19 ampere hour battery would last in an actual situation which would depend on some variables not easily known. It seems that you have succeeded in finding a nice way to power your CPAP without spending the huge bucks that a medical equivalent would cost. More power to you (pun intended) and I salute you for your success.Well-done

BTW, You could get another Stanley unit and connect it in series with your existing one and it would power the CPAP for more than twice as long considering you wouldn't need the 12 by 24 ResMed device with its losses..
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#8
Just for kicks, I'll re-post my reply that I made to someone else who was thinking of using a residential-grade UPS battery backup to power their CPAP machine:


(11-24-2012, 08:32 PM)SuperSleeper Wrote:
Crog Welly Wrote:Its an 18Ah SLA battery and should go for at least 10 hours. It ran for 5.5 hours last night camping. At 3:30am I woke to the machine trying to "boot up" over and over again but would not get up and running. Booster pack was fully charged at the beginning of the evening.

Hi Crog, welcome to Apnea Board! Smile

An 18 amp-hour sealed lead acid battery is woefully inadequate for CPAP usage with a heated humidifier and is close to the unacceptable level for a modern CPAP even running at 11 cmH2O of pressure without heated humidifier. With your humidifier on, there's no way that it will last for anywhere near 10 hours. Your battery is the weakest point, not the inverter.

Read my post HERE about this issue.

You need to get a battery with a much higher amp-hour rating. There are issues with the Peukert's Equation (posted in that link above) and the fact that you'll ruin your lead-acid battery if you discharge it beyond a certain percentage (again, read that post).

Look at the equation in that linked post. Assuming you're using your heated humidifier and drawing about 3 amps of 12 volt power, the formula would be:

18 AH x 90% = 16.2 AH / 3 amps draw = 5.4 hours absolute max run time x 60% (max safe discharge number) = 3.24 hours (safe level of discharge).

In your situation, you ran down your batter below the maximum safe discharge rate, and thus, got about what this equation predicts: 5.4 hours of use from an 18 amp-hour battery.

But, you've got a problem - you discharged the battery down to a level that may have done damage to the battery capacity. Do that one or two more times and you may end up with a dead brick instead of a battery.

On top of that, you used a small DC to AC inverter - and that sucked about 10-20% of your available power out before you even got the electricity to the CPAP machine, which reduces your available electricity even further.

If you can, as JJJ wrote, it's best to run on direct 12-volt DC if your CPAP will allow it. No inversion loss that way.

Also, you need to get a battery that has a minimum of around 60 AH (Amp-hours). That way, you'll get a good solid 10 hours of use before discharging the battery to a level that will not cause harm to battery capacity.

This is why if folks are going to use lead-acid batteries, many of us get the large, honkin' "marine deep cycle" batteries like you'd get from AutoZone, WalMart or K-Mart, etc. Those inexpensive deep cycle batteries are about the size and weight of an average auto battery and usually have an amp-hour rating of at least 110 AH or so. More expensive deep cycles have higher ratings. I personally have two 6 volts wired in series for a total of 12 volts. Those batteries have an 220 AH rating.

Here's some more threads that talk about these subjects:

http://www.apneaboard.com/forums/Thread-...-with-CPAP

http://www.apneaboard.com/forums/Thread-...wer-outage

http://www.apneaboard.com/forums/Thread-...or-camping

http://www.apneaboard.com/forums/Thread-...up-problem

Hope this helps!

Coffee

SuperSleeper
Apnea Board Administrator
www.ApneaBoard.com


INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.



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#9
And when you consider the fudge factor that some manufacturers use to rate their batteries, we can only marvel at any little degree of satisfaction we get from using them. A battery is an inherently low efficiency device that already involves a lossy conversion process before we get anything out of it and even then half of the circuit power is dissipated in the battery itself due to its internal resistance.

My hat is off to drgrimes since his success flies in the face of most other users results. More power to him (there, I did it again).Lolabove
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