CPAP on battery power issues? - Fiberglass RV


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Old 08-04-2014, 10:58 AM   #1
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CPAP on battery power issues?

Well, we finally got the Casita out for an actual camping trip. We had used it three times already. Just one to two night trips, plugged in. This was the first time in the boonies on battery power. After backing nearly a 1/2 mile down a twisty narrow dirt track. I was thoroughly convinced we made a wise choice in such a compact trailer.

On the first night the CPAP worked fine.
The second night it beeped twice and shut down shortly before restarting immediately, a couple times.
Then on the third night, after about 3 hours it just kept beeping and shutting down.
The fourth night only beeped and shut down.
What should have been the fifth night, found us at home. I was in pretty rough shape from no CPAP, so we went home early.

I use a CPAP, not only for airway obstruction but true sleep apnea (I stop breathing for extended periods, repeatedly).
It's a Resperonics M series. I normally use a humidifier but remove it when camping.
I've got the, manufacturers OEM, direct 12V power supply.
The Casita has the typical Interstate Marine/Deep Cycle Battery.

I know that this isn't the optimal Battery. I plan on switching over to two 6-volt true deep cycle Batteries by next year. However, in the past I've used whatever Battery was in what we were driving, with an inverter (which draws even more power). This would get me 3-4 nights consistently, and still be able to start the vehicle.

Also the Casita Battery still had power. We used LED lights very sparingly. After the CPAP quit I let my wife use the MaxxFan (AWESOME!) on the lowest two settings all night. Plus I turned it on when we got home, out of curiosity. After 36 hours it was still going so I turned it off and plugged in the converter.

It didn't occur to me to try the inverter once the direct supply didn't work. We're going out again in a couple weeks. I'll try it then.

I don't know much about testing and such beyond "is there power or not?"

So questions:

Does the CPAP have such a narrow operating range that it can't work once the battery is out of optimal charge?

Should I just run larger wires from the battery to the power supply?

Or...?

Thanks!
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Old 08-04-2014, 11:28 AM   #2
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Originally Posted by hariph creek View Post
Also the Casita Battery still had power.
What are you basing this on? The ability of the lights to turn on and the fan to run? Without some kind of voltage indicator (either a battery gauge or an actual voltage display) you are really just guessing at it. The lights and fan would still be able to function on a nearly "dead" battery when the CPAP won't because that may be pulling more power (amps) than the battery is capable of providing. The lights and fan may still appear to work fine since they draw less current.

It may also be that your battery is aging and not able to hold a charge as well as it had in the past.
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Old 08-04-2014, 11:57 AM   #3
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Sorry to hear of your issues. I have a couple of folks in my family who use CPAP and not having it available can be a pain.

It sounds like you are flat running out of power. If you are using a standard Interstate marine battery, it is flooded, and can recover a bit through electrochemical action when off, which is likely why, with steadily reduced outcomes, you were able to use it by spells.

There are a whole series of questions that must be asked. The two big ones are power demand and power available.

What is the wattage or amp requirements on the CPAP? What other DC appliances are you using, so we can get a better idea of your amp draw on those?

What specific battery are you using, and what is the amp hour rating on it?

Also, have you done a open circuit voltage test or specific gravity test on the battery when it was run down? If so, what were your readings?

Assuming, based on your statement that you have the standard Casita Interstate marine battery, let's use a SRM-27 as an example. Interstate says this battery will provide 5 amps for 17.2 hours. That's running it down to 10.5 volts, open circuit voltage. Real life, a battery should not be pulled down on a regular basis below 50% capacity, which for this battery would be roughly 8.6 hrs at 5 amps draw. Assuming 8 hours of sleep, the beeper originally went off at the 11 hour mark. At this point you were likely approaching that 10.5 volt mark. This is excluding any other loads you had on.

Now you have some idea of what you are dealing with. It sounds complicated, but there are many here who can help you, but first we need the data.
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Old 08-04-2014, 12:23 PM   #4
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Does the CPAP have such a narrow operating range that it can't work once the battery is out of optimal charge?

Or...?
It could be. What does your manual say for power consumption?

I found one manual on line that says 12V DC 3.0 Amps max
http://www.apria.com/wps/wcm/connect...df?MOD=AJPERES

Reading between the lines, maximum draw could be 24 Amp hours for an 8 hour night. That is a fairly large draw on a battery that is not being charged daily. I would expect an older battery to run out after a while.

I suggest you get this book and learn about the 12 V side of life in a trailer.
Managing 12 Volts: How to Upgrade, Operate, and Troubleshoot 12 Volt Electrical Systems: Harold Barre: 9780964738621: Amazon.com: Books

It will give you insight to solar charging while travelling and how to choose what you need for your system requirements.
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Old 08-04-2014, 12:53 PM   #5
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I use a CPAP myself. These are voltage sensitive devices. If the voltage drops it won't work--particularly with the heater humidifier power demand--which I use all the time, and you should too. Run it off an inverter. But that wont fix your problem.

What is probably happening is your batteries are being drawn down too low by the CPAP. Most likely you need new and a larger (higher Amp Hour) capacity batteries. A solar panel and a charge controller will ensure your batteries are topped up every night. I recommend a Victron Battery monitor also, which will help you feel more confident in the batteries state of charge. Although with a good solar charge controller and adequate battery capacity, would probably maintain your batteries well enough that you could ignore monitoring.
The more you keep it topped up, the longer they will last.

FYI. Using my inverter all night, which is a pure sine wave 500 watt x2 inverter (I use only half of it), and other AC loads like a TV, DVR, WiFi router, and other DC loads I use 50-60 AH in colder weather, charging my laptop or running my MAc Mini draws a bit more. If you are just running an inverter, lights, and a CPAP I would think a 200AH battery bank would be more than sufficient, and the batteries would last a long time. You could probably get by with less than this-- you would want at least 100AH capacity.

The smaller the battery bank, the shorter the lifespan of the batteries, but if you doubled the battery bank, while it would last longer, you would be carrying around a lot of weight and it might be better to replace a smaller half as big bank twice as often. I opted for the larger bank and I've needed it a few times with extended cloudy periods.

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Old 08-04-2014, 12:55 PM   #6
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Originally Posted by Roy in TO View Post

In the meantime, for the quick and dirty, go here.

The 12volt Side of Life (Part 1)

Be sure to go to Part 2 at the bottom of the page as well.
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Old 08-04-2014, 01:04 PM   #7
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I recommend a Victron Battery monitor also, which will help you feel more confident in the batteries state of charge.
In the interim, a simple Volt meter would start to give you a good idea of your batteries state of charge and your power usagaes over time.
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Old 08-04-2014, 01:08 PM   #8
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My original thought was that the battery was dropping below the operating range of the CPAP.
It does make sense that the LED's and MaxxFan could still work after the CPAP has reached its minimum threshold.
I just wanted some verification/clarification on my supposition.

As stated earlier, I don't know about testing and such. I don't have money in the budget to buy equipment right now.

The battery is a SRM-27, has enough electrolyte, is kept on a battery tender (when stored), beyond that...?

While I can't remember the draw on the CPAP? I did the math (without factoring minimal voltage range) a while back and figured I should be good for a few nights. Given optimal battery conditions. So I guess 2-1/2 nights isn't such a surprise.
It's that my wife's tiny Honda Battery did just as well. And my old pick up's Battery did slightly better. Both while running a inverter in addition to the CPAP. However I won't be using regular Batteries again, understanding what that type of depletion does to them.

Is there some aspect of an inverter that compensates for the low 12V voltage (even though it's drawing even more power) to accommodate the CPAP's needs?

No other devices or accessories were used on the trip. LED's were used VERY sparingly, one at a time.

Solar panels were not brought up as they are an expenditure set for about two years from now. Next year is better Batteries (6V deep cycle).
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Old 08-04-2014, 01:14 PM   #9
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I'll check out the link, thanks.
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Old 08-04-2014, 01:26 PM   #10
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As stated earlier, I don't know about testing and such. I don't have money in the budget to buy equipment right now.
You've been given a couple of good links to information that is easily learnt and a simple meter can be had for less than the cost of a 6 pack.
Search results for: 'volt meter'

Besides they are handy to have in your tool kit.
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Old 08-04-2014, 02:13 PM   #11
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"As stated earlier, I don't know about testing and such. I don't have money in the budget to buy equipment right now. "

You don't need to spend much and it is an investment.

A digital volt ohm meter can be had for -$7.00 at Harbor Freight. A useable battery hydrometer can be had for about the same price at HF or most parts store. You don't need fancy, and the VOM will serve you well in any number of travel trailer situations.

One note of caution, using either of these tools, you should check the battery after several hours of rest after any charging or substantial discharge.

If the tools don't come with adequate directions for use, you can easily find instruction on line, free.
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Old 08-04-2014, 03:34 PM   #12
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If you consider a car with a low battery it may not be able to start the car but the dome lights, radio or even blower will still run.

I would expect the CPAP has a low voltage warning/shutoff so it stops at a point where there is still enough power to run the other items.

If even a small solar panel might fit in the budget it would really help offset the high demand every night. Using the numbers supplied by Roy even a $75 - $95 for a 40 or 50 watt panel would put 1/2 a night worth of power back into battery each day.

Those cheap volt meters can really be a useful tool, plus throwing them into the tool bag in the camper you don't have worry about them being damaged at those prices.
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Old 08-04-2014, 03:40 PM   #13
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I too use a CPAP in the trailer and have a Marine battery. The main reason for the 12v system is the CPAP. While camping I don't use the humidifier because of the power draw but always set up the solar panels after each night on the battery. I also have a small battery pack that I spent a small fortune on that is especially for the CPAP. It lasts about a night plus. If I am in a campground with electricity I will have that recharged at the ranger or campground entrance. But with boon docking i would always recharge. I also have an inexpensive volt meter that I plug in the socket to test the power in the battery.

Good luck.
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Old 08-04-2014, 03:55 PM   #14
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You know, at the mention of "Harbor Freight." I recall I do have a volt meter thingy, from H.F.
I have no idea how to use it, but I'll start reading up on it.

I did have the battery electrolyte Hydro test thing done at a shop. Was told A-OK. Sorry I forgot to mention that. I'll look into getting a tester for myself.

Solar is still going to have to wait. We get by on a pretty modest income. $75-$100 dollars is not a small amount of "discretionary spending" for us. Add in enough solar capacity to be sufficient and a controller thing and...? As with most things, the initial cost isn't the whole cost.

I've looked at those Battery packs. "Small fortune" about sums it up!
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