CPAP on battery power issues?

[Steve Outlaw]

Most newer Respironics CPAP machines will draw between five and six amps per hour on 12VDC. Eight hours of use will draw down you battery system by nearly 50 amps. Powering your machine off an inverter will take even more due to the power overhead of the inverter itself.

First you need to know the Amp Hour (AH) rating of your batteries at 20 hours. This is the standard and should always be used when comparing different battery systems. In order to get an AH rating, the battery has to be drained down to zero over the course of a specified amount of time. The amount of amperage that it took to get it down to zero, over that specified amount of time constitutes the AH rating. Because of the Peukert effect (the faster a battery is drained, the less overall amperage is available), if you discharge a battery over the course of 100 hours, the AH rating looks higher that if you discharge that same battery over the course of 1 hour. So, there has to be a standard. For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20 Hr rate, then that battery was discharged over 20 hours with a 5 amp load. So, that weird 20Hr rate that you see after the AH rating on batteries, tells you that the rating in question is the realistic, common rating; rather than an over-inflated number to make the battery look better than it really is. Of course, we NEVER want to drain our battery below 50% in actual use.

Second you have to have some way to replace the amps that you've used overnight. Simply put, you have got to recharge your battery(s). There are really only two practical ways to achieve this - with Solar or thru your onboard converter/charger. Other than solar, you've got to have an external source of electricity; hookup or generator.

Being in the medical field, I know the CPAP machine really is a life-line. As you well know, it does much more than just give everyone a peaceful night's sleep. With that in mind, you need to come up with a method to assure that you can use your machine thru the night (EVERY NIGHT) without having to worry so much about how much power it is consuming.

With tight financial resources, you will need to prioritize. If your battery is rated at 100 AH, most likely you will bring it down to at or near 50% with just your machine. This will work (barely) but your battery will suffer for the max draw down being placed on it. If you had 200 AH you could also run some lights and the fan. An additional battery would probably provide that buffer, but without some way to recharge them you’re still losing ground.

The other choice is coming up with a method to recharge your existing system by either buying a generator and carrying extra gasoline around with you or buying a small solar system with the idea to expand it in the future as monies allow.

Have faith that there are ways out there to camp comfortably.

[Roy in TO]

Good post Steve.

[Steve L.]

Put another way, if you consume 48 amp hours (6 amps for 8 hours is 48 amp hours, for example), you will need to replace ~48 amp hours plus some for normal system losses. A 100 watt solar panel will give you about 7 amps (100÷14≈7) of 14 vDC charging current. That means you would need at least 7 hours (48÷7≈7) of good sun. Just to make our lives miserable, battery chargers don’t return the max amount right up until the battery is recharged, instead it runs full out for about 80% recharge then tapers off to approach 100% which means it takes significantly longer than 7 hours (in this example) to return to full charge. Many/most/some panels recharge in partial sunlight but not at their rated output. This is part of why I use a generator (Honda 1000) when boondocking. It doesn’t recharge any faster than a solar panel in full sun, but it runs the same rain, shine or shade, at no more than an idle. That and I rarely have more than a patch of sun in my campsite and I’m not going to spend my time chasing it around to maximize a panel’s output.

[RogerDat]

While those small Honda generators are a good option in terms of dependability they are probably not cost effective if working within a tight budget.

Given the budget considerations and the draw outlined in the post by Steve (Outlaw) the least expensive option for multi night safe camping is second battery plus solar with some way to monitor the current state of charge in order to bail to a campsite with power if the battery charge in late afternoon indicates insufficient power to get through the night.

My thinking is a second battery gives you two nights with possibly a third if you abuse the batteries by discharging them past 50%. Even if solar only replaces 1/2 of daily usage in two days it will have still have provided enough for a 3rd night. I will say this stuff is too expensive to purchase it twice. Figure out your power needs for solar and don't purchase until you can purchase enough panel to provide a good margin of surplus to deal with cloudy or inclement weather.


That ability to monitor available battery power before going to bed and knowing the power draw of the device is critical to making an informed decision that will protect ones health without it becoming an urgent issue in the middle of the night.

[War Eagle]

Jorah, One aspect of your original post that has not been addressed yet by others is appropriate wiring. To set the stage, the original owners of our EggCamper BOTH needed CPAP while boondocking their way up the Alaskan Highway. They mounted three 12V batteries on the tongue to power a 1000W pure sine wave inverter that they both plugged in to at night. They were on the move most days, so they relied on a high amp alternator installed in their tow vehicle (Hundai Sante Fe) to recharge the camper battery bank while driving. I don't know all the power flow data details. That's just how they rolled, and that's how we purchased it.

Now back to the wiring. They installed a pair (+/-) of dedicated 8 gauge tinned marine grade wires (expensive) from the battery bank on the tongue directly to the input terminals on the inverter. I'm guessing it is about a 13' run from battery terminals to inverter terminals. I'm also guessing it was not easy to thread that heavy gauge wire around corners, behind walls and cabinets, etc.

My point in sharing all that is, when the original owners made this very important modification, they were just as serious about the quality of the wiring as they did the quality of batteries and inverter. I'm guessing it can make a big difference in CPAP performance. So you might want to spend some time researching and getting advice from this forum about the most appropriate wiring for your situation, too. Lots of knowledge to be had on this forum. Best of luck....

[rainjer]

I use a CPAP also. I have a 115 amp hour deep cycle battery to run mine in my teardrop trailer. I can get up to 5 nights of use with this battery if I minimize my light usage. My CPAP will shut off if my battery drops below 11.5V. Also I have notice if it is above 80 degrees or below 40 degrees it will affect my battery life.

Jeremy

[Timon]

The main issue here is the dropout voltage of the CPAP vrs the cutoff voltage for the battery. Typically the battery is considered used up at 10.5 but the CPAP cuts out at 11.5 so even though you have battery power left the voltage is too low for the CPAP. The other issue is the wire gauge used to supply power to the CPAP. You could easily loose another 0.5V there.

I'd do a couple of things. First with a fully charger battery and the CPAP running measure the voltage both at the battery and the CPAP. Make sure to do this while the CPAP is running at its highest level which may take a while to reach. This will give you the voltage drop. The only way to reduce this voltage is a larger gauge wire along the path.

Now let say this drop is 0.5V then you know that anytime the battery get to 12V your CPAP will drop out. What can you do?

One solution is what's called a Buck-Boost DC to DC conveter. These will take in 10 to 15V, some are 10 to 30V, and output a clean 12V out, it's adjustable so you could up that a little. Now your CPAP is always getting 12V all the way down to the battery cutoff point.

Nothing comes for free and there is a downside to these. They are typically only 75-85% efficient so you'll loose some capacity.

Here is one good for 100W, I'll see if I can find another one that's less expensive.

Buck-Boost DC/DC converters 12 volts to 12 volts, 24 volts to 24 volts, 24 volts to 19 volts, 12 volts to 6 volts. DC/DC conversion

[Jared J]

Is it possible to run on dc the first night to conserve battery power, and off an inverter the second night (assuming the inverter can handle more dc drop than the cpap)? I realize the inverter has more losses, but maybe it will still run the cpap on ac at 10-10.5 vdc battery power?

It still won't get you five days, but might help some.


Sent from my iPhone using Fiberglass RV

[hariph creek]

I agree that solar needs to wait until I can get the "proper" set up. Even then? Where we camp, shade is an issue. Heavily forested and secluded areas.
Maybe a generator? I don't know? It's a lot of money to only get used 3-4 days at a time 1-2 times per year. Otherwise we can just plug in.

It was particularly hot for around here, in the upper 90's, that week.

Driving to a "campground" to recharge isn't an option. It would mean a couple hours drive each way. We get pretty far out. It would defeat the whole point of camping, for me. I need a few days of... No where to go, no one to see, no schedules, no traffic, no cell phones, etc...

I am looking to convert over to a tongue mount. When I get better batteries. Purely for weight distribution. My Casita doesn't have a bathroom and likes to rock on the axle, if I don't get enough weight forward.
I'll be sure upgrade to an adequate gauge wiring when I do. To avoid voltage drop.

I appreciate everyone's insight. I think I've got an idea of what I need to do. Now I just need to wait until I can.
We're looking at one more camping at the end of the month. Only three nights. I'll try the 12v supply the first two nights then the inverter on the third. By next year I'll at least have better batteries, wiring and... Who knows?

[Roy in TO]

Originally Posted by hariph creek

Where we camp, shade is an issue. Heavily forested and secluded areas.
... We get pretty far out.

If you are that far out, I'd be comfortable using a free standing solar panel in an open spot, somewhere nearby.

[War Eagle]

Jorah, I know it's a big up-front expense, but if you swallow hard and go ahead and buy a little generator (Honda and Yamaha both have efficient, very low noise models with a few different convenience features), you'll be surprised at how many other uses you'll find for it. We live in the country and lose power from time to time (storms, trees falling on lines, etc.), and it's a comfort knowing it will only take a minute or two and we can fire up some lights, use the microwave, keep the refrigerator and freezer going, etc., until the power comes back on. If I need to work on a distant structure or piece of equipment, I can take the generator and power tools to the work site and get the job done faster with less sweat and grumbling. As far as our camper's 5000 A/C unit, our little 2,000W Honda generator starts the compressor fine on full generator power, then once the A/C is running, we can back the generator off to efficiency mode, and it purrs right along. In efficiency mode, the generator makes only about as much noise outside as the A/C compressor makes inside. It will be worth your time to at least check them out.....

[RogerDat]

I know what you mean about getting out away from it all. If figure if I can't pee on a tree without worrying about "cover" from anything but our camper it's too crowded.

You might consider taking your volt meter, check the battery voltage after the second night. I think you want at least a small light or something on so you don't get a surface charge reading from the battery at rest. Voltage getting too low slap a jumper cable on and charge from the TV for an hour or two, remove cables and re-check voltage with small load.

Not fuel efficient or a good long term solution but having the jumper cables will at least give you an option so your trip is not spoiled. Deep cycle do not charge all that fast but if running vehicle at idle through a gallon or even two of gas gets you through the rest of your trip it might be worth it.

[hariph creek]

War Eagle, we literally have no savings. We live from check to check. I do want to get one of the little Honda or Yamaha inverter generators. I'd like to convert it over to propane, too. We can get by pretty well without power or water (grid failure). But I worry about not having my CPAP. I will start saving for Batteries this fall. Maybe I can work out a 1000 watt suit case too? We have to be careful and plan ahead to buy anything. It took years to be able to by this trailer. It's been nothing but a little money pit ever since.
Solar would be nice. However in this part of the country, most the year, overcast and rainy is what we get. The 3-4 hours per day of direct sun we get in the deep woods won't work either. That's assuming it's not raining, we do camp in the rain. I put in for time off and I'm taking it. Rain or shine!

RogerDat, yep that's what I mean.
I'm thinking about a mod I saw on one of these forums. My truck used to be set up for 5th wheel. It's still got the 7-pin receptacle in the bed. I may modify a spare plug I have so we can charge the battery on the inevitable trip to town for ice or tampons (never fails every year, wife or daughter forget them). That way even with two 6-volt deep cycles. I can still have this big Interstate as a backup. The PO converted the trailer over to a 4-pin. I'll fix that when I relocate the batteries to the tongue.

[Huck]

Maybe you could find a cheap, used battery to take for emergencies. Charge it up at home before leaving.

[RogerDat]

Originally Posted by hariph creek

RogerDat, yep that's what I mean.
I'm thinking about a mod I saw on one of these forums. My truck used to be set up for 5th wheel. It's still got the 7-pin receptacle in the bed. I may modify a spare plug I have so we can charge the battery on the inevitable trip to town for ice or tampons (never fails every year, wife or daughter forget them). That way even with two 6-volt deep cycles. I can still have this big Interstate as a backup. The PO converted the trailer over to a 4-pin. I'll fix that when I relocate the batteries to the tongue.

Just make sure the wire running back to charge the batteries is pretty heavy. 8 gauge works pretty well. Current only flows into battery due to the difference in voltage between charge line and battery voltage. Thin wire has more voltage drop so delivers less charge. Got a tip on the forum to check local scrap yards for the heavy wire, can be had there cheap compared to 57 cents a foot at big box store.

Wish I was closer, I built a frame to mount a small gas engine and school bus alternator on to charge my batteries. Going into the garage sale. Louder than a Honda that is for sure about like a lawn mower at idle, but it worked and I had the parts so I could afford it.

The fact that you were able over time to get the money to purchase the camper says that you can handle long term plan and delayed gratification. I'm sure you will figure out something that will work for you and get there eventually.

Until a rainy weekend camping is worse than a rainy weekend of chores at home or a Friday at work. I'm a gonno go camping.

[ruscal]

I use a Respironics CPAP. I can watch the current draw on the Trimetrics meter while in use. When I exhale the current drops way down near 1 amp, and during inhalation peaks about 2 amps. The average is what the real draw is. Mine is set at 14 cmwc which is a fairly high setting. The higher the pressure the more current will be needed to run. I do not heat the water chamber, just fill it for passive humidification.
On night 1 assuming my Number 27 rv battery is at 100%, it will usually be in the high 80's % in the morning. I can usually get 2 nights without charging. Don't just assume you need 5 amps continuous to run your CPAP.
Convert your lighting to led for good cheap power savings. I purchased some led's from china for cheap and installed them into the existing fixtures.
Another free thing you could do would be to take the tiny battery out of your wife's car and pack it with you. It would provide a one night extension for those 3 day trips.
Russ

[hariph creek]

Hey Russ, I'm already running LED's. After learning more about batteries, since buying this thing. I will no longer be using regular car starting batteries for this application.
This weekend I'll rig up the "charging station" in the bed I mentioned earlier.

[fusedlight]

One reason we try for an AC site is that I really suffer when I don't use my CPAP. Not to mention my Lovely Wife having to put up with my snoring. Here's a link I found for ResMed, the brand I have:

http://www.resmed.com/assets/documen...de_glo_eng.pdf

What they recommend is that you do NOT use the humidifier if you are on battery. Considering that this is mainly a resistive load, this makes a lot of sense.

[Timber Wolf]

Good thread, looks like there are a lot of CPAP users out there. I have only tried mine (120V Resmed) for two nights through a 150W inverter (modified Sine wave) and the group 27 battery out of my tractor, which is the old battery out of my pickup. It worked fine but was in the sub-12V range after two nights. I did this just to confirm I could get by in a power outage and for short camping trips. The next mod/purchase for the old Scamp is two 6V batteries. I also need a new converter/charger to ensure that this nice new battery bank is appropriately charged. I already have a solar system (two 100W panels and MPPT controller) but have yet to get it all together. I think I got the cart ahead of the horse and should have bought the controller/charger and batteries first (and now I am broke).

[warezdog]

find your local interstate battery warehouse distributor, not a retailer, and walk in and ask for a blem. I've been buying group 31's for $45 for years and smaller ones like 27s are about $35. They're basically batteries that have a blemish, dented corner, scratch across the top etc.

I have one 27 that is now pushing 5 years this November, had nothing but great luck with them.