Hi Folks,A little background. Not new to RVing, but starting to deal with battery issues more, now that I have a week long music festival coming up w/ no hookups. I decided that after last year, not knowing what I should about battery use, maintenance, etc, I figured I would learn EVERYTHING I could this year, by reading this forum. After about, oh maybe 1000 or so posts, I feel I have a really good knowledge basis, but a question looms.My brother is visiting next week, and he wants to use his CPAP machine. He linked to good article, explaining the different models, and giving their current draw. His machine on 120AC, draws approx 2 amps. Thereis no DC mode, and it needs to be powered by a small inverter. My first question relates to the efficiency of various inverters on the market. I have read some posts giving detailed info on some of the better quality units, obtained from actual testing by the posters. I, however am stuck with one of 2 units. The first is a 300 W Radio Shack brand that I have had for many years. I use to use it for running a small TV and DVD player (in my cruiser when I was a police officer!!, before smartphones.)The other is a 1000 watt off brand I bought from BJ's about 10 years ago to run a few lights when the power went off. I just purchased 2 brand new Exide Group 27 Marine batteries to use( in parallel) exclusively for the unit. I know, Exides probably suck, just like every other Marine dual-purpose, but for 85 bucks a piece, I figured what the heck. I couldn't justify the more than double that for a true Deep Cycle like a Trojan, especially only using them maybe two or three times a year for boondock camping. For this festival, I am also borrowing a nice little Honda 1000w generator from a friend, to top off these, as well as my house battery on the fifth wheel. I would like to use my Schumacher 3 stage charger ( 2-20-75 watt) on the 20 watt setting, to do this.So, just how much battery do you think I will go through with just the CPAP running, IOW, how much more current are these inverters gonna eat up on me? I would do a test, but I am afraid to run the batteries down below 50 or 40 percent or more in the process, shortening their already crappy lifespan. I will be getting a new digital multimeter ( Radio Shack again?,other suggestions welcomed) to monitor the voltage of the bats, but I don't think its practical to try to wire it up to measure current draw in line with the inverters right now. As the weather appears to be continuing a hot humid pattern the next week, I would love to run an AC window fan at night ( low setting of 3 speeds) also, but I know I am asking for trouble with that. I don't have the time right now to purchase a DC one, that would be more efficient. So a few questions.1. Do you think I am good with the 2 Group 27s for a good 7 hours straight, with one or the other inverter, w/ the CPAP?2. Should I even think about the window fan?3. Bonus question. If I run the bats down to 50 percent every night, ( I assume that would be about 100AH, one battery's worth) how long will I have to run my Honda 1000W gen w/ charger on 20 AMP setting to get back to 90%. Thanks to all on this forum, I have gained a tremendous amount of knowledge this past year from all of youedit: reading some other posts, looks like I'm gonna need a bigger charger, 20 amps aint enough. I don't wanna hear the gen for more than 3 hours

I can't answer the recharge questions, but two group 27's will be plenty to power the CPAP. I would advise your brother to run without the heat and humidifier on because those will use considerably more power. I use a CPAP with no heat/humidifier and we could camp for two nights in the winter with heater, lights and TV before we needed the generator.With two batteries, you should be able to run the fan and CPAP with no problems for a few nights before needing a recharge in my opinion.

Camp, ForestDo you run the CPAP with an inverter? He said he will not be using the heat function.

If your brother doesn't use the humidifier on his CPAP, your two batteries should power it for several days without recharging. The Honda generator will take several hours to charge up the two batteries, unless you use two battery chargers, one for each battery. The problem with recharging batteries is it takes quite a bit of time to top them off and you really want to do that if possible. As for your other electrical demands, I don't know.I use a CPAP in our coach, and don't have any problems dry camping, but I have four deep cycle batteries that also power up everything in the coach.

BEWARE! ! !I tried a Respironics with a Samlex PURE SINE inverter and the machine went nuts. Hieroglyphics on the machine's screen. Lost all settings. The stored data on the data card was corrupted.Because of the WATER ISSUE (humification) most machines use LOW VOLTAGE direct current which is converted by a brick just like a laptop computer.My machine is TWELVE VOLTS D.C. (battery power). With a proper plug to fit the socket, a direct cigarette lighter socket/plug connection to the machine is appropriate. Most manufacturers in fact offer an ACCESSORY cigarette lighter cord to power machines. Operation is identical to that when using the brick and AC.Another caveat. CPAP and BiPAP manufacturers warn against leaving the machines connected to 12 volts DC when a charging source higher than recommended is seen in the 12 volt power system. So converter power is OK, but manufacturers do not like to see machines connected when the rig's engine is started. I would say 14.0 volts is about the limit for comfort.HUMIDIFIERS draw FOUR TIMES as much power as the blower and hardware. Around one amp with the humidifier shut off and five amps BATTERY POWER when the humidifier is HEATING. Unless it is very cold, humidifier heaters turn on and off as needed.Just for drill, the SAMLEX powering the Respironics with humidifier on drew EIGHT POINT FOUR amperes out of the battery bank. That is quite a penalty to pay for using an inverter, never mind the software scrambling.Not all inverters will do this, but I challenge anyone to come up with a MODIFIED sine wave inverter that will work with MY Respironics BiPAP.

FYI, the charger in my rig charges the batteries faster then the 12VDC output of my Honda generator. This past weekend it took about 15 minutes to go from 50% to 90%.I have friends that can get 6 nights with their CPAP off a single group24 battery, with NO other loads.

CPAP machine w/ inverter. How much power? | Good Sam Community

27 Replies

rickst29
Explorer
Jul 20, 2013
Fully Regulated CPAP Power, using a Vicor Converter module

MEXICOWANDERER wrote:
BEWARE! ! !
....(short snip).... My machine is TWELVE VOLTS D.C. (battery power). With a proper plug to fit the socket, a direct cigarette lighter socket/plug connection to the machine is appropriate. Most manufacturers in fact offer an ACCESSORY cigarette lighter cord to power machines. Operation is identical to that when using the brick and AC.

No- and I'm sure that you agree because of your next point: With such a cord, you are running "12V" directly at RV system voltage. Maybe as high as 14.6 volts during "bulk" charge; even 13.7V during "trickle" charge is a lot more than the output from the 120V power brick you use at home. As you say:
MEXICOWANDERER wrote:
,CPAP and BiPAP manufacturers warn against leaving the machines connected to 12 volts DC when a charging source higher than recommended is seen in the 12 volt power system. So converter power is OK, but manufacturers do not like to see machines connected when the rig's engine is started. I would say 14.0 volts is about the limit for comfort.

This is not merely when "the rig is started"; you will be exceeding that figure at any time a Power Converter decides to switch into "bulk" or "boost" mode. Example: Your RV, parked in a campground with full plugins.

So I agree with your post, warning people about this issue. But let me offer a completely different alternative: Vicor makes "power converter modules" which accept anything between 10-20 volts input from your RV "12V" power distribution, while creating out EXACTLY 12.0 volts on it's output terminals. They are even tunable! If you check the exact output ("12VDC") of your 120V "power brick" at home, you can match the voltage at plug-in end of your module and cord making up for the fact that your cord, downstream from the Vicor and into your CPAP, will have at least some power loss.)

My own 120VAC brick actually puts out 12.15 Volts at the CPAP connector, not 12.0. I added a resistor (4M) to the "tune for increased voltage", matching that figure. It will maintain that voltage under any possible state of 12V or discharging (except, of course, for totally dead batteries). 14.6V "Bulk" charging? the CPAP gets 12.15 Volts. 12.5V, or even 11.9V, discharging? the CPAP still gets 12.15 volts :B (Although it's a really bad idea to use it if your Trailer 12V system is rall the way down to 11.9 Volts.)

You can buy such a Converter (used) on EBay, for about $30. Just take care to be sure that the particular "converter" which you are getting is for 12V in AND 12V out - most of them are built for different voltage combinations. If you can find one with multiple outputs, you can "bridge" them together and achieve output wattage well in excess of the requirements for running with the humidifier. But that wiring is a bit more complicated, adding some "control bus" connectors between the multiple internal units. I bought a single-output 70 Watt module, and simply skip the humidifer when we're camping without 120VAC plugins.

The Vicor module, encolsoed in a plastic "project box", becomes a 12V alternative to your 120VAC power brick. Should I call mine "Brick, Jr."? ;)
Searching_Ut
Explorer
Jul 14, 2013
mena661 wrote:
Searching_Ut wrote:
With the smaller inverter you're looking at, you'll probably be looking at using at most 25 to 30 percent more than you would if you had to run straight off the battery.
Where do you get those numbers from? My 300W doesn't waste anywhere near that amount unless you're talking about the really cheap inverters. Mine is rated at 80-90% efficient.

I think you're leaving the CPAP power supply out of the equation.

To begin with, most inverters don't deliver the advertised efficiency under anything but ideal load conditions. That said, assuming your advertised 80 to 90 percent efficiency of the inverter, you lost 10 to 20 percent of your energy converting from DC to AC. The internals of the CPAP machine run on DC. Most modern units use a brick type power supply to convert AC to somewhere between 12 and 24 volts DC depending on manufacture. A few of the machines built the DC converter into the machine, so with those there isn't a practical way to bypass this portion of the equation, but you are still going to experience another energy loss converting from AC to DC. If you have a unit that can be plugged directly into the DC power supply you'll bypass the two conversion processes and realistically save somewhere between 25 to 30 percent over a comparable machine which requires putting the inverter and brick into the mix. With really good electronics, you might do a little better, but I wouldn't expect it to be all that significant of an improvement.

Note: For the technical types out there, before anyone points it out, I realize that the energy isn't "lost", but for our purposes that term works as good as any in my humble opinion....
mena661
Explorer
Jul 12, 2013
Searching_Ut wrote:
With the smaller inverter you're looking at, you'll probably be looking at using at most 25 to 30 percent more than you would if you had to run straight off the battery.
Where do you get those numbers from? My 300W doesn't waste anywhere near that amount unless you're talking about the really cheap inverters. Mine is rated at 80-90% efficient.
pianotuna
Nomad III
Jul 12, 2013
Hi bgrass,

You need to think in "watts". 2 amps X 120 volts = 240 watts.

240 watts divided by 12 volts = 20 amps.

Those numbers assume 100% efficiency. You will never get that. 90% is considered good, so draw may be 22 to 23 amps @ 12 volts.

It is entirely possible that the 2 amps is a maximum draw and that the CPAP uses less. The only way to tell is to plug it into a watt meter. My favorite is a kill-a-watt

watt meter

bgrasspkr wrote:

I, will be using a cheap Radio Shack 300 W inverter. How much MORE( than the approx 2 amp rating) current will this draw.

I have 2 Group 27's paralleled. If I could go direct w/o inverter I could use it for a week! Are these inverters really that inefficient so as to double or even triple the current draw. Doesn't make sense
Searching_Ut
Explorer
Jul 12, 2013
With the smaller inverter you're looking at, you'll probably be looking at using at most 25 to 30 percent more than you would if you had to run straight off the battery. This will be due to the efficiency loss in the inverter, and then again within the CPAP machine which internally runs off DC voltage. You'll be converting DC to AC, then back to DC. Unless you're using a lot of other stuff through the night as well, you shouldn't have any trouble running things off the batteries you have provided you run the generator 2 to 3 hours a day to charge things up. You'll want to of course charge the batteries all the way up when you get home from your trip but you should do fine for the week.

If you do a google search, there are several companies that sell DC power cords, and stand alone battery power supplies to run CPAP type machines on. I can't remember where I saw it, but one of them has a pretty good chart that will tell you how long the assorted brands will run off their battery power supply at different pressure settings. As I recall most of the units will supposedly make it though the night on an 8 amp hour battery, even the ones that need to run on an inverter, providing of course you don't run the humidifier heater.

For others looking in to camping a lot with a CPAP, the modern Respironics machines run on 12vdc and just need a power cord to hook to a cigarette lighter etc. ResMed machines mostly run off 24vdc and are easiest run off of inverters. With other brands, its worth looking in to if you intend to camp without hookups often.
bgrasspkr
Explorer
Jul 12, 2013
Searching, and wbbeasly,

From your posts, seems you are running units 12vdc directly from your bats, WITHOUT an inverter. I do not have that option. Thus, my original question on inverter efficiency. Looking back, while people have given, a lot of info, none have seemed to know how efficient these inverters are.

Bottom line: Run a CPAP without heat and humidity direct from battery, no problem w/ 2 or 3 nights on a group 24 battery, according to actual users who have posted.

I, will be using a cheap Radio Shack 300 W inverter. How much MORE( than the approx 2 amp rating) current will this draw.

I have 2 Group 27's paralleled. If I could go direct w/o inverter I could use it for a week! Are these inverters really that inefficient so as to double or even triple the current draw. Doesn't make sense
Searching_Ut
Explorer
Jul 12, 2013
I think the 120 volt 2 amp thing is with the humidifier heater running. My Respironics Bipap machine runs off 12vdc and draws less than 2amps DC without the humidifier heat on. With a Bipap the current draw fluctuates depending on whether you're inhaling or exhaling and it also varies if your machine automatically changes pressure settings as required. I run my machine off it's own group 24 battery which works fine without recharging for 3 nights. When we go on longer trips I have to charge the battery every couple days, which is no problem as I also have to put at least a couple hours of charge on the main TT batteries every couple days anyway.

As for living on batteries, you just have to figure out what you can and can't have. It's amazing how much power it takes to run a lot of the modern devices we now have available to us. That said, I found we can live fine in our TT for several days on a couple group 24 batteries without feeling we're giving anything up.
wbbeesley
Explorer
Jul 12, 2013
I run a cpap machine off a group 24 battery for three days. Machine had a 12v input and was able to buy a cord with alligator clips on one end and cig liter plug on other at auto store, picked up a cord on ebay from machine to the cord and away we go. during the day a run a solar trickle charge that is probably minor but if it helps a little in the daytime it helps lot at nighttime. Using a battery just for the machine provides a backup battery for camper as well.

if i need to run the generator I plug the camper in to the generator and auto charger for the cpap battery, thus charging both same time.
pianotuna
Nomad III
Jul 12, 2013
Hi bgrass,

Look in the owner's manuals. My 2004 had a PD 40 amp--but I had to add a wizard (thanks to smkettner's comments).

If you have a volt meter check battery voltage before you plug in--and then immediately after you plug in check again--then check a 3rd time in about an hour.
bgrasspkr
Explorer
Jul 12, 2013
Thanks PianoTuna

I will know a lot more this weekend after some tests. What are the chances my 2006 Keystone Outback 5th wheel has decent 3 stage converter installed. I will check it tonight, but not sure what to look for, or if I can see the model from the electrical panel without taking it apart.