Help CPAP and Inverter Dilemma

[Excesses]

Tried to post to an older thread but it was, apparently, too old to reply in. Thank goodness for the back buttons, copy and paste.

Hi

I’ve read the threads on CPAP and inverters, but I don’t understand the lingo very well. My wife and I both use CPAP and we’d like to be sure we have the right product to do the job and maybe a bit more. We know we need a pure sine wave inverter for the CPAPs and for sensitive electronics we might want to use. Our CPAP machines are both ResMed Air Sense 10s. Different models, but the tag underneath both say _____ DC 24v. ————3.75 A.

As I said, I know nothing about electronics but I know our new to us coach has 2 6 volt house batteries (122 mins @75 amps, 232AH Interstate GC2) wired to 150 watt solar panels on a 20 A controller. A 12 volt ignition battery as well. So I think we may have enough power for 2 CPAPs for 8 hour nights. We also have an on board a gas powered generator that we seldom use (so far). Late thought, all batteries are up front of the MH.

My questions are this

How big a sine wave inverter do we need and can anyone recommend a good brand? I’m thinking it should be an aluminum case to help dissipate heat. Is 1000W too much for both CPAPs Plus maybe a TV or laptop?

I read that I should install an inverter up front near the batteries but how do we get the power back to the rear bedroom? Install a couple plugs under the bed and run some 120 v house cable under the rig from the inverter to the plugs to power them?

I’ve even thought of putting a couple of batteries in the rear pass through and wiring them to additional solar (thinking of portable panels) and hooking the inverter up in the rear. This would avoid the necessity of running cable from the front and it would add additional capacity for the CPAPs without using the house batteries. But it is costly with 2 batteries, solar etc.

Am I over thinking this whole CPAP thing? Are we ok with our present house battery capacity? We don’t use A/C much if at all, and we don’t boondock much in cold conditions even though the Triple E products are really well insulated.

Our main focus is to be ready in the next month or so in case we have to boondock a night or two on our way back to a Wisconsin wedding in June. Am holding off on a purchase of a 10 amp
https://www.amazon.ca/Power-TechON-P...ct_top?ie=UTF8

for $149.

Lastly, do I need to buy a circuit breaker for this unit or, as well, a Marine type switch battery selector for the two battery groups? 12v and the 2 x 6v. One video recommended it.

Really confused as to what to do here.

Thanks in advance.

[firedoc]

I have been using the same chap on a non sine wave 2000 watt inverter for 5 years with no problem and I use an humidifier which draws much more than just the coal. We have 4 6 v batteries for our house setup and we run a residential refrigerator on it also.

[jarwiebe]

I use the same cpap and run off of a modified sine wave inverter, no issues.

[wa8yxm]

Ok 24v by 3.75 amps= 90 watts adding in inverter loss about 100 watts (NOE ABOUT)
You have 22o amp hours at teh 20 hr rate total of which you can use about 110. so 110 times 10 (Figure on some other loads) is 1100 watts or a total of 11 hours of CPAP use

You have two CPAP's so you will be using 2 hours every hour

I'd suggest adding a 2nd pair of GC-2 Bateries.

[BCam]

Watts = Volts x Amps, so 24 x 3.75 = 90 Watts per CPAP

Therefore, both of your CPAPs together will consume 180 watts of power. An LCD TV will be about 90 watts and a laptop about 60 watts, so, in total, you are at about 330 watts if all are running. A 1,000 watt inverter is more than enough. It's your battery bank's size that will be the limiting factor, depending on how long each is running.

At 180 watts total, the two CPAPs will draw 180w/12v = 15 amps at 12 volts (twice as much as the amps drawn at 24 volts).

In an 8 hour night, they would consume 8hrs x 15 amps = 120 amp hours. At 60 watts, the laptop will draw 5 amps at 12v and, at 90 watts, the TV will draw 7.5 amps at 12v.

Note that I've ignored the fact that the two CPAPs are only drawing 180w/120v = 1.5 amps from the inverter, since that's only an intermediate step. It's the 15 amp draw from the 12v battery bank that's the important number in computing amp hours.

The reason you want the inverter close to the batteries is that the amps flowing through the wires to the inverter will be ten times that flowing from the inverter to your CPAP, TV or whatever. The fact that the inverter is putting out 120v now becomes important. A device that consumes 120 watts of power will draw one amp at 120v but the same device will draw 10 amps at 12v. (Amps = Watts/Volts). This means that the wire gauge from the inverter to the battery needs to be much larger than that from the device to the inverter. Your inverter will come with appropriately sized cables for this.

I wouldn't run "120v house cable" from the inverter to the bedroom. Typically "house cable" is Romex, which consists of solid wires. Properly sized stranded wire is preferred due to the vibration inherent in an RV that can cause solid wire connections to fail. Your rig may, however already be wired with solid wiring. You can, therefore, duplicate your rig's existing 120v wiring. You should be able to find somewhere that you can read the specs printed on it.

This all, is in a perfect world, so the actual consumption could be more or less since the inverter won't be 100% efficient and your CPAPs may not draw the rated wattage 100% of the time.

I'd suggest getting an inexpensive multimeter with a clamp on ammeter and take some real life readings to be sure.

I certainly wouldn't invest in a separate battery bank and solar until you try the simple approach of an inverter for a night or two. You can attach the inverter to your batteries with the cables provided with it and use a decent extension cord between the inverter and the CPAP.

[Excesses]

Originally Posted by wa8yxm

Ok 24v by 3.75 amps= 90 watts adding in inverter loss about 100 watts (NOE ABOUT)
You have 22o amp hours at teh 20 hr rate total of which you can use about 110. so 110 times 10 (Figure on some other loads) is 1100 watts or a total of 11 hours of CPAP use

You have two CPAP's so you will be using 2 hours every hour

I'd suggest adding a 2nd pair of GC-2 Bateries.

Thanks. Just trying to follow the math and the online manual. Please bear with me. Without a humidifier, the manual says that at my pressure setting (12 cm H2O):
current draw @12v DC (amps) = 0.61v.
It doesn’t make sense to me but it also says that the “battery size” and I’m quoting here...for 8 hrs use (amp hrs) is 8 and this includes a 50% safety margin.....

Does this change the math at all?

And what size inverter do you think we need?

[msturtz]

The placard you’re likely reading is the CPAP unit itself since it is referring to DC. You will need the placard off the AC power supply that plugs into the wall.
If your CPAP machines power supply lists something like this: Xx amps at 120 - 240 VAC then it’s a switching power supply and can tolerate a modified sine wave inverter.

As to sizing an inverter. I believe the simple approach is usually best. Determine everything you want or need to run and add up the wattage. Then add 50% for starting if you have loads such as refrigerators etc.

You will want an inverter with an automatic internal transfer switch so the outlets are on shore or generator power if available. This type of inverter is very common.

You can then route the existing circuits to the inverter.

Locate the inverter as close to the batteries as possible without being in the same compartment.

[Martind4]

I believe your CPAP actually uses 12VDC and a power cord that plugs into a cigarette lighter type receptacle is available. Why waste the energy to convert 12VDC to 120VAC and then back to 12VDC? Our rig already has a 12VDC receptacle in the bedroom area, yours might also.

[BCam]

Originally Posted by Martind4

I believe your CPAP actually uses 12VDC and a power cord that plugs into a cigarette lighter type receptacle is available. Why waste the energy to convert 12VDC to 120VAC and then back to 12VDC? Our rig already has a 12VDC receptacle in the bedroom area, yours might also.

That was my first thought, but the OP said the label said 24v which is why I went on to the inverter track. Maybe he misread 120/240v as 24v.

That being the case, you are 100% correct, plug directly into 12v DC and forget everything else. That's what I do with mine and have never used much battery power. 0.61A at 12v is insignificant (6.48 amp hours over 8 hours) as is the 8 amp hours suggested with safety margin.

Maybe the OP can clarify things a bit (his last post was confusing) so we know the facts.

[Excesses]

Originally Posted by Martind4

I believe your CPAP actually uses 12VDC and a power cord that plugs into a cigarette lighter type receptacle is available. Why waste the energy to convert 12VDC to 120VAC and then back to 12VDC? Our rig already has a 12VDC receptacle in the bedroom area, yours might also.

Nope. It plugs into a house receptacle only. No other input in back. The manufacturer wants to sell me a converter. I’d rather an Inverter. But thanks for your input.

[Excesses]

Hi guys. Thanks all for the help. Here is the power unit that plugs into the machine then into the wall. There are no other places on the machine for another plug or adapter of any kind.
Attachment 202544

I’m sorry it’s posted upside down. I tried twice to get it right side up, even turning the original picture upside down before uploading. It just wants to post upside down and I have no idea why.

Anyway, I guess we could use a lighter plug converter and run a heavy extension cord for 30 feet or so. But we then need to plug in 2 machines into the extension cord....Is there an associated hazard with doing this?

I really don’t want to have to pay an auto electric tech $95+ to do something I could do with the right instructions and information.

Thanks again.

[Sbrownstein]

It uses 90 watts. Get a little 300 watt inverter (PSW if possible) and plug both of them in. If you turn off the humidifiers I bet you will use less than 90 watts for both of them! Plug the inverter in as close to the batteries as possible. If it has alligator clips, use them. Use an extension cord with a multiple plug adapter and run it as far as you need.

No fire hazard there. I used to plug an exteninson cord int my 2000 watt inverter that runs my fridge. Ran it in the window in the bedroom and plugged in.

[BCam]

The photo confirms it, 24v DC is required so direct connection to your 12v DC won't work. I agree with SBrownstein's advice. I don't think you'll have a problem and I don't think you'll need any additional battery power than you already have onboard.

You may have found this already, but there's a ton of information from ResMed here, including the power requirements and recommended inverter sizes for various ResMed CPAP models.

https://www.resmed.com/us/dam/docume...de_glo_eng.pdf

I don't think you're going to draw anything close to 90 watts, that's the max rating of your power supply, not the average draw of your CPAP. Your CPAP is only going to draw what it needs, not the full 90 watts. The above ResMed document will give you the power draw for your particular CPAP model. Also, not using the humidifier will reduce the power draw if it's not an issue for you.

I notice that ResMed does make a 12v to 24v DC converter that they say is more efficient than an inverter but they're relatively expensive ($85+), and you'd need two.

Good luck.

[mrboyer]

Originally Posted by BCam

The photo confirms it, 24v DC is required so direct connection to your 12v DC won't work. I agree with SBrownstein's advice. I don't think you'll have a problem and I don't think you'll need any additional battery power than you already have onboard.

You may have found this already, but there's a ton of information from ResMed here, including the power requirements and recommended inverter sizes for various ResMed CPAP models.

https://www.resmed.com/us/dam/docume...de_glo_eng.pdf

I don't think you're going to draw anything close to 90 watts, that's the max rating of your power supply, not the average draw of your CPAP. Your CPAP is only going to draw what it needs, not the full 90 watts. The above ResMed document will give you the power draw for your particular CPAP model. Also, not using the humidifier will reduce the power draw if it's not an issue for you.

I notice that ResMed does make a 12v to 24v DC converter that they say is more efficient than an inverter but they're relatively expensive ($85+), and you'd need two.

Good luck.

I used a ResMed with their 12vdc to 24vdc power brick. Worked very well. I installed a lighter receptacle in the side skirt of the bed. My 12v fuse panel is in the bed base. Ran #12 wire from spare fuse to receptacle. I leave the 120vac to 24vdc at home.