converter vs inverter battery setup

[Schomer]

It looks like my AirSense 10 requires a 12v->24v converter. I've read through a lot of the threads here about battery backups, but I haven't found any info about how an *inverter* compares to the *converter* that is recommended by ResMed.

Inverters are cheap, compared to the $85 converter ResMed sells, so I'd like to hear from anyone who is using an AirSense 10 with an inverter battery setup, instead of the converter, to better understand what kind of efficiency they're getting. The inverter is going to use more energy - right? Is there enough of a difference that it even matters though?

[ver2go]

Let's back up for a second. Can you describe what you are trying to do rather than what you think you need?

In context of what I see so far

Inverters - invert.. take DC to AC.. typically take DC output of Solar Panels (48v) to make into 110V AC
Converters - in context of what is see here are DC - DC converters, step up in your scenario. Looks like you want to take a DC 12V source to step up to 24V for the Resmed. Make sure the amperage output is enough as well.

*Note on inverters, there are different types. Cheaper ones are "Square Wave". I think a CPAP will require a more expensive "Sine Wave" output inverter if that is what you're looking for.

[Schomer]

yes, i'm planning to put a battery setup together. sounds like i need to get a 12v->24v converter. but why not use an inverter off the battery instead? from what i've read, there isn't an issue with square/sine waves, but i need to double-check that as well. maybe i'm just being cheap here and should go ahead with the converter. it just looks kind of silly to spend $90 on something, if an inverter setup works just as well and saves money.

[Pugsy]

How long are your camping trips? One night? More than one night? Will there be available power to recharge the battery during the day if more than one night is needed?

[ver2go]

Still not making complete sense to me. You want to run your Resmed off batteries?

All you need is 24V dc out put then. What is your battery source? Car battery? You going to buy them?
There is a property call "series". If you have two 12V batteries and put them in a series, you get 24V output.

If this was my project I'd probably ask these questions of myself

How portable? More portable would mean I would use LiPos rather than acid based batteries
How long do I need to run it for off the battery?
* Note, I'm new here and to CPAP so I don't know what an S10 draws to help you figure out how many mah batteries you will need.

Anyway to sum it up, you don't need a converter or an inverter. The S10 takes DC voltage. Batteries put out DC voltage.
I would however put in a regulator (keeps voltage stable). Fully charged batteries put out higher voltage and drops according to a curve. So at full charge it will be more than the 24V the s10 needs. I'm not sure how regulated the S10 is, but those laptop power supplies they come with are regulated.

[Schomer]

Okay, sorry, a little background… I'm going to be staying in my RV for a week. I can run the generator during the day but not at night. So I was thinking of picking up another RV house battery (deep cycle?) for about $75 (local Costco has them) and then going with either an inverter or converter to power the unit off the battery. During the day, I'd recharge the battery with a battery charger, while running the generator.

[Pugsy]

Okay...wait for CapnLoki to drop by as he has first hand experience with prolonged cpap machine usage running off batteries (lives on a boat for a good part of the year).
He can explain the pros and cons a lot better than I can. I asked those questions because I knew he would ask them because how one uses or expects to use a battery makes a big difference in what is recommended.
The converter is usually recommended because it will give you more battery life but since you can recharge the battery during the day it may not be that critical.

Wait for CapnLoki though...he knows his stuff and my eyes glaze over when I get into anything beyond the basics on battery use.

[ver2go]

Makes a lot more sense now.

You have a couple of options

1) Inverter - You plug your existing power supply that big wall wart thing into the inverter. Inverter attached a 12V.
Probably the least efficient way of doing it but you get all the regulation provided by the power supply.
You'll still have charge that battery some how.

2) Do 2 x 12 batteries in series, get a cord with the correct barrel connector to go in the S10. I would strongly advising you put in a regulator for this
Something like this. I just assumed 24V, 90W(?) so that's about 3.75 AMP
http://www.ebay.com/itm/Automatic-Boost ... SwEetV7e7f

3) I think this would be the best and easiest compromise. 1 x 12V battery with a step up converter. Nice thing is regulator is built in. You'll still need a cord with the right barrel connector to plug into the S10
Just as an example
http://www.ebay.com/itm/DC12V-Step-Up-t ... SwMmBVzP1H

So what you want to look for, assuming my guess is correct is 24V at least 3.75 amp. I based this on 90W power supply at 24V to give me the 3.75 amp

[Schomer]

ver2go,

Well, that $16 converter looks a lot better than a ResMed $85 one, but I don't know how to wire things up like that… I guess I'd need to find the other end that goes from the converter wires to the CPAP unit…

You mentioned that an inverter is not as efficient… that was my original question. How much different is it? If it's like 10% less, then that doesn't seem like an issue. If it's 50% less efficient, then that cancels the thought out.

Pugsy,

In addition to my RV I also stay on my sailboat, so I can't wait to connect with CapnLoki and hear what he thinks!

Wow, what a great discussion forum… you guys are quick to help out. Thank you so much.

[ver2go]

I'm not smart enough to figure out how much more in efficient. But lets put it this way.
1) Convert 12V dc to 110V ac
2) Power Supply from the Resmed then takes 110V AC and converts it back to 24V dc

Step up converter is easy. Think of it this way. Hi and low side (which is typical of all transformers)

Low side is your positive and negative from the 12V battery. 2 Wires
High Side is the 24V dc output of Positive and Negative to the resmed.
* There is usually some adjustment on those converters. You turn a pot and measure with a meter to make sure the output is 24V

4 wires.

On the high side if you can find just a replacement cord just splice them into that high side output of the converter. Make sure to align "tip" and "ring" to the correct polarity. Meaning positive to the correct wire. I can look later if you need it. That barrel connector is a standard, just can't remember how many mm it is right now. It is used on laptops.

I assume you can charge the car battery easily? I think RV's have an aux battery just for that purpose.

[Schomer]

yeah, i have a battery charger i'll bring along in the rv to recharge the battery during the day…

i'll do some looking around for the barrel connector. gosh, it would be nice to find a cheap converter setup like this and save people money.

[Pugsy]

I think the converter vs inverter efficiency issue is considerably more than 10% loss and there's the "what pressure do you use" factor and "do you want to use the humidifier" factor.

This is where my eyes really glaze over so I will defer to CapnLoki.

So I guess you should also answer the "what pressure" and "do you want/need to use a humidifier" questions.

I know I simply could never use my cpap without my humidifier myself. I tried a lower setting with disastrous results once.

Heated hoses pretty much are out though...they really suck the power but we can usually figure out some what to do without a heated hose.

[Schomer]

i've only been using the machine for two weeks and i have never tried it with the humidifier off, so no idea how well that works for me. i live in santa cruz… not a dry climate, so it might be fine.

[Schomer]

possible dc power plug… need to figure out the barrel size...

http://www.amazon.com/Philmore-Right-An ... NJYPHZGHFE

[ver2go]

I tried looking up the barrel connector for you and found some hearsay that worries me. Not sure the rule about linking, so I'll leave that out.
There is some talk that Resmed does some thing proprietary to their power output, which is why they sell their own DC-DC converter.

Side note - that really irks me that companies put in silly artificial limitations.

So we come full circle. The safest options I see for you now
1) Inverter to take the 12V to 110V ac so you that power supply from Resmed and all is happy.
2) buy their proprietary DC to DC converter instead of a $16 one

* Also I saw some mention a sine inverter is required.

This is all based on a quick google search, but having seen that, I would not recommend you do anything that doesn't go through the Redmed power supplies since they seem to want to lock you in from what I've read.

Sorry. I tried to help, but didn't know RedMed was one of those evil companies that put in artificial limitations.