converter vs inverter battery setup

[Pugsy]

If you want to back track a little and read what CapnLoki has already said (while waiting for him) you might find your answers in is previous posts.
Everything you are wondering about has already been wondered about and usually covered.

search.php?author_id=67433&sr=posts

ResMed makes it extra difficult...no way around it.
If one of the other brands it is so much simpler.

[WindCpap]

Don't use an inverter. They are terribly inefficient. As for the converter, I would suck it up and pay for the Resmed DC To DC converter rather than risking voiding the warranty. if something happens, you will have proof that you own a Resmed approved power supply.

with the approved converter you can use any input voltage from 12-24v

[Schomer]

i've spent hours reading through those old threads and none of them explain the difference in efficiency between an inverter and converter - other than to say "an inverter uses more".

so, that was my original question. if i go with an inverter, which looks cheaper, then how much more battery will it require? i haven't seen this question answered yet…

[grayghost4]

the common idea around here is if you want to go camping .... get a Phillips machine

[ver2go]

My thoughts on this now (since I'm upset with the stupid DRM ResMed put into a $%^& power supply) is to stop giving them money. At this moment in time, I would buy a inverter even though I said they are inefficient from the start. This way I'm not locked into RedMed and more and not giving them any more of my money. Also you can use the inverter for other things in the future.

I really hate artificial locks on something I paid for.

[grayghost4]

most of the newer RV have Inverters in them .... to power a TV and other things.

[sptrout]

i've spent hours reading through those old threads and none of them explain the difference in efficiency between an inverter and converter - other than to say "an inverter uses more".

so, that was my original question. if i go with an inverter, which looks cheaper, then how much more battery will it require? i haven't seen this question answered yet…

I am a new CPAP user (as of April) and I have yet to take on the "power backup" problem. However, I am a retired radio systems engineer that spent many, many hours designing very large power backup systems (mostly UPSs or AC to DC power systems). Any time you convert one type of power to another (AC to DC, or DC to AC) you lose big time due to the power required to make the conversion. Many times this loss of power can easily be as high as 50% or more (although this problem is getting better). This large loss really kills the amount of backup time that can be provided by the backup power system. For example, you do not want to convert from AC to DC then back to AC as an inverter does. You lose every time the voltage/power is converted.

A UPS is an example of an inverter. It takes AC power, converters it to DC power that is stored in batteries, and then if the AC power fails, it takes the DC power stored in the batteries and converts it back to AC. This process is very lossy, but it is very commonly used in large power systems that have an AC generator on site to quickly restore AC power. Without the generator, a UPS uses large amounts of stored battery power to be able to keep the desired system(s) powered. Without generator backup it is usually better to use DC as the power required by the end device. In this situation, AC power from the utility company is converted one time to DC to charge a bank of batteries. The device powered (CPAP) runs off DC all the time, either from the battery charger, or during an AC failure, directly from the batteries. As you can see, in this case, the end device (CPAP) has the full capacity of the batteries available for its use without any conversions eating up your needed backup time.

The above discussion applies very well in large commercial systems. In the small world of backup power of a CPAP machine (or a more common device; a computer) it is more complicated. Money is usually the biggest driver as well as physical room for any backup system. I have a UPS for my computer network, and another one for my home theatre system. None of the devices in these two systems can run off of DC power, so UPSs are the only options. With CPAP it is a different story, 1) all models use AC power, and 2) some have the capability of using either AC or DC power full time. My CPAP is AC only, so I am leaning towards a large UPS for it (but remember, it is the battery capacity that is important in this case since the AC power required is very low, a small (in Watts rating) UPS with a large battery would be ideal, but likely impractical in a home situation (unlike a commercial site) and very uncommon to find for home use.

So, I am sure that I have confused the situation. I still have not decided which type of system is best for my specific equipment or its placement (remember the physical room issue). I hope to decide one day soon, although I plan on getting a better CPAP machine soon that will have a DC input. If so, that changes what kind of backup power I would consider.

[CapnLoki]

i've spent hours reading through those old threads and none of them explain the difference in efficiency between an inverter and converter - other than to say "an inverter uses more".

so, that was my original question. if i go with an inverter, which looks cheaper, then how much more battery will it require? i haven't seen this question answered yet…

OK already! Here's your answer: A cheap inverter, combined with the "brick" power supply that actually supplies the 24 Volts will be about 60% efficient, maybe even worse. The 12 to 24 volt converter from Resmed will be (I'm guessing) about 85-90% efficient.

Does this make a difference? Maybe - if you are running without humidity you might use about 6 Amp-hours from the house bank with the efficient converter setup, and that be maybe 10 with the inverter. This is not a huge difference if your RV has big batteries and you plan to run the genset the following day. However, if you run the humidifier and worse, the heated hose, then your usage could go as high as 35 amp-hours with the converter and 50 with the inverter. If you have a small battery, this might be a large hit, especially on top of the evening TV/computer usage.

Only you can determine the best course here - If you camp in a humid environment (like on a boat!) then you don't need extra humidity and it all gets easier. You could also argue that its better to spend the $100 on an extra battery than the converter, and that an inverter will come in handy. I'm glad I have a Philips cpap - they run on 12V directly, but if I had Resmed I would consider the 12-24 converter essential, even a bargain, because the failure rate of cheap inverters is high. (And they tend to be noisy!)

Here's a link to a thread where I log the usage with different configurations:
viewtopic.php?t=102179
About 2 years ago I had a post where I measured several inverters:
viewtopic.php?f=1&t=92230&p=854963#p854963

BTW, my understanding of the ResMed connector is that it is proprietary and not trivial to simulate. This would imply that the ResMed power bricks and the ResMed 12-24 converter are the only games in town. If you find a way around this I'm sure there are folks here that would appreciate it.

[Schomer]

Well, after researching everything, and hearing everyone's feedback, I sucked it up and ordered the converter. It is $80 and comes with a battery connection. I found a 10% off coupon online, for 1800CPAP.com. Next, I'll pickup a battery from Costco, then do some in-home testing before my trip! Thanks again, for everyone's help evaluating things. I think the short answer, next time this comes up is, just get the converter, pay a little more, get a more efficient setup, and know it works.

[idamtnboy]

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

Won't work with S9, so probably not with AS10 either. Resmed has some sort of communication between the power brick and the blower so plain 24vdc won't cut it. If you go back to posts 4 to 5 years ago you'll see quite a bit of discussion about this. No one was ever able to crack the code so for the S9, and like I say probably for the AS10, you have no choice but to use the Resmed power brick, be it the 120/240 volt ac one, or the 12vdc to 24 vdc one.

Schomer, if you're buying this setup new seriously consider the Resmed power converter. Why? A couple of reasons. First, a GOOD inverter will cost $30 to $40 or more, or about 1/2 the cost of the Resmed unit. Your CPAP is much too valuable to trust it to a "cheap" inverter. Second, you will have less bulk to deal with. The Resmed unit is more compact than the inverter/Resmed power brick combo. If you expect to have use of an inverter for other reasons, like powering your laptop while camping, or something of the sort, then the decision can easily tip in favor of the inverter. If you go that route don't buy a cheap unit. If the AS10 requirement is the same as the S9 then you can use a modified sine wave inverter with no problem. DO NOT use a square wave inverter, and that's what most cheap ones are.

[CapnLoki]

Well, after researching everything, and hearing everyone's feedback, I sucked it up and ordered the converter. It is $80 and comes with a battery connection. I found a 10% off coupon online, for 1800CPAP.com. Next, I'll pickup a battery from Costco, then do some in-home testing before my trip! Thanks again, for everyone's help evaluating things. I think the short answer, next time this comes up is, just get the converter, pay a little more, get a more efficient setup, and know it works.

This is the common wisdom. Good cpap solutions tend to look at the long haul - a cpap is good for about 5 years, and today's primary machine will become tomorrow's backup, so your new converter will likely find use in a variety of situations over the years.

BTW, make sure the battery you get is labeled "deep cycle." "Dual purpose" will also work, especially since you're probably only using it a dozen times a year, or less. Don't assume that any "marine" battery is deep cycle.

[idamtnboy]

BTW, make sure the battery you get is labeled "deep cycle." "Dual purpose" will also work, especially since you're probably only using it a dozen times a year, or less. Don't assume that any "marine" battery is deep cycle.

That's right. The ones at Costco are labeled marine/deep cycle I believe. Anyway, that's what I have. Get the larger 115 amp-hour one. Only a few $$ more than the smaller one and will run your CPAP longer.

Two things to keep in mind. Deep cycle batteries are designed for lower and more sustained output than are car batteries. Because of this they will not recharge nearly as quickly. Since you won't be using it constantly it will sit for weeks or months at a time. You need to get a decent battery maintainer (NOT a trickle charger) like a unit from NAPA or Battery Tender brand. A 1 1/2 to 2 amp model will work quite well.