2 cpap machines with one backup battery

My wife and I both use RemStar Pro's (Resperonics System One) and are wondering if it is possible to attach both machines to one larger battery rather than two for backup and vehicle camping. With the humidifier they are 12V 6.67A and I am working on the assumption they would be closer to 2.5-3A without it. Also are considering Lithium-ion which delivers constant 12V rather than having to have an inverter to cover the voltage drop in lead-acid (glass-mat). If anyone has helpful information as to whether 1) it's possible to attach both to one power source 2) the actual System One's draw in Amps with no humidifier 3) whether it's preferable to stick with an inverter/coverter for any reason, we would be welcome to hear what you have to say.

milo wrote:My wife and I both use RemStar Pro's (Resperonics System One) and are wondering if it is possible to attach both machines to one larger battery rather than two for backup and vehicle camping. With the humidifier they are 12V 6.67A and I am working on the assumption they would be closer to 2.5-3A without it. Also are considering Lithium-ion which delivers constant 12V rather than having to have an inverter to cover the voltage drop in lead-acid (glass-mat). If anyone has helpful information as to whether 1) it's possible to attach both to one power source 2) the actual System One's draw in Amps with no humidifier 3) whether it's preferable to stick with an inverter/coverter for any reason, we would be welcome to hear what you have to say.

The usage you're quoting is the power supply rating, not the actual draw averaged over time. My 560 uses about 0.5 Amps at pressure 10 at sea level. High pressure plus altitude can raise that as high as 1 Amp. The humidifier can use as much as 5 Amps and is not recommended with a battery.

There is no problem using 2 cpaps off of one battery, though obviously it has to be twice as large. It often works out better to have one large bank rather than 2 smaller batteries, especially if the weight is not a big issue.

Since your machines can run on 12V with an inexpensive cable, definitely don't use an inverter, which is probably about 60% efficient.

The setup I would recommend would be an appropriate size AGM battery and a BatteryTender charger. The charger comes with a cable to bolt on to the battery. Add a 1 to 5 splitter cable and 2 cigarette lighter sockets plus 2 respironics DC cables appropriate for your pumps. I normally advise a 35 Amp-hour battery, which with last about 5 nights for one user, but you may want a larger battery, perhaps 60 to 100 Amp-hours depending on your plans.

Lithium batteries can be used, but they are very expensive. For most people, it is only worth the extra cost if the light weight is significant, such as long hikes or air travel. For car camping or home backup AGM batteries are usually preferred.

Refer to the link in my signature for a lot of supporting info.

Thank you, CapnLoki, for your insight.
Your advice has always been proven accurate,
and your sources are reasonably affordable.

It would work fine, as long as the battery was big enough, BUT realize the power will go down TWICE as fast. You would be better served to have both machines 12 volt, so you would not to have to convert to 110 volt a/c. Two deep cycle batteries would be better, that way you could run them in parallel, or hook each XPAP to its own battery, or you could hook them in series (24 Volt) to run that type of XPAP. Don't forget to fuse the leads and correct polarity for safety. Jim


Unless your last name is Gates, a Lith Ion battery is probably not a reasonable solution, unless you own a Tesla or Prius.

The only advantage to Lith Ion, I can see is Weight per amp.

milo wrote:If anyone has helpful information as to whether 1) it's possible to attach both to one power source 2) the actual System One's draw in Amps with no humidifier 3) whether it's preferable to stick with an inverter/coverter for any reason, we would be welcome to hear what you have to sa

As has been said avoid the inverter.

IMO Lithium has no advantage for your application unless making your wallet light has some value for you.

No matter what you will need a DC cable for each cpap so get that done and out of the way. cpap dot com has them for a reasonable price.

Then it comes down to battery size and 1, 2 or more batteries which will certainly be up to you. Personally, I like the idea of 2 batteries (per person) so you can use one while the other is charging - they can take a while to charge back up. How will you be doing that?

I think a 35A will be fine for 1 cpap but not for both over the long haul. Certainly you could power two it just won't last as long and when it dies out there will be two people tired AND cranky. So if you get two 35A then you also run into charging coordination confusion.

So would you want to get 4 - 35A batteries? Then rotate use and charging?
or 2 35 A + 1 -60 or 100A (with 60 or 100 you should be able to run 2 cpap for at least a couple days but then you run into extended charging times when you could use 2 -35A during the charge time).

When planning and organizing all of this please do not forget to get 5 or 6 fuses or when they blow you will be out of luck.

Time to sit down w/pen, paper, and calculator then do the math to see what you want to do.

Whatever you decide make sure you try it out at home to work out any problems.

Do let us know how it turns out.

So the question of whether to use one large battery or several small ones is worth discussing. First of all, there is no advantage to using 2 batteries for two cpaps in this situation - that just leaves you with two partially discharged batteries. If you like the idea of a "backup" then use two batteries sequentially, and you'll hopefully know when the first runs out and that will be the half way point. This also means the battery is half the weight, which helps if it has to be carried a distance. It also gives more options for short trips, one person, etc.

I favor the one large bank because it tends to be more cost effective and more energy efficient. In the 2 battery case, you end up timing the switch to the second either early or late, while in the one large battery situation you just have to make sure the total is big enough. The large battery will accept a proportionately larger charge - if you're charging with a car alternator, for example, a 35 AH will accept about 9 Amps, while a 70 AH will accept 18 Amps. This "acceptance" issue is something I deal with daily if I have to recharge with an engine.

There is also the possibility of one large and one small battery. This gives you more options - a large bank that will efficiently cover the most time, plus a smaller backup. BTW, the connectors used by the BatteryTender charger have fuses and quick connects, so switching over only takes a few seconds.

Not sure this is what you meant to say?

CapnLoki wrote:So the question of whether to use one large battery or several small ones is worth discussing. First of all, there is no advantage to using 2 batteries for two cpaps in this situation - that just leaves you with two partially discharged batteries. If you like the idea of a "backup" then use two batteries sequentially, and you'll hopefully know when the first runs out and that will be the half way point. This also means the battery is half the weight, which helps if it has to be carried a distance. It also gives more options for short trips, one person, etc.

How can using one larger battery be half the weight?

While the implemented plan is up to the OP I do like to be able to use a battery while charging another. Depending on the choosen battery size it could be more money but I think it is more flexible esp. if one is sleeping here and the other sleeping there.

I was just pointing out options for the OP to consider as there are choices here to be made.

Guest wrote:Not sure this is what you meant to say?

CapnLoki wrote:So the question of whether to use one large battery or several small ones is worth discussing. First of all, there is no advantage to using 2 batteries for two cpaps in this situation - that just leaves you with two partially discharged batteries. If you like the idea of a "backup" then use two batteries sequentially, and you'll hopefully know when the first runs out and that will be the half way point. This also means the battery is half the weight, which helps if it has to be carried a distance. It also gives more options for short trips, one person, etc.

How can using one larger battery be half the weight?

Sorry if I wasn't clear - the half weight is for one of two smaller batteries rather than a single large one.

Guest wrote:While the implemented plan is up to the OP I do like to be able to use a battery while charging another. Depending on the choosen battery size it could be more money but I think it is more flexible esp. if one is sleeping here and the other sleeping there.

I was just pointing out options for the OP to consider as there are choices here to be made.

Well, duh!. If the two users are apart then two batteries are probably appropriate! Your point about charging one while using the other assumes you have a means to charge while you're sleeping. Solar probably won't work, nor will a self driving car, yet! Yes, there can certainly be a situation where a battery could be charged at the ranger station down the road a piece, so there are lots of scenarios that could lead you to wanting multiple batteries. I'm just pointing out that one large battery often works out better because it maximizes the usable power.

In the boating world, the "common wisdom" for many years was to have two identical banks, with the common "Bat1/Bat2" switch. Unfortunately this often led to forgetting to charge one bank and other mischief and ending up with two dead banks. Now the preferred approach a dedicated starter bank and a dedicated house bank which maximizes the chance of success.

CapnLoki wrote:Well, duh!. Your point about charging one while using the other assumes you have a means to charge while you're sleeping.

Can you count the # of ass um ptions made?

Guest wrote:they can take a while to charge back up. How will you be doing that?

Guest wrote:

CapnLoki wrote:Well, duh!. Your point about charging one while using the other assumes you have a means to charge while you're sleeping.

Can you count the # of ass um ptions made?

Guest wrote:they can take a while to charge back up. How will you be doing that?

Your battery seems to have died ...

I'm making no assumptions, only considering the theoretical case of dividing battery capacity into two batteries or keeping it as one. As I said, there are possible situations that make it useful to have two batteries. But barring those, one large battery often works out better. The OP gave us no information beyond "backup and vehicle camping" and asked if one large battery can be used.

Either the OP plans to bring enough power to cover the entire trip, or has some means to charge. Either way, one large battery is usually more efficient, unless there is some need to have batteries in two different locations.

If you want to discuss charging protocols, I'm more than happy to do that.

CapnLoki wrote:Your battery seems to have died ...

I'm making no assumptions,

First you nix the idea of more than 1 battery.

CapnLoki wrote: First of all, there is no advantage to using 2 batteries for two cpaps in this situation

Then go on to point out the advantages of having more than one battery.
I was not trying to force an idea on the OP but only point out some options.

CapnLoki wrote:So the question of whether to use one large battery or several small ones is worth discussing. First of all, there is no advantage to using 2 batteries for two cpaps in this situation - that just leaves you with two partially discharged batteries. If you like the idea of a "backup" then use two batteries sequentially, and you'll hopefully know when the first runs out and that will be the half way point. This also means the battery is half the weight, which helps if it has to be carried a distance. It also gives more options for short trips, one person, etc.

CapnLoki wrote:Your point about charging one while using the other assumes you have a means to charge while you're sleeping.

How did you assume they would charge one large battery? And when?

Guest wrote:

CapnLoki wrote:Your battery seems to have died ...
I'm making no assumptions,

First you nix the idea of more than 1 battery.

CapnLoki wrote: First of all, there is no advantage to using 2 batteries for two cpaps in this situation

Then go on to point out the advantages of having more than one battery.
I was not trying to force an idea on the OP but only point out some options.

There are reasons to use first one battery, and then another. As you said there might a possibility to charge one. Or you might want a backup. But I can see no purpose to use two simultaneously. They will undoubtedly discharge at different rates and then you end up with one dead while the other has plenty. Its better if they were combined.

Guest wrote:

CapnLoki wrote:Your point about charging one while using the other assumes you have a means to charge while you're sleeping.

How did you assume they would charge one large battery? And when?

Are you claiming that a large battery is impossible to charge while two smaller ones will magically recharge with no effort? My only assumption is that if and when recharging is needed during the trip, it will likely be easier if its one large battery.

One possibility, actually the first thing that came to mind, is that the big battery is charged at home with a large charger and it lasts for the whole trip. Otherwise, perhaps its charged while driving to the next destination or to a local market. Or maybe a generator is fired up. In these cases the high acceptance rate of the large battery means it will accept more amps. Or maybe a solar panel is charging during the day. Again, the larger battery will accept a larger charge. It is possible to charge two smaller batteries at the same time, but if their charge levels are different (which is inevitable) the charging is less than optimal. And I don't know how many times I have to say this: If you have a situation where it pays to have multiple batteries, then by all means, do that. But all things being equal, it pays to have one large battery.

Ya know I felt if I mentioned using just one battery that for sure you would say I was making assumptions.
So to me, not having a lot of info and making no assumptions at least two batteries was logical.

While there are many possible options so much is not known like sleeping conditions -
will the 6ft DC cord reach the battery when/if there is only one batt?
Where will the batt be placed?
Charging method is unknown - how will you charge the battery if/when there is only one batt?
Are there other health conditions that would make carrying a 60-70 batt a problem?
If not get two. <smile> You won't be sorry. You have two cpaps, right?

I could go on and on but as I said it is up to the OP to make the choices that will best fit their situ.