DIY Battery box question

[JethroBowdien]

Hello, I'm new to the forum but not to CPAP. I've been using an Airsense S11 Autoset for a few years now. Last weekend I went camping for the first time in a long time, something I used to do a lot. I had to go without my machine, I believe the first night I've slept without it since starting therapy and it was MISERABLE. I can't have that happen again and I may need to go off grid again in a few weeks. I searched the forum and can't seem to find an answer to my specific question.

A few years ago I built myself a pretty serious, portable, rechargeable battery box to power an ice fishing house I have, one of my winter addictions. It's made with three, 20ah LeFePo4 batteries wired in parallel to power all my 12v items in my fish house (sonar, bait bubbler, lights, underwater camera, a flat panel 12v screen, charging devices, a fan... etc.) Anyway... I want to use this battery box to power my CPAP for future off grid use and in the event of an emergency. I see that I can buy a 12v to 24v step up converter to power my S11, and my battery box was built with a "cigarette" style plug that I can plug into. I also see that I can buy a cord that does not step up the power if you have a 24v connector. Currently the battery box is wired 12v with the three 20ah batteries in parallel, but I can very easily wire the batteries in series to achieve 24v natively. My question is this- is the 12v to 24v step up converter less efficient than using the batteries in series? The cords are not far off from each other in price, so I'm not worried about that, although the 12v to 24v step up converter would be an easier solution so I can keep my battery box wiring as it is currently.

I know enough about electricity to be dangerous, but a step up voltage converter- that is beyond me how it works. If it's more efficient to run the 24v cord than the 12v to 24v step up converter, than that's what I'd probably do. For what it's worth, I will likely not need more than 2 nights sleep off grid.

Thank you!

[ChicagoGranny]

Do you require the humidifier?

[JethroBowdien]

I would prefer it, but I guess I could live without it.

[Dog Slobber]

Don't worry about stepping the from 12v to 24.

Get the ResMed or off-brand DC-DC converter. They accept 12 from a cigarette lighter adapter.

• And you don't have to source the power adapter connector
• Determine the pin-outs
• I believe, but may be wrong, this is based on the Air10 power connecter. But, it also had a third pin 3.3v control pin.
• Don't have to worry about building a 12-24 converter

When estimating power consumption, The rule of thumb I've used is about an amp an hour, your milage may vary. So you should be able to get about 7 nights, assuming no heated humidity or hose. If you use heated humidity, your going to use a lot more. 2 maybe 3 nights. Consider pass-over humidity, water in the tank but heat turned off.

Refer to the ResMed battery chart, for battery usage.
https://document.resmed.com/en-us/docum ... lo_eng.pdf

It doesn't list the Air11, and the capacity usages have builtin tolerances to assuming Lead-Acid and prevent over discharge.

The nice thing about using LFP batteries is they can be discharged very deep.

[ChicagoGranny]

I would prefer it (humidifier), but I guess I could live without it.

Better try it at home first. Some people are miserable without it. Others of us don't ever use it.

[amenite]

...
and my battery box was built with a "cigarette" style plug that I can plug into.

I've not noticed any serious/noticeable degradation from the 12/24v converter. I think it's pretty minimal, way less than an inverter for sure. What I have noticed is alot of variation in the female cigarette style jacks. Some seem like they barely can accept the plugs and will barely stay plugged in. One of these days I am going to snip the cigarette plug off my AS10 DC/DC converter and put Anderson power pole connectors on there so I can mix and match what I plug into for 12V. Most of my battery units now have 5.5 mm barrel connectors for 12V output (and some for input also) and I would feel alot better about that staying plugged in overnight.

[RNeil]

The 12 volt to 24 volt converter can not be 100% efficient. It is probably 80 to 90% efficient. If you wire the batteries in series, you get 24 volts without any loss. On the other hand, the three batteries will have more energy than two.

[JethroBowdien]

...
and my battery box was built with a "cigarette" style plug that I can plug into.

I've not noticed any serious/noticeable degradation from the 12/24v converter. I think it's pretty minimal, way less than an inverter for sure. What I have noticed is alot of variation in the female cigarette style jacks. Some seem like they barely can accept the plugs and will barely stay plugged in. One of these days I am going to snip the cigarette plug off my AS10 DC/DC converter and put Anderson power pole connectors on there so I can mix and match what I plug into for 12V. Most of my battery units now have 5.5 mm barrel connectors for 12V output (and some for input also) and I would feel alot better about that staying plugged in overnight.

That's exactly how I am going to wire it. All the things I plug in to this box are wired with 5.5mm barrel connectors, they are perfect.

I'll be able to camp on the ice this year again!! so excited.