CPAP gets 18650ed.

rebelrider.mike

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I got to go camping this weekend, at a primitive (no electricity) campground. Having my CPAP machine would have been really nice. My wife got up in the middle of the night to answer nature's call. When she got back, she thought something in the bushes was growling at her. Turns out it was just me, snoring away.

I have an enquiry to my insurance company to see if they'll buy me a battery, but I doubt it. Most insurance companies consider a battery to be an unessential sort of thing, and won't cover it. The batteries I've seen online are woefully inadequate for a 2-3 night camping trip. Not to mention horrendously expensive.

For anyone who doesn't know, a CPAP is a small machine used to treat folks with sleep apnea. Most apnea patients don't want one, until they get one. But once we get one, we don't want to do without! (Plus, you can pretend you're a spaceman, or deep sea diver, or whatever.)

Being a battery nerd, I mustat least consider building my own. I've seen a couple threads here discussing the possibility, but I haven't seen anyone actually build one.

So first thing, I have checked the power requirements of my machine. I have a ResMed AirSense 10, which uses 24VDC. The easiest (safest) way for me to find the Amps, was to measure the AC power going into the power brick. With the heater and humidifier off, the Amps bounced from 0.25 to 0.07 depending on whether I was breathing in or out. The average of that is 0.16A. The wall socket was 122V, so the power brick was using 19.52W average. The range is 8.54-30.5W.

122V x 0.07A = 8.54W (low)
122V x 0.16A = 19.52W (avg)
122V x 0.25A = 30.5W (high)

This translates, at 24V to:

8.54W / 24V = 0.36A (low)
19.52W / 24V = 0.81A (avg)
30.5W /24 = 1.27A (high)

I've looked around, but haven't found a Voltage range for my machine. All references I could find simply say 24V. So I'll use a regulator to make sure the machine is constantly given 24V. Then next choice then, is how many Volts will the battery be? The closest Voltage range without going over 24V is 5s. (21-14V) The closest without going under 24V is 9s (37.8-25.2V) I like 5s better. Easier to charge. So it will need a boost converter.

Using a 5s battery, the Amps will be different depending on where the battery charge is. At full charge, 21V, in order to give 30.5W, the battery will have to deliver 1.45A. At its lowest acceptable Voltage, 14V,to give the maximum power required, 30.5V,it will need to deliver 2.18A. So after the battery is all designed, it must be able to produce at least 2.18A safely. At this point, I don't really care about the lower values.

30.5W / 14V = 2.18A (Maximum)

The average continuous use of the battery will be the average 19.52W, and the nominal Voltage of the battery will be 18.5V. This will help determine how much energy the battery will need in Wh. I also need to start with a number of hours that the battery will be used between charges. Let's say 8 hours per day, for 4 days: 32h. And I need the average Amps the battery will be using.

19.52W / 18.5V = 1.06A (avg continuous current)
19.5W x 32h = 624Wh (minimum energy required)

So to sum up, the battery must meet the following requirements:
18.5V (nominal)
1.06A (continuous)
2.18A (peak)
624Wh

For the cells making up the battery, I'm looking at a particular make and model LiIon 18650with the following attributes:
3.7V (nominal)
2.6Ah (probably)
9.62Wh (V x Ah)
0.5A (continuous)
5A (peak)

Volts is determined by cells in series, so to get 18.5V nominal, 5 cells will be needed.

18.5V / 3.7V = 5 cells

The total number of cells needed for the whole battery, is the energy of the battery, 624Wh, divided by the energy of each cell, 9.62Wh. Of course, the resulting number needs to be rounded up to the nearest whole cell.

624Wh / 9.62Wh = 64.86 (65)cells.

Now 65 cells are needed in total, and 5 must be put in series. To get how many in parallel, 2 things must be considered: the total energy of the battery (again) and the current the battery needs to deliver. To get the number in parallel to satisfy the energy requirement, The total cells calculated earlier, 65, is divided by how many cells must be in parallel for the Voltage, 5. But Amps must also be taken into consideration. Each cell can deliver 0.5A, and the battery must deliver 1.06. Additionally, each cell can deliver a short burst of 5A safely, and the battery will need to deliver up to 2.18A for short periods of time. (This won't be a problem!)

65 cells / 5 cells = 13 cells parallel (satisfies Wh)
1.06A / 0.5A = 2.11 (3) cells parallel (satisfies Amps)
2.18A / 5A = 0.436 (1) cell parallel (satisfies peak Amps)

Pick the highest number, 13, and all the requirements will be met. This gives a battery that is 5s13p, and should last up to 4 days. (Maybe a little less due to inefficiencies.)

Now I can go back and check what the actual battery should do. (Using the same math as above.)

21-14V range
18.5V nominal
6.5A continuous current
65A peak current
625Wh total energy
32h runtime

Of course, the battery will not get to deliver such high currents because I'm putting fuses on each cell to prevent that sort of thing. But you can see the battery meets or exceeds all the original requirements. So we done good. :)

As for cost, I should be able to get 99 cells for about $165, a 5s BMS for $5, a boost converter for nothin' (already have one), and 100 fuses for around $10. So about $180. I already have 5s balance wires, and an iMax charger. I've also got fancy tape, foam, cell top insulators, and other stuff handy from past projects.

Maybe I could fit this into one of those old school metal lunch boxes. I'll also want a Voltmeter and power switch. I bet insurance will never come up with a 4-day battery for $180!
 

Korishan

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rebelrider.mike said:
My wife got up in the middle of the night to answer nature's call. When she got back, she thought something in the bushes was growling at her. Turns out it was just me, snoring away.

HAHAHAH :p


You could go 10s, 36V pack, then buck down. ????
That way you wouldn't have to worry about the amps slowly creeping up as the voltage drops.
 

rebelrider.mike

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I told her I dreamt I were a motorcycle!

Once I get past 6s I don't really have a way to charge stuff. There's chargers out there, but the iMax is free, and the number of cells in parallel means Amps really aren't a problem. :)
 

jdeadman

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rebelrider.mike said:
I told her I dreamt I were a motorcycle!

Once I get past 6s I don't really have a way to charge stuff. There's chargers out there, but the iMax is free, and the number of cells in parallel means Amps really aren't a problem. :)

You could always Break it up into 2 Packs that you charge separately but use together. Like 2 5s packs in series.
 

Geek

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jdeadman said:
rebelrider.mike said:
I told her I dreamt I were a motorcycle!

Once I get past 6s I don't really have a way to charge stuff. There's chargers out there, but the iMax is free, and the number of cells in parallel means Amps really aren't a problem. :)

You could always Break it up into 2 Packs that you charge separately but use together. Like 2 5s packs in series.

So long as the packs are used equally in series, you will find that they discharge at similar rates, allowing you to charge them in Parallel.
 

Lucan

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I also use a CPAP, and also use the ResMed A10. PLEASE DON'T USE AN INVERTER with the standard AC/DC power supply. Terribly inefficient, and some have reported power supply failures with cheap modified sine inverters. Resmed makes a DC/DC power supplyfor the A10 unit that will run on 12V/24V systems.
https://www.cpap.com/productpage/resmed-dc-converter-airsense-aircurve-10-machines Note that the Resmed power supply uses a "smart" connector.I considered building my own power supply after reading this thread, but in the end decided the $85 was worth it.

It will run between 12V and 24V.I have used this power supply with a 12V lead acid, 18V tool batteries, and my 24V custom pack, described below.

I built a 7s8p pack usinga 20A BMS (ebay). The pack fits nicely in the small Apache casefrom Harbor Freight (a clone of aPelican case). I addeda 20A fuse, Anderson PowerPole connectors, a 3d printed connector holder to the side of the case, and a simple volt meter. Below are some pictures of the V1.0 build. I'm building a second pack that will have abattery state of chargemeter, plus a USB outlet for phone charging.

I've taken my pack camping a couple times this year and it will run the CPAP for 3 nights, with the humidifier on!

I forgot to add, for charging I picked up a dedicated 7s, 2A charger. 29.4Vmax voltage. Less than $10 shipped from eBay. It seems like most/all of the parts needed for this build came from China/eBay.
image_afbski.jpg
image_uzrdjf.jpg

image_suzeqo.jpg
 

rebelrider.mike

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Thanks folks, but I think all that stuff is a bit complicated just to get rid of a simple boost converter. Unless there's a particular reason that boost converters are bad?

Lucan: thanks for sharing your setup, and the link. After you charge your battery, are you running your CPAP on 29.4V? Or do you have some kind of regulator? Or do you mean that you're running the battery through the 12/24V DC-DC power supply?

If I can run my ResMed on a range of Voltages, that would definitely change how I design the battery!
 

Korishan

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rebelrider.mike said:
Thanks folks, but I think all that stuff is a bit complicated just to get rid of a simple boost converter. Unless there's a particular reason that boost converters are bad?

They aren't inherently bad. The issue is that boosting a voltage takes more "amps" from your battery to give you the voltage you need at the required wattage.

Example:
Device uses 5W @ 12V

5W / 12V = 0.42A
5W / 4.1V = 1.22A

Whereas if we use a buck converter and run the battery voltage up to 7s (25.9V nominal, 28.7V top end), we get:
5W / 25.9 = 0.19A

These numbers are just examples and not to match your specific application, but it shows the large difference in amp draw based on voltage. Since you need at least 24V, your numbers will adjusted accordingly.
Remember, higher amps requires heavier wire. Since your CPAP can consume 30W peak
8.54W / 24V = 0.36A (low)
19.52W / 24V = 0.81A (avg)
30.5W /24 = 1.27A (high)
you'll need to be able to handle the 1.2A load. This doesn't seem like much, but most "boost" converters have harder time achieving the higher stable amps. Using a buck converter would reduce the amp requirements.
Also, as your battery pack decreases in voltage, the amp draw will increase. At 21V (3V/cell) to get 30W, you'll need 1.43A. As the night goes along, the boost converter will be working harder and harder to keep the load constant.
This is why I would recommend the buck converter.

However, if you didn't skimp out on the boost converter, you would probably be just fine.
 

Lucan

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rebelrider.mike said:
Lucan: thanks for sharing your setup, and the link. After you charge your battery, are you running your CPAP on 29.4V? Or do you have some kind of regulator? Or do you mean that you're running the battery through the 12/24V DC-DC power supply?

If I can run my ResMed on a range of Voltages, that would definitely change how I design the battery!
The power supply I linked to above takes the place of the power "brick" that plugs into the wall. It outputs a steady 24v to the CPAP, and works over a range of input voltage. It is fed from my 7s battery, who's voltage ranges 29.4v down to 21v. The power supply also has the ResMed proprietary plug that tells the CPAP that there is a "official" 90w supply plugged in.

It is marketed for 12v and 24v systems, so should work from about 29V down to 10V. I'm sure it is just a buck/boost converter at heart, maybe with some additional filtering.
 

rebelrider.mike

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Korishan: Ok, that makes sense. My (theoretical) battery as it is now could handle 6.5A continuously, so the boost converter would be the limiting factor more than the battery.

Lucan: I will look into that. The DC-CD brick would probably (hopefully) be better than the little boost converters I can find. And I wouldn't have to figure out what plug to get. Maybe my battery won't need a boost converter. :)
 

Korishan

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rebelrider.mike said:
Korishan: Ok, that makes sense. My (theoretical) battery as it is now could handle 6.5A continuously, so the boost converter would be the limiting factor more than the battery.

Yes, the converter is the limiting factor, that's what I'm referring to ;)
Depending on which one you get, it could be burned out from constant current load. The one Lucan is referring to actually might be the best route, especially considering that it also sends the authenticating signal to the CPAP to make sure it works.
 

rebelrider.mike

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I've been thinking...
I'm sold on the 12/24V adapter rather than using my own regulator. I also really like the ABS box idea. I'm going to try to find one that will hold the battery and the cpap both. And since I won't be using a booster, I can make a 6s battery instead of a 5s, and be within the Voltage range of the adapter. That means more Volts, less Amps, and can still be balanced by my iMax charger.

I've got cells coming from Battery Hookup, and I've priced everything but the box. I'll have to get everything together and see what size I need.
 

rebelrider.mike

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Batteries arrived!

image_raymrf.jpg


I had to clean up a bit, because the testing station has been buried in other project stuff. These are modem batteries made by Pegatron. Never heard of them before, but they've used legit LG cells.

image_libqbf.jpg


Dug out my favorite tools for cell harvesting, but after the first box, I found I didn't need the mallet.

image_jsdzip.jpg


These are pretty easy to open without destroying, and since they're compact, they may be useful for future project.

image_awdpnk.jpg


The first 4 cells came in at just under 2600 mAh. And it's so helpful to have the cat sit on top of the tester instead of trying to steal my tools as I'm using them!

image_rgrbki.jpg


I've added a couple features to the testing station since I last used it. I got a UPS just for the station; mostly to make sure the Opus doesn't lose power during the discharge test. Also, I discovered that there is such a thing as a GFCI adaptor that works just like the outlet, but you don't have to change out the outlet. Inexpensive, and just too cool not to get one!

Next thing is, I want to get the DC-DC adaptor made for the cpap, and test the output under different input Voltages to see exactly how it behaves. Then I can nail down exactly how I want to make the battery.
 

norknid

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If you regulate to 24V, there is a simple way to use a Thevenin resistor equivalent for the center pin. I have the values somewhere if you want them. That connector is unique even though it looks standard. I just cut it off the power pack as I had a spare.
 

Geek

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For my UPS solution for the Opus, and my other chargers, I use an old lead acid battery. A car battery that is no good for starting anymore usually still holds a substantial amount of power. I pick them up for scrap price, and when I have used all the useful life from them, I sell them as scrap. Cost of ownership is $0. (last time I even made small profit, as the price of lead had risen) I usually get several years out of them. They are also great for running LED lights in a blackout.
 

rebelrider.mike

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I plan to get the DC-DC adaptor that's made specifically for the cpap. I've devised a way to test the output Voltages of the two + pins while I change the input Voltage. This should tell me if it really is acting as a boost/buck regulator or not.

The UPS was second hand. Usually, I pick them up for cheap and the battery needs replacing. In this case, the battery was in perfect condition, and so is the rest of the UPS. So rather than modify it, I left it as-is. Around here, we rarely get long outages, but we to get a lot of brief interruptions that are long enough to reset electronics. Only problem I have is the cat likes to walk across it and step on the power button. LOL.

I've discovered that TP4056 boards, buck converters, and small bms boards fit in the little space above the cells in those gray boxes. I've got 9 of them apart now, and only one broke during disassembly.
 

norknid

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To power the center pin from 24V to make it look like a 85W pack use a 22K resistor to the +24V and a 3.3K for the bottom leg of the voltage divider. This is the Theveninof a 2.7K resistor sourced from 3.3V for the center pin. Actual values are 19.6K and 3.16K if you used 1% resistors, but the other values of standard resistance are close enough. DO NOT USE ANYTHING ELSE YOU MAY HAVE SEEN ON THE INTERNET. This information comes directly from the patent and will not damage your machine. I in the day. just use a car battery and a boost converter. Typical usage is between 100 and 120WH a night. I have mine on a timer so I don't forget to turn it off when I get up. It acts like a very rude alarm clock if I sleep late.
 

rebelrider.mike

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The DC-DC adapter is here! Finally get to experiment a little.

here is the DIY adapter I made to test the output.

image_ugdlhn.jpg


A simple loop for the outer ground ring. Inner ring is a wire wrapped around another wire and held in place with the insulated wire with tape around it to get the width right. That will contact the inner 24V ring. Finally, the center pin connects with a short insulated wire, where I've taken a few strands out to make room for the pin. That would be3.3V.


image_ujqele.jpg


With that in place, I made a temporary 3s battery to test the adapter's ability to boost Voltage and keep it at 24V under a lode. The battery connects to the adapter, which connects to a CV CC regulator, which connects to a 12V 21W light bulb.

image_fsepau.jpg


With the battery at just under 11V, the output is at 24V without a load. Not pictured is the center pin at 3.2V.

Under a load of about 20W, the battery drops to around 9V, and yet the adapter's output remains at 24V, and 3.2v.

image_bouggt.jpg


So the adapter is a good boost regulator and can go as low as 9V input, maybe even lower.

Next I put together a 7s battery to test the adapter's ability to reduce Voltage. Same setup as last time, but with the bigger battery.

image_tntdxh.jpg

With no load, the battery is about 26V, and the adapter still outputs 24V and 3.2V.


image_asneev.jpg

Again, under a 21W load, The battery drops in voltage, but is still above 24V. The adapter's output remains at 24V, and 3.2V.

So between this and Lucan's experience, with a fully charged 29.4V battery,I'm convinced that this adapter is also a buck/boost regulator. Which means I don't have to install one on the battery.

I just ordered a bunch of stuff to start building the permanent battery. In the meantime, I'm charging up a temporary 6s battery to actually run the cpap off it for a couple hours.
 

Lucan

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Glad to hear it is working out for you!

The center pin (3.3v) does some handshaking with the CPAP to tell it that it is a authentic power supply, so you might not be able to catch the signal with a multi meter. I know there was someone on the CPAP board that was reverse engineering the supplies, and was able to catch the signal and timing. I don't know much more than it is required for the CPAP to start up.
 
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