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CPAP gets 18650ed, maybe.
#1
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 must at 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 18650 with 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. Smile

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!
-Mike G
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#2
(08-15-2019, 03:42 AM)rebelrider.mike Wrote: 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 Tongue


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.
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#3
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. Smile
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-Mike G
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#4
(08-15-2019, 04:13 AM)rebelrider.mike Wrote: 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. Smile

You could always Break it up into 2 Packs that you charge separately but use together. Like 2 5s packs in series.
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#5
(08-15-2019, 12:22 PM)jdeadman Wrote:
(08-15-2019, 04:13 AM)rebelrider.mike Wrote: 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. Smile

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.
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#6
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 supply for the A10 unit that will run on 12V/24V systems.
https://www.cpap.com/productpage/resmed-...0-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 using a 20A BMS (ebay).  The pack fits nicely in the small Apache case from Harbor Freight (a clone of a Pelican case).  I added a 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 a battery state of charge meter, 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.


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