#### rebelrider.mike

##### Active member

- Joined
- May 25, 2017

- Messages
- 373

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!