CPAP gets 18650ed.

rebelrider.mike

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May 25, 2017
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382
Now that I have the wires and fuses off the battery, I was able to test the fuses for continuity. Turns out there are 4 bad fuses. Even though they look fine, they may have been defective right out of the bag.


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I've marked them with a sharpie for replacement later. That explains series 4. But all the rest of the fuses are good, and I still don't know why series 6 was also low. Every connection was good and solid.

But all the cells are free now, and I'm testing them individually. I'll take the best 30 out of these 66, and make a new battery. With pre-tested fuses!
 

ajw22

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Nov 16, 2018
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4 broken fuses concentrated on just that one side sounds like they blew correctly due to overloading. You may simply need a higher rated fuses.
They probably blew when all the bad conditions stacked up:
* Batteryhad low voltage, and regulators/converters drew more current to make up.
* Current draw spiked when CPAP turned on with Heater and humidifier on. A heater, when cold, is basically a short circuit. Huge current draw.
* Cell with highest capacity and/or lowest IR supplied more current than the others
* One broken fuse -> more load on the other fuses and they break

It may also have been a gradual event. Like incandescent light bulb filaments, fuses will weaken over time when repeatedly heated. Then break even though it wasn't overloaded.
 

rebelrider.mike

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They don't seem to be blown as much as broken somehow. The failure isn't visible to me. Before I marked them with a sharpie, they looked just like all the good fuses. I must have broken them somehow during assembly. Or they were defective out of the bag. Even a single good fuse in the series would still be able to handle the current that the CPAP machine uses. The connections were all good.

Pretty sure the cells are all good too. But I'm retesting them just in case. 12 cells so far are in perfect condition. Still testing the rest...
 

rebelrider.mike

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I've finished re-testing all the cells, and they're all good. The fuses were the issue with series 4, and it turns out, series 6 was just fine. I messed up the test, and read the wrong numbers. I can't re-test as it's been dismantled, but I'm confident that the capacity would have been just fine if I'd done the test right. Testing each individual cell is more accurate and less prone to mistakes anyway.

I decided on a 6s6p battery this time, which will make the new battery a little larger than half the size of the old battery. 36 cells instead of 66. That will make the overall case weight a little lighter, the smaller battery will charge faster, and I'll have a little more room inside the case. The smaller battery will still provide around 47 hours of use.I'll share pictures as I make progress. :)
 

rebelrider.mike

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I got the 6s6p battery assembled. This time it goes through the whole charge cycle without getting imbalanced.

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rebelrider.mike

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Thanks. :)

I got the case put back together with the new battery. With the smaller battery, the whole thing weighs a bit less now at about 17 lbs. And there's a little room for an extra pocket.

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I've used the CPAP on the battery for the last couple nights with the hose heater on. This uses a little extra power, so I can find out the actual Wh of the battery in use, a little faster.

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So far, I've used 188 Wh, and the battery is still balanced within a few mV. I figure I'll have part of one more night until the battery is drained. Still looking like I'll have 4-5 nights per charge with the heaters off.
 

rebelrider.mike

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Yeah, my insurance considers it an unnecessary accessory, and won't pay for any of one. When I went shopping, I found mostly 6-hr batteries for about $500. One could certainly make a more compact version, by keeping the CPAP separate.
 

Electricme

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Dec 3, 2018
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Wow, this is really great information, well done.

One thing I have noticed on my system, the current drawn from my GZ Yeti400 seems to vary a fair bit, took a while until I understood why.
Seems the Heated Hose has a thermostat in the end that sensors the outside air temp, the colder it gets during the night the more current is supplied to the heated hose, this is variable so there is no way it can be pinned down to fixed figures.

Winter time, more current drawn out of the battery.
Freezing nights the current drawn is deeper than usual.
Summer time and less power needed to heat the air in the hose.

All up this thread is awsome :)
 

rebelrider.mike

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May 25, 2017
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Thanks! I'm glad you found it helpful. Yes, the hose heater will draw a lot more current than just the basic machine function. I did all my calculations with the hose and humidifier heaters off.

People's preferences will vary, but I prefer my hose heater off, and I only use the humidifier if I'm sick. I figure when camping, I can do without either, but just in case, I made sure that all the individual components could handle the load if everything is at full power. Just means the battery will discharge faster. :)
 

Micko

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Oct 19, 2020
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Lucan said:
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.
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Lucan, this info is brilliant!!
I started playing the 18650 game because I need to power my Resmed Airsense 10 while I'm away shooting in the state forest.Had I not read this and found out about the resmed DC/DC power supply, I don't know what I would have done once I got to the stage of powering the unit.
I'm interested in the "20A BMS (ebay)", that you chose for your setup and the choice to go for 7s.
I'm also wondering how everything is running, after over a year of use. Cheers.
 
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