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CPAP gets 18650ed.
#41
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.



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!
-Mike G
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#42
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:
* Battery had 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.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#43
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...
-Mike G
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#44
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. Smile
-Mike G
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#45
I got the 6s6p battery assembled. This time it goes through the whole charge cycle without getting imbalanced.
[Image: 01ff39f9a390be32d8fc8b6bfd376770.jpg]
-Mike G
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#46
Hey,Thats great information!! Valuable content
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#47
Thanks. Smile

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.
[Image: 60d58d972166a68ecdd3e965e3196f42.jpg]

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.
[Image: 143560319c1be28ed55db25e582ea222.jpg]

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.
Korishan likes this post
-Mike G
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#48
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!
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#49
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.
-Mike G
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