Homemade Resmed A10 camping battery

rebelrider.mike wrote: ↑

Wed Dec 04, 2019 5:27 pm

No, I'm planning to camp with it.

Looks good.

rebelrider.mike wrote: ↑

Wed Dec 04, 2019 2:06 pm

Started fitting all the parts into the case:

Unfortunately, there's no room for the charging hardware, so that will have to stay out. Not a huge loss, just would have been nice.

The power meter and switch still need a box to hold their wires and position in the case. And there's still wiring and other details to sort out. But it's nearing completion!

That looks awesome!! Would you be so kind as to post the weight of the whole kit once you get it completed?

Thanks,
-JD

I'll be happy to. Remind me if I forget.

Haven't made much progress lately, but here is a pic that I don't think I've shared yet:


I need to build a little box for the Watt meter and switch to fit into, and then it's just hooking up all the wires to wherever they're supposed to go.

Finished!

I finally took an afternoon to build the little power box.





The Watt meter will keep track of the total energy used, and there is a low Voltage alarm that I can set myself that should go off before the BMS low Voltage cutoff does. I think I'll keep the little battery checker in the box too. It gives some peace of mind knowing the series are all balanced.

I'm hoping to get around 32 hours of sleep between charges. I'll start using it tonight and see what happens. Also, someone asked how much the whole thing weighs. It is 21 lbs.

Looks Good.

I ran the CPAP off the battery for 12 days, I think. It shut off some time at night, but I was able to get some numbers.

It looks like one of my series discharged faster than the others. When it reached 2.5V the BMS did its job and shut the whole thing down. So I didn't get the full 632 Wh I was expecting. And there appears to be a fault somewhere in the battery.

Here's what I got though. The CPAP itself recorded 69.97 hours of total use. The energy meter recorded 412 Wh total. An average of 6.34W. I also noticed that the regulator uses about 1W even when the CPAP is unplugged. So that seems consistent with the observations of a fellow on another forum who also runs his CPAP on a battery.

My first thought about the battery is that one or more fuses might be blown or broken. That would reduce the capacity of a series, and those glass axial fuses are pretty fragile. After visually inspecting each one, and checking continuity between one cell and all the others, It seems the fuses are all intact. I'll have to open the other side of the battery and check the solder joints.

If I can't find anything physically wrong, I'll test each series individually for capacity. Depending on what I find, the whole battery may have to come apart. All these cells are the same make/model and all tested with similar capacities. I packed them with repacker, so it seems unlikely that the cells would be at fault. More likely I did something wrong. But I can't have a battery that's going imbalanced after a single discharge. I'll just have to test everything until I find the culprit.

But on the bright side, I got 12 nights out of this battery when I was initially expecting only 4. That means if I need to rebuild the battery, I can make it smaller and still have plenty of capacity.

That's awesome!! It's always nice when you can confidently check off, "exceeds expectations by a country mile" on the evaluation report!

Bravo!
-JD

I got all 6 series tested on the CPAP battery, and here's the results:
1: 23.2 Ah
2: 27.1 Ah
3: 26.1 Ah
4: 15.2 Ah
5: 26.6 Ah
6: 18.5 Ah


Series 4 was definitely the one that caused the BMS to shut off the whole battery. But series 6 wouldn't have been far behind. I'm not sure why there is such a large variation in capacities. I'm definitely going to pull the battery apart and retest each cell individually.


When I rebuild it though, I'll be making it smaller. Probably 5p instead of 11p. That'll still give me about 5 nights sleep, reduce weight, and take up less space in the box.

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!

This is awesome, kudos to you!

Too cool! It sounds like this has much more capacity than the current batteries on the market (which all seem to promise 1-2 nights without humidifier...). How much did the supplies for this cost?

I don't usually count up the cost of my various weird projects, as it spoils the fun. But in this case, one of the goals was to make a battery just as safe, but way cheaper than what's on the market. After adding up all the parts costs, it looks like the whole thing, case and all, will be a bit under $200.

The cheapest battery i could find for sale was somewhere around $250, and provided only 6 hours of use. Once I rebuild my battery, it will provide 47 hours of use. Now, I happen to already have a few things, like wires, cell holders, and a universal charger that can safely charge this battery. If someone were to have none of those things, I'd guess the cost would be more like $250-300.

Also, 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. I'll share pictures as I make progress.

Would you be willing to share your favorite vendor(s) for reliable cells and holders? When it comes to batteries, I've just been shopping lowest price because, it's a battery -- they're all the same, right? Well, I've learned the hard way (the best way, sometimes) that they are NOT all the same... The 18650's I've bought have been a fairly mixed-bag when it comes to reliability. So far, my risk has been limited to nuisance and aggravation when a cell stops taking a charge... But this project of yours has me thinking of other ideas and I'm wanting better odds of getting reasonably reliable cells.

Thanks!
-JD

I'm sorry you had to learn the hard way that not all batteries are the same. In the case of Li-Ion, that can be a dangerous lesson, LOL.

So by far the most useful place I've been to as far as information is secondlifestorage.com. This is a forum that focuses on DIY powerwalls and collecting used 18650 cells, so as to reduce waste and cost of making batteries. But lots of folks like me, prefer to make smaller stuff like the CPAP battery. Lots of info there and knowledgeable people.

What I know about finding good cells, is first off, never buy a cell with "fire" in it's name. Specifically UltraFire. It's a terrible brand that makes unsafe cells and lies about their capacity.

A few general things that help me find good cells:
- If the price looks too good to be true, it almost certainly is.
- 18650 cells typically have capacities of between 1,500 mAh, and 3,600 mAh, depending on how they're build and what they're for. Anyone advertising 4,000+ mAh is lying to you.
- LG and Samsung tend to make the best quality (safest) cells. Panasonic and Sanyo are ok, but tend to have a higher failure rate.
- Be careful who you buy from. Cells can be re-wrapped and sold as something they're not.
- Test everything. It's the only way to really be sure.

For used cells I buy from a fellow at batteryhookup.com. He's an honest guy who does his best to sell good used cells and provides all the info he can. His stock varies though depending on what he can find. I've made several purchases from there and am quite satisfied.

I've only ever bought new cells once. From https://www.18650batterystore.com. I got 6 cells from them to rebuild a power tool battery. They tested great and work great so far. A little early to recommend them, but I'd definitely buy from them again.

I've mentioned several times about testing. My favorite tester is the Opus BT-C3100. It will charge, discharge, and test up to 4 Li-Ion 18650 cells at a time, and give individual results. It's also about the cheapest on the market. I got mine from Amazon.

The cell holders are pretty much all the same wherever you buy them. I've gotten mine from Amazon and eBay. They come in lots of different sizes do you can make your battery whatever size or shape you want. Search for 18650 cell holder, and plenty of options should show up.

I hope this helps, and is not too long winded.

rebelrider.mike wrote: ↑

Sat Mar 07, 2020 1:27 am

I hope this helps, and is not too long winded.

Nope, not too long winded at all. That's very helpful!! Thank you Sir!
-JD