The top springs are moving, but the terminals aren't moving in relation to each other.
Cinergi's 28 kWh / 4 kW Solar / 10 kW inverter RV build
- Thread starter cinergi
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The top springs are moving, but the terminals aren't moving in relation to each other.
85% SoC: 0.7, 1.1, 0.8, 1.4 spring movement
91% SoC: 0.9, 1.2, 1.2, 1.7 spring movement. I can now measure .15mm, .14mm, and .19mm of movement between the terminals with the flexible bus bars on them. Cool, the CALB bus bars work! I'll measure again at 100% and again after disconnecting them.
I've confirmed with NHRV that I have 77" of width, so I'm good!
100% SoC
Spring travel: 1.0, 1.4, 1.5, 2.0
2 other things I did -- I tried using the CALB bus bars on my main battery to see how it would fit ... it's cleaner-looking and it's easier to compress the cells by hand (and I can observe the bus bar flexing up, as it should).
Second, I arranged an 8s pack in the spring fixture so I could get a sense of maneuverability and whether the cells would move/fall out under the spring pressures involved at 0% SoC. It's heavy for sure (and I didn't have cables & hardware attached) but I think doable for the 2 times I intend to move them. The square rod moved when I picked it up -- so not enough pressure against the plywood to keep it in place. I think the cells moved a little because after I set it back down, the left and right cells weren't quite touching the table. Not that I'll be moving these at 0% SoC (hopefully) but it was a good worst-case test. I'm not sure I'd walk around with them like that. Maybe I'll put a strap around the contraption for safety and/or see if there's an opportunity to combine that with mounting to the RV. That was the nice thing about 4 cells -- it was really solid even at 0% SoC (less weight and less spring movement). ?
Spring travel: 1.0, 1.4, 1.5, 2.0
2 other things I did -- I tried using the CALB bus bars on my main battery to see how it would fit ... it's cleaner-looking and it's easier to compress the cells by hand (and I can observe the bus bar flexing up, as it should).
Second, I arranged an 8s pack in the spring fixture so I could get a sense of maneuverability and whether the cells would move/fall out under the spring pressures involved at 0% SoC. It's heavy for sure (and I didn't have cables & hardware attached) but I think doable for the 2 times I intend to move them. The square rod moved when I picked it up -- so not enough pressure against the plywood to keep it in place. I think the cells moved a little because after I set it back down, the left and right cells weren't quite touching the table. Not that I'll be moving these at 0% SoC (hopefully) but it was a good worst-case test. I'm not sure I'd walk around with them like that. Maybe I'll put a strap around the contraption for safety and/or see if there's an opportunity to combine that with mounting to the RV. That was the nice thing about 4 cells -- it was really solid even at 0% SoC (less weight and less spring movement). ?
Another revision of my compression fixture ...
Goals
I used a steel angle bracket with one long side and two 1/2" square tubes to create a floating floor. The square tubes simply sit on top of the angle bracket and are cut to a length that allows maximum compression (empty SoC). I need a better way to prevent side-to-side movement of those square tubes (my original design didn't work) but I think it'll be similar to the bolts that you can see on both sides.
The 2x6's are not part of the design but they're pretending to be the feet of the system. I hope to use something like this:
which mount to the L bracket and will give me vibration resistance and hopefully enough lateral movement to allow proper compression. These are pretty stiff so I'm not sure. I have other sizes on order.
This increases my overall height by at least 2 inches which I'm not wild about but I can probably get away with it. Note that this is test plywood and isn't cut to proper size (finished product will be a little bigger).
Also note that I don't plan to rest the cells directly on the square tube. I'll likely have thin plywood there as a shelf (which I'll also have to prevent from moving sideways and falling out).
Goals
- I want to be able to secure the fixture to the floor without compromising its ability to expand and contract
- I want to protect against vibration (this is mounted in a 5th wheel)
I used a steel angle bracket with one long side and two 1/2" square tubes to create a floating floor. The square tubes simply sit on top of the angle bracket and are cut to a length that allows maximum compression (empty SoC). I need a better way to prevent side-to-side movement of those square tubes (my original design didn't work) but I think it'll be similar to the bolts that you can see on both sides.
The 2x6's are not part of the design but they're pretending to be the feet of the system. I hope to use something like this:
which mount to the L bracket and will give me vibration resistance and hopefully enough lateral movement to allow proper compression. These are pretty stiff so I'm not sure. I have other sizes on order.
This increases my overall height by at least 2 inches which I'm not wild about but I can probably get away with it. Note that this is test plywood and isn't cut to proper size (finished product will be a little bigger).
Also note that I don't plan to rest the cells directly on the square tube. I'll likely have thin plywood there as a shelf (which I'll also have to prevent from moving sideways and falling out).
Do you have numbers behind the compression concept? I was just planning to bolt mine up but I am curious about the need for spring loading.
In mine, I've added some PEX around the allthread to protect the batterys from rubbing on it. (yes I read the flex numbers, I'm more concerned if I need to allow for cell movement)
From what I can see, the biggest concern is terminal movement delta from the pack postion. I'm thinking that flexible bus bars -calb style or just terminated cables is likely sufficient. It's still a significant investment in battery pack, so I'm certainly interested as I'm pretty close to battery installation on mine. (Today it's a tesla module, soon to be the 16x 280ah cells)
In mine, I've added some PEX around the allthread to protect the batterys from rubbing on it. (yes I read the flex numbers, I'm more concerned if I need to allow for cell movement)
From what I can see, the biggest concern is terminal movement delta from the pack postion. I'm thinking that flexible bus bars -calb style or just terminated cables is likely sufficient. It's still a significant investment in battery pack, so I'm certainly interested as I'm pretty close to battery installation on mine. (Today it's a tesla module, soon to be the 16x 280ah cells)
The math is in this post.
I believe flexible bus bars are required for this setup, yes. They don't move much, but they do move.
So you used 16 of those $4 springs? How many per pack (single-row)?
What I mean is if you were going to use these springs for a 4S or 8S pack with all of the cells abutting in one row, how many Springs would you use, 4? 8?
4 per pack, regardless of 4s or 8s -- it can handle both because the fully-compressed set of 4 springs is 728 pounds and subtract 4mm for 8 cells (0.5mm each) or 0.15748 inches at a spring rate of 380 pounds per inch so 60 pounds per spring or 240 for all 4 .. which means 0% SoC is at 728 - 240 = 488 pounds. 728 pounds = ~13.5 PSI and 488 pounds = 9 PSI. I plan to back off the spring a little so I'm closer to 8.5 at 0% SoC and 12.5 PSI at 100% SoC.
Thanks.
So I understand you are using 5 springs per pack and that each spring applies 182 lbs fully-compresssd (since 4x182=728lbs).
And I understand that from that fully-compressed state near 100% SOC, you are assuming 0.5mm compression per cell (so each spring will expand 8x0.5mm = 4mm at 0% SOC).
And by ‘spring-rate’ I believe you mean the force applied by each spring will reduce from it’s maximum of 182 lbs fully-compressed at a rate of 380 lbs per inch meaning that after expanding 4mm = 0.1575”, each spring will have been reduced by 60lbs to 122lbs remaining (so 488 lbs remaining in total at 0% SOC).
What I’m not understanding is what is the maximum travel of the spring at this spring-rate of 380 lbs /inch? Is it 182 lbs / 380 lbs-per-inch = 0.479” (12.17mm)?
And also, how confident are you of the 0.5mm per cell travel figure is correct? I’ve seen others claim travel if 0.75mm / cell or even 1.0mm / cell between 0% SOC and 100% SOC.
Could it be that those larger numbers were measured on the first charge/discharge cycle under pressure and you are getting this lower number after completing several cycles?
I started going down the Belleville washer route but had problems with inexpensive washers getting caught on threads, so when I look at the cost of more expensive washers versus the $16 needed for 4 of these, between being cheaper and easier because of no issues with threaded-rod threads, I’m starting to re-evaluate...
You've got it!
This particular spring can travel 0.5 inches. The rate is constant.
I have repeatedly measured a maximum of 2mm of movement in a 4s configuration. The first run of 100% to 0% SoC will result in larger movement but after that, I've not seen more than 2mm.
Note that they charge shipping if you only order $16 worth.
Shipping to the Bay Area is only $6.40, so not that big of a deal. But for orders under $40 there is a $20 ‘handling fee’ which is a showstopper for me.
So $42.62 for 4 springs ($10.66 each) or $48.36 for 12 springs ($4.03 each)
Now I understand why you bought 16 and I may need to try pull together a Bay Area Group Buy. Anybody else interested in 4 or 8 of these springs from Lee Springs?
I dig the spring solution, but has anyone considered just spacing the cells out? If we were to add a spacer that's like 1/8 inch thick between each cell, and cut out the middle so that it's maybe 1/2 inch around the edges of the cell, wouldn't that allow for the expansion?
Just a thought...
Just a thought...
Bob B
Emperor Of Solar
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- Sep 21, 2019
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The point of the compression is to prevent the cells from bloating .... and at the same time add to the cycle life of the battery .... if the compression is in the correct range.
There are actually several threads on the subject of cell compression .... a quick search will provide more reading than you want.
The cells won't really noticeably "bloat" under normal charge conditions, except maybe at very high cycle counts or C rates. The compression helps prevent delamination over thousands of cycles, and also provides support which reduces micro cracking of the carbon anode material.
If bloating conditions (excessive cell voltage occur), the compression rig should prevent the cell from swelling, the damage is still happening as the electrolyte breaks down, and the byproducts poison the SEI and form voids.
I suspect there's a small terminology issue here. The cells aren't bloating like you see when they're overcharged. But they're absolutely expanding and contracting no matter what the charge/discharge rate is and you can see and feel it. The compression fixture is actually containing this process significantly. The cells expand a LOT more when they're not under compression. I have cells that are at 100% SoC that were in the fixture which are now flat but all my cells at 100% SoC that are not in a fixture are a bit fat. Even putting those fat 100% SoC cells into a compression fixture does not cause them to become flat again until I take them way down in SoC. From that point forward, they'll remain flat so long as they're in the compression fixture.
Contrary to intuition, there is actually some free space in the casing. This isn't the cells in question, but gives an idea. The laminate roll cannot conform to sharp corners, so there is some free space there. As the laminate grows/shrinks in thickness, it has a tendency to cause the roll to ripple slightly, resulting in the case/roll growing. Keeping a bit of pressure on the case discourages this rippling, by reducing roll gaps, especially at the short edges. The high density cells which are popular here have even less free space at the sides and top, so compression appears to be more important for those seeking a decade of service.
Also unrelated, but some battery makers are putting all their cells through a CT scanner as part of the QA process. Probably for mobile device and EV applications.
Here is a side view of a different cell roll. The red areas are gas formation after an abusive undervoltage. At high age and cycles similar gas voids can appear, especially at high C rates.
I thought this figure (unrelated) was interesting. These are cells which have been crushed until they short circuit inside.
Also unrelated, but some battery makers are putting all their cells through a CT scanner as part of the QA process. Probably for mobile device and EV applications.
Here is a side view of a different cell roll. The red areas are gas formation after an abusive undervoltage. At high age and cycles similar gas voids can appear, especially at high C rates.
I thought this figure (unrelated) was interesting. These are cells which have been crushed until they short circuit inside.
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