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EVE-280 cells should these be clamped tight or spaced for expansion?

Just realised my torque wrench only goes down to 20 ft/lb, so i am going to need to look around for a wrench that can to much lower torque.
Are such wrenches common? I am having trouble finding any that read below 28nm which is far higher than 8 inch pounds.
 
Just realised my torque wrench only goes down to 20 ft/lb, so i am going to need to look around for a wrench that can to much lower torque.
Are such wrenches common? I am having trouble finding any that read below 28nm which is far higher than 8 inch pounds.
A 1/4" drive one should go pretty low. I have a cheap chinese one that I used, but I tested it in a vise with a pull scale to calibrate it first. IIRC, I used just less than 6 in-lbs.
 
Just realised my torque wrench only goes down to 20 ft/lb, so i am going to need to look around for a wrench that can to much lower torque.
Are such wrenches common? I am having trouble finding any that read below 28nm which is far higher than 8 inch pounds.

Look for a 1/4" torque wrench, digital if you can get it. Not all torque wrenches are equal. But they're usually close enough unless something internal is broken. The problem with close enough is that we're dealing with very low torque on the cell terminals. Torque wrenches may not be as accurate at the lower end of their range. Just a suspicion of mine that makes me use extra care when at the extremes of the range.

My first digital torque wrench was a Snap-On that I found in a pawn shop. It's 1/4" and goes down to 12 inch pounds. My second digital was also a Snap-On but it's 1/2", so not relevant to this thread. My latest is a micro 1/4" digital torque wrench, also from Snap-On (see the trend here?), that goes down to 15 inch pounds.

My 3/8" non-digital Snap-On torque wrench was gifted to my son who likes to do most of the work on his cars.

My very first torque wrench was a 1/2" Craftsman, which I still have. I used it one time too many for low torque fasteners and broke a bolt in a transmission pan. That prompted the search for the 1/4" digital.

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I read a report that 'compression' slowed the capacity-degradation-rate marginally, when cells were new.
However, after a year or two there was almost no difference between compressed and loose cells, with regard to capacity-degradation.
Perhaps this is what I wanted to hear, but I do not compress and so far, I've got no regrets.
 
I read a report that 'compression' slowed the capacity-degradation-rate marginally, when cells were new.
However, after a year or two there was almost no difference between compressed and loose cells, with regard to capacity-degradation.
Perhaps this is what I wanted to hear, but I do not compress and so far, I've got no regrets.
How old was the report?
 
How old was the report?
I can't find what I read, It had graphs showing capacity vs time.
FYI i just spotted this article that discusses the pros and cons: "Note CON #3 which I did not know.
  • Reduced reliability: In some cases, compressing the cells may actually reduce the overall reliability of the battery pack. This is because the compression process can increase the risk of internal shorts or other types of damage to the cells."

Pros of Compressing LiFePO4 Cells

The lithium battery industry has undergone tremendous growth in recent years, creating a need for more efficient and reliable batteries. A popular type of lithium battery is the Lifepo4 cell, which can be compressed to enhance its performance. Compressing cells results in longer battery life, faster charging time, easier storage and transportation due to the smaller size, increased power output, and reduced maintenance and lifecycle costs. Compression is not mandatory but can ensure long battery life if cell connections are strong and secure. If you want a longer-lasting battery and fewer problems, consider compressing your Lifpo Cells!

There are several potential benefits to compressing LiFePO4 cells, including:

  • Improved electrical conductivity: Applying pressure to a battery cell can help to improve the electrical conductivity between the cell internal layers. This can lead to better performance and efficiency, especially in high-drain applications.
  • Longer cycle life: LiFePO4 batteries are known for their long cycle life, and compressing the cells can help to extend this even further. This is because compression can help to prevent the electrode materials from foaming in the electrolyte, separating, cracking and forming over time, which can reduce the battery’s overall capacity.
  • Increased safety: LiFePO4 batteries are already considered to be some of the safest lithium-ion batteries available, but compression can further enhance this safety by reducing the risk of thermal runaway or other types of battery failure.

Cons of Compressing LiFePO4 Cells

While there are certainly some potential benefits to compressing LiFePO4 cells, there are also a few potential drawbacks to consider, including:

  1. Increased cost: Depending on the size and complexity of your battery pack, compressing the cells may require additional materials and manufacturing processes, which can add to the overall cost of the battery.
  2. Decreased flexibility: Compressing the cells can limit the flexibility of the battery pack, making it more difficult to fit into certain types of devices or applications.
  3. Reduced reliability: In some cases, compressing the cells may actually reduce the overall reliability of the battery pack. This is because the compression process can increase the risk of internal shorts or other types of damage to the cells.

Compressed Cell Safety and Longevity Considerations

To start, it is vital to understand LiFePO4 battery technology and how compression works. Compressing LiFePO4 cells involves firmly combining multiple cells together to maintain consistent contact and prevent movement when faced with external forces, such as vibrations or shocks. Additionally, compressed LiFePO4 batteries maintain their performance and run for extended periods without degradation caused by cell swelling or delamination of internal components due to uneven stress on electrical terminals.

When evaluating whether to compress your LiFePO4 cells, safety risks and the expected lifespan of compressed versus uncompressed cells must be considered. Failure to compress your batteries can lead to cell swelling, increasing pressure inside the battery pack and causing potential short-circuiting. Uncompressed cells can also cause delamination of internal components and additional stress on electrical terminals over time. It is essential to understand any required regulatory changes before performing modifications to avoid exposing yourself or others to unnecessary risks during use.

Overall, compressing your LiFePO4 batteries according to manufacturer recommendations can be an important maintenance step. It helps ensure maximum performance while keeping you safe from potentially hazardous incidents resulting from improper handling practices in the future.

Should You Compress LiFePO4 Cells?

So, with all of that in mind, should you compress LiFePO4 cells? As with many engineering decisions, the answer will depend on your specific application and requirements. If you need the highest possible performance and efficiency from your battery, and cost is not a major concern, then compression may be a good option to consider. On the other hand, if you need a more flexible or cost-effective solution, then compression may not be needed.

All of our LiFePO4 Battery packs come with compressed cells mounted inside to ensure the highest efficiency and reliability.
From
I apologize - the link below was the wrong one. The link above is the one I should have given you.

 
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I thought LiFePO4 is not capable of thermal runaway.
I agree. There is a lot of confusion everywhere. I like Will P's position on SOC and longevity - just use the cells for maximum benefit right now, because 'shelf-life' is a bigger issue.
 
I agree. There is a lot of confusion everywhere. I like Will P's position on SOC and longevity - just use the cells for maximum benefit right now, because 'shelf-life' is a bigger issue.
I think its more the fact that EVE have published specifications for compression that suggests it's recommended or at least won't cause harm if done correctly.
 
My BS flag is flying fully erect on "cons" two and three.

For number two, you would build a pack to fit your application, not try to shove some random pack into it. Besides, my compressed pack only takes about 1/4" more space than if I had just built a tray for the cells to slip into. Poron foam seems to work well for its intended application.

For number three, letting the plates and electrolyte move around is a much better way to get an internal short than by trying to constrain the motion. The big issue is that the electrolyte changes density with SOC. That means that the volume of the electrolyte changes with SOC. Without compression or some other way to constrain the flow of electrolyte, you could end up with nothing between the cathode and anode in a spot, creating a short.

The manufacturers specify compression for a reason, and you certainly won't shorten cell life with proper compression.

PXL_20221025_222453918.jpg
 
300kgf is a LOT of force! 300 x 2.2 = 660 lbs!
So I would definitely not apply 660 lbs on a single cell IMHO.
Now if they are talking about pressure, then 300 kgf / m2 = 0.42 psi so that is a light pressure.
Typical Chinese technical writing. They either don't care or don't know the difference and don't bother in proof reading and reviewing their manuals or specs. Same with asking about Neutral to Ground bonding. I can never get a coherent answer.
 
I strongly agree with Gary's complaint against "Con #3" (listed within SandyMcC's post, and "Con #2" doesn't make a lot of sense. Some of his post is copied from an "Evlithium" information page, but that original page does not list any "Con" items at all. (SandyMcC did give us a link to that page.)

"Evlithium" is a reseller, with no special relationship to EVE (the manufacturer). Both of the photos on their information page show badly built designs. The spacers in the first photo leave ARE NOT TALL ENOUGH. About 15mm of height near the top of each cell is without compression. With swelling and shrinkage, that area bends in relation the the larger compressed area below - and the bending is focused along a horizontal line within the upper face of cell "front" and "back" surfaces. Unlike the edges, the middle areas of the THIN faces are super thin, and they are unsupported by the thinner sides which provide "depth" to the shape (including the top side, with the terminals).

Their lower photo is even worse. It has two even smaller sections of foam before compressing, which will cause even more lines of distortion and stress within the large cell faces.

The large faces need be compressed with EQUAL PRESSURES over the ENTIRE surface. (At least 12 PSI, never to exceed 25 PSI at the "highest spot" under full SOC. When tightened bars will provide the compressive force, with or without springs, those "hot spots" will generally be along the edges of cells near the locations bars.)
 
300kgf is a LOT of force! 300 x 2.2 = 660 lbs!
So I would definitely not apply 660 lbs on a single cell IMHO.
Now if they are talking about pressure, then 300 kgf / m2 = 0.42 psi so that is a light pressure.
Typical Chinese technical writing. They either don't care or don't know the difference and don't bother in proof reading and reviewing their manuals or specs. Same with asking about Neutral to Ground bonding. I can never get a coherent answer.
The specification is in Newtons (3000n minimum, 7000n maximum). In standard gravity at sea level, that is 674.4268 units of pound-force. Your math is a decent approximation under standard gravity, but (let me be a tiny bit pendantic) 'kg' is actually a unit of mass (and not "weight").

You find that value surprising, but that IS the recommendation for EVE's 280 V3 cells,. The area of the large faces on those cells is less than 60 square inches. In order to match EVE's specification, you will need to apply more than 12 PSI (minimum) on each square inch.

Within an assembled "pack" of multiple cells, the force is carried through the cells. From the first surface, the interior volume feels all of the force and pushes it to the opposing surface - where it is applied the the "first" face of the next cell (with only a thin insulator between them.) The purpose is creating high pressure within the interior.
 
Ive got a question on the pack ive assembled.
I dont think ive over compressed the cells or anything, but after giving a full charge they definitely have noticeably expanded.
There appears to be more of a gap in the inner edge of both rows, but nice and tight on the outer edge.
My main concerns are I dont think the pressure is even across all the bolts, ive had to guess somewhat as the torque wrench ive got only goes down to 2nm.
Its a digital wrench and it would beep once it hit that level, i did the 2 centre bolts at 2nm which is close to 19 inch pounds, and i did the outer bolts to that and then backed off a turn, they honestly felt pretty loose to me at that torque, but as the battery charged you could see they had gotten pretty tight with the expansion, but the outer plates were still able to slide up prior to charging the bank if I was to lift it by the clamps as I wanted to be safer than sorry than over clamping the pack.

Im thinking I will get some of this memory foam everyone talks about, but I couldnt get any in time before christmas and I needed to get this bank online.
I was expecting the cells expansion might have gone back down when I discharged the battery, but it didnt seem to have reduced anything noticeably and I didnt want to tighten it up to attempt to take it out or anything, note that the expansion I experienced after the top balance appears to be within the acceptable limits, but is noticeable.
Im not too sure if I will bother about getting too fussy about torque and instead just install the foam packers and tighten just enough to hold the cells in place?
Im still looking for a torque wrench that can go low enough, but I see others using higher torque than I was going to go with (8 inch pounds outer bolts and 16 on the inner ones)
 
Once again, torque on a threaded fastener is a poor choice when the specification is for applied force.
 
Once again, torque on a threaded fastener is a poor choice when the specification is for applied force.
So should I use springs instead? Those pre-made enclosures don't appear to have any springs or anything, they just appear to hold them in place so they have no room to expand?
Seems most people bolt them together without any fancy springs etc.
 
So should I use springs instead? Those pre-made enclosures don't appear to have any springs or anything, they just appear to hold them in place so they have no room to expand?
Seems most people bolt them together without any fancy springs etc.
I don't know what any premade enclosures might do. You said you torqued the screws and then backed some of them off one turn. You had no compression, or at least not uniform compression. I guess it is easy to worry about too much if you don't now how much you have in the first place.

Any chance you can post a photo of your swollen cells? You didn't say what brand you have, or what the manufacturer specifies for compression of your cells.
 
I don't know what any premade enclosures might do. You said you torqued the screws and then backed some of them off one turn. You had no compression, or at least not uniform compression. I guess it is easy to worry about too much if you don't now how much you have in the first place.

Any chance you can post a photo of your swollen cells? You didn't say what brand you have, or what the manufacturer specifies for compression of your cells.
I would consider it barely any compression, but rather kept firm. I wanted to stay on the side of caution and just wind them up enough to feel like the cells were grabbing, when I lifted the bank by the bolts, the cells slipped down on one end, so I know it was not ultra tight.
Now after performing a deep cycle, basically if I hold a straight edge on the end plate, you can see it has bowed, particularly in the centre of each cell.
Its a solid 20mm thick plastic material that felt stiff enough but has still been able to flex in the middle between each row of bolts.
The fr4 sheets feel uniformily tight with no gap on the outer edges, but the edges on the centre bolts i can wiggle the fr4 a bit in places.
I don't think the widest gap between the cells would be much more than 1mm.
This was largely how it was before I assembled the pack, I took a risk top balancing before clamping, only reason was because I didn't have jumper cables long enough or else I would have done so.
My friend advised it would be ok if applying little current, but it appears they exapand regardless while approaching full SOC.

I'm not there right now to grab a photo but most of the expansion occurred during my top balance.

How can you even tell they've noticeably expanded? What voltage did you charge to?

3.65V was the max for the final balance, I didn't notice any difference in expansion or any tension going from 3.6 to 3.65V
 
My friend advised it would be ok if applying little current, but it appears they exapand regardless while approaching full SOC.
The electrolyte changes density with SOC, so compression helps to keep it where it belongs rather than letting it migrate and cause thick or thin areas. It seems that the current EVE specification is not all that worried with the upper limit on compression force.

300kgf (660 pounds) is the recommended amount of force (applied at ~30% SOC) across the broad face of the cells. Building a fixture that allows the cells to slip out is clearly not anywhere near enough, particularly if the compression is done with the cells at 100% SOC.
 
Cell expansion is due to fully charged negative graphite anode. (lithium-ions stuffing of graphite). It expands by about 11% in volume between 0% and 100% fully charged. Graphite anode is about 20% of cell. That is less than 2 mm thickness for a 280 AH cell, but it adds up if you stack 16 cells in a single row.

The positive and negative electrodes are separated by about 0.5 to 1.0 mils (thousandths of inch) thickness perforated plastic sheets. Crush it and you short out cell.
 
Cell expansion is due to fully charged negative graphite anode. (lithium-ions stuffing of graphite). It expands by about 11% in volume between 0% and 100% fully charged. Graphite anode is about 20% of cell. That is less than 2 mm thickness for a 280 AH cell, but it adds up if you stack 16 cells in a single row.

The positive and negative electrodes are separated by about 0.5 to 1.0 mils (thousandths of inch) thickness perforated plastic sheets. Crush it and you short out cell.
So does that mean one side of the cell pack is expanding more, uneven pressure from one side to the other?
 
The electrolyte changes density with SOC, so compression helps to keep it where it belongs rather than letting it migrate and cause thick or thin areas. It seems that the current EVE specification is not all that worried with the upper limit on compression force.

300kgf (660 pounds) is the recommended amount of force (applied at ~30% SOC) across the broad face of the cells. Building a fixture that allows the cells to slip out is clearly not anywhere near enough, particularly if the compression is done with the cells at 100% SOC.
I forgot to add that these are indeed EVE cells and i discharged the bank to about 30% the first cycle and checked the tension of the bolts a second time, they were a little looser and I went over enough to just grab the cells.
They most certainly are locked in tight after a full charge.
 

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