The Newest Specification Sheets of EVE Cells

Jaxonwood said:

I wonder if anybody has made some of those 6 rod plates for this DIY crowd to replicate that apparent requirement...
If a ton or two or three of force will contain one cell, my physics says it will also contain any number of the same in a row. (Right?)

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There have been 3 basic approaches I have seen.
1st is to use springs to try to keep the cells in a pressure range ..... it has generally been thought that 12 PSI is the sweet spot .... which is pretty close to what the 300 kgf EVE used to recommend in their older spec sheets .... based on conversations with EVE techs by some members, 17 PSI is the max the cell should be subjected to without having negative effects on cycle life.
I'm a fan of the spring method.

The 2nd approach is to just put them in a rigid fixture and snug them up.

The 3rd approach has been to utilize a special foam that compresses and somewhat maintains the desired pressure.

By now recommending 300 kgf at 30 to 40% charge in a rigid fixture ... EVE has totally changed the game. I don't know if their new recommendation is due to differences in expansion of the newer cells .... but they seem to have completely changed the game by seemingly having no concern over what happens at higher SOC when the cells expand.

Do not want to derail this thread, but while on spring compression, have any informative build threads using them?

What would be helpful is to share the test methods and data they used to arrive at these specs.

Kornbread said:

Do not want to derail this thread, but while on spring compression, have any informative build threads using them?

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If you use the search tool you should be able to find some dedicated to springs and compression in general.

Bob B said:

The recommended fixture is described in the specs ..... it is rigid.

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I know but they have some way to measure the pressure when they tighten the rods to set the correct pressure on the cells, without having the ability to do that how do you set the pressure?

With die springs you can set the pressure on the cells fairly easily, the springs I got are 160lb compressed to .63” with four you are just below the 12psi recommend pressure on a single stack of cells.

S Davis said:

I know but they have some way to measure the pressure when they tighten the rods to set the correct pressure on the cells, without having the ability to do that how do you set the pressure?

With die springs you can set the pressure on the cells fairly easily, the springs I got are 160lb compressed to .63” with four you are just below the 12psi recommend pressure on a single stack of cells.

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I still like the spring method also .... takes the guesswork out. It would be VERY hard to do a fixed rigid fixture and get the desired pressure.
Kinda seems like these new specs are designed for EV manufacturers.

I've read this new specification sheet for the EVE LF280K battery cell several times, at varying levels of sleep deprivation and alcohol consumption, and I'm just not convinced we are understanding it entirely. (please note that there is a difference in important details on the spec sheets for different battery cells. Don't apply the wrong sheet to the wrong cell.)

First, I don't want to confuse the test conditions for determining cell failure under abuse with instructions on how to use the battery. For instance, section 3 of the document is dedicated to test conditions. There are parts of this section that tell you to discharge the battery to 0v (section 3.8.4.2) or heat it to 300°C (3.8.4.8 step 4). These are not operating instructions, these are testing conditions for various failure modes. Yet, in this same section, people are seeing the Fixture design in section 3.3 and assuming it is the recommended fixture for normal use. However, if you read the text, it is called a "Test Fixture" designed for use with a "single cell" not a battery pack.

It is section 4 of the document that outline the limits for normal use of the cell in a battery, and in section 5 we can find the absolute limits where the BMS must shut down the use of the battery completely.

So, it is not until section 6 that they begin talking about the recommendations for battery module design. (note that the first section 6.1, about "Battery Directions" is blank. The term "battery directions" is referring to the definition of the x, y, and z axis, and the diagram that goes in this section is at the top of the same page. It appears to have gotten mis-located when they laid out the document.)

So then I asked myself what I can understand with certainty from Section 6 of the EVE LF280K-

• From the table at the top of page 27. If at any point in the charge cycle, your battery cell holder applies more than 50kN or 11240 lbs of force (or 240 psi) to the broad face of the cell, you will cause internal damage.
• Confusingly, in section 6.3 there is another test, but I think this is a test of your battery module holder. The holder is supposed to be setup to apply the same 300kgf at 30-40% state of charge as the test holder does. At the beginning of life (BOL), after cycling the cell fully and then returning it to a 60% state of charge, your module holder should not be applying more than 3kN or 674 lbs of force to the battery pack.
• Section 6.4 just tells us that the battery conducts heat 10 times better to the ends of the cell as it does to the broad face of the cell. This makes sense because the metal foils will carry heat to the ends of the cell whereas the heat going to the broad face of the cell has to cross layers inside the battery.
• Section 6.5 tells us to monitor the center of the broad face of the cells within the module and to monitor the cell terminal temperature.

Those are the only things I can say for certain.

Also, just to say it again, the specs for the 304ah battery are much different. This is especially true for the pressure applied to the cell. Those cells will get damaged at only 9kN, which is 2032 lbs of force on the broad face, or 36.7psi. I'm sticking with my compressive foam design on module holders for these cells.

Kornbread said:

Do not want to derail this thread, but while on spring compression, have any informative build threads using them?

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Here is 45 pages of compression madness :


Also, @cinergi has a 37 page thread in which his spring loaded compression is detailed IIRC.

I noticed that the specification sheets at the top of this thread have not been added to the Battery Datasheets section of the Resources tab in the forum.

I thought about adding them, but I don't want to appear to take credit for another members contribution, and since the posting member is associated with a vendor, I don't know if it is best that they put them in the Resources section for fear that the resources get deleted if the member loses their account again.

@upnorthandpersonal, since you are a moderator here, what is the appropriate action to take?

The only concern I have with the new twin stud/terminal design, and the most recent Eve specs which further define the benefits of compression, are how to ideally tie the terminals/cell packs while not over stressing the terminals. With the new design (while seemingly a step forward regarding better connection), it complicates the use of flexible busbars.

Obviously Eve seems to suggest using compression in their latest specs, yet the busbars I received with my cells (new twin post design) are flat/rigid and seemingly will not allow for any "give" between cells/terminals if compressed... Yet it's known the cells expand and contract a significant amount depending on their SoC. One would think if the MFG is recommending compression for maximum life, a flexible busbar design would either be spelled out, or provided, but the busbars I received clearly are not optimal in this regard and would likely result in failure if used in a compressed cell packs as the MFG recommends.

I will likely use the suggested compression and custom twin wired busbars per terminal to allow the required "give" at the busbars, but having a solution provided would save me a lot of time and money designing my own solution!

If the cells are in a rigid fixture then there should be minimal stress at the terminals as the cells won't be moving.

SparkyJJO said:

If the cells are in a rigid fixture then there should be minimal stress at the terminals as the cells won't be moving.

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Based on from what I've read into their specs, they are not recommending a truly rigid fixture, unless testing for failure.

Since commercial applications of these cells appear to use welded busbars, they must not be expecting the cells to move much if at all.

hankcurt said:

... First, I don't want to confuse the test conditions for determining cell failure under abuse with instructions on how to use the battery....

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I agree - the clamping only seems to relate to a test procedure. Have cells on order, so still no idea whether they should be clamped or not? And if clamped do they need insulation between each cell? I remember Will P did a video showing cells all separated on a rack, with a BMS hanging in the air - i.e. with no compression at all. Will read the other (lengthy) threads on compression over the next week or so!

SeaGal said:

I agree - the clamping only seems to relate to a test procedure. Have cells on order, so still no idea whether they should be clamped or not? And if clamped do they need insulation between each cell? I remember Will P did a video showing cells all separated on a rack, with a BMS hanging in the air - i.e. with no compression at all. Will read the other (lengthy) threads on compression over the next week or so!

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Yes, the cells should be in a compression fixture. Cells shown for testing without a compression fixture is he exception, not the rule.

Kornbread said:

Do not want to derail this thread, but while on spring compression, have any informative build threads using them?

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I can see the two plates as described in the new spec sheet, and twelve washers and six springs between nuts and plate. Selecting springs, if I have the engineering figuring right, six springs each applying 18lbs of force on new cells at an SOC around 30% would approximate the 50kgf described in Testing. Using springs that compressed with less than 1/6th the maximum force would ensure that damaging force never be exceeded. Looking around at the multitude of retail boxes with these cells in them, they must either be terribly lacking in design for longevity or we are overthinking this.

Jaxonwood said:

Looking around at the multitude of retail boxes with these cells in them, they must either be terribly lacking in design for longevity or we are overthinking this.

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Because age of cells likely has bigger impact than elaborate compression mechanism.

I think we need to step back from all this and think about what we are really trying to achieve.

These cells are built into a relatively thin gauge aluminium box enclosure, which by itself does not have sufficient built in strength to resist the high internal gas pressure that can sometimes arise during certain modes of operation.
Hence the cells can swell up, and not return to the original shape as the gas is reabsorbed.

I do not see that applying huge initial clamping pressure to new unswelled cells is going to help very much, in fact it could buckle the cells if you get really carried away. What seems to be needed is a very stiff rigid frame that supports and contains the aluminum cells to prevent any increase in size.

Springs will allow further size increase, but may be suggested to prevent initial crushing from overtightening. But I don't see springs as being ideal. No doubt the battery manufacturer has thought the whole thing right through, and decided springs may be the safest suggestion for non technical users.

My own battery is sandwitched between two quarter inch thick steel plates with quarter inch threaded rods top and bottom for each row of cells.
The nuts are just snugged up, not torqued down. Basically finger tight plus one or two turns, or something like that.

I don't like the rigid links supplied with the batteries, any slight swelling or movement places tremendous stress into the cell posts. I much prefer flexible multistanded cable (bent into an S shape) and heavy crimp lugs.

As a matter of interest, these cells can be stacked five by three into a standard filing cabinet drawer, with just sufficient space for steel clamping plates. That is only fifteen cells, not sixteen, but it makes for a very convenient battery housing (that can be locked) with excellent accessibility.

Warpspeed said:

My own battery is sandwitched between two quarter inch thick steel plates with quarter inch threaded rods top and bottom for each row of cells.
The nuts are just snugged up, not torqued down. Basically finger tight plus one or two turns, or something like that.

I don't like the rigid links supplied with the batteries, any slight swelling or movement places tremendous stress into the cell posts. I much prefer flexible multistanded cable (bent into an S shape) and heavy crimp lugs.

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What you said.
I like it. Perhaps once a year with a low SOC, a check of any built up pressure on the nuts is plenty of due diligence.

Jaxonwood said:

Using springs that compressed with less than 1/6th the maximum force would ensure that damaging force never be exceeded.

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That depends entirely on the spring rate and the expected expansion of the cells.

The goal with many spring designs/implementations is to have a relatively constant spring load over the expected displacement. This implies a low spring rate, which usually means longer springs, since getting a low rate from a short spring at some point will exceed the yield strength of the spring causing it to permanently deform.

You can also stack springs in series; doubling the length will cut the spring rate in half, but again..things get long. So for a row of 4 cells getting a 30% SOC spring load of X and a fully charged load of Y, you would need to double the spring length in an 8 cell pack for the same loads...and so on.

Also keep in mind that bolting cells between plates isn't "rigid" in real life.....your threaded rods are the (extremely high rate) springs. Smaller diameter rods will have a lower spring rate than larger diameter rods, and the same diameter rod twice as long will have half the spring rate of the shorter rod, just like a spring.