Best practice for 300kgf ‘fixture’ 280Ah cells

Tradewinds63 · May 29, 2024

After reading the EVE test data amongst the several LF model batteries and comparing it to this chart...

It's clear that the EVE experimental test subject was the EVE LF 304. If you compare the surface areas of the various batteries, only one, the LF 304 comes to approximately 660 lbs (300Kg) total force upon its surface (55.86 in²) when using the optimal 12 PSI as the clamping force/axial force applied to the battery.

So, should someone use the 300kgf for the LF105 (surface area 40.5 in²) as an example. They would be applying 16.3 psi, which is still in the peak zone but does reduce it by a projected potential 5k cycles, from nearly 20k cycles to about 15k cycles, when compared to 12 psi. An LF 105 would produce best results not with 300 kgf but rather 220 kgf.

It's too bad EVE didn't express their unit as 2.874 N/cm² Y axial force, which is 12 psi Y axial force. Then, they'd have avoided all the confusion with respect to the applied force and been far more accurate.

upnorthandpersonal · May 29, 2024

The fact that they use an outdated, non standard unit like the kgf has always puzzled me...

Tradewinds63 · May 29, 2024

upnorthandpersonal said:
The fact that they use an outdated, non standard unit like the kgf has always puzzled me...

I'm going to guess they contracted the test out to some old geezer in China who provided them with that?

What's sad is that they just slapped it into the literature for all their LF line-up, altering only the surface applied to data and battery model info. Not realizing it became inaccurate information when they applied it to batteries other than the LF304, solely because the unit expressed wasn't a standardized unit but rather a custom unit for the LF304, only.

HRTKD · May 29, 2024

Tradewinds63 said:
Since we are discussing an applied mass force upon a surface area of the battery, the second unit of measure is typically expressed in either mm², cm² or m², etc. If it represented mm², the cells would be crushed if the force applied was equal to 300 kgf/mm² and the cells would also be crushed by 300 kgf/cm². it would be like you standing on a tin can and it buckling under your body wiegth.

While I haven't done the math, wouldn't 300 kg/m² be the more likely measure?

Tradewinds63 · May 30, 2024

HRTKD said:
While I haven't done the math, wouldn't 300 kg/m² be the more likely measure?

That's what I thought also as mm² and cm² would be insane amounts of pressure. But the literature expresses within a single snippet mention, the area in which the force is applied to, referencing the plane perpendicular to the Y axis and providing that areas dimensions. So it isn't applied to any standardized unit, making it non-transferable upon batteries with different surface areas. Yet, they do just that throughout the many manufacturer specification docs.

Even companies such as CATL references the same experimental results using the informal non-standardized 300kgf for their lifepo4 prismatic batteries axial compression force. Which if used will result in fairly different results among the various battery models, in some cases, failure. For example the LF32ah with effectivly a 148x91 mm surface area or 20.875 in² almost half the LF105 and a mere 37% the surface area of the LF304. Providing a total 300kgf axial clamping force to the 20.875 in² surface would result in 14.37 k/in² or 31.68 psi! That would eventually likely result in cell damage as they say 9 kN will cause internal failure. On the LF304 that 300kgf comes to about 3 kN on the planar surface which on the LF32 is 2.7 times the force of 3 kN, at nearly 9 kN or more presicely 8.1kN and the literature clearly states that 9kN force will cause battery failure. So, 8.1kN force through repetitive charging would very likely eventually cause internal failure also.

What I don't like, I had to read through the literature of several models to realize what they had done, copy, paste, edit the same experimental literature. Then it took a while to determine which model was most likely used in the referenced experimental setup (LF304). Between that and the other independent experimental test results it became clear what EVE and others have done with respect to that single experiment they did. They dropped the ball and didn't convert the measurment to a standardized unit.

Unfortunately, that's going to result in many improper applications of the "fixture." It's pretty clear they don't care. LOL, Otherwise they'd issue a standardized unit update to clear the air on the matter.

So, after an exhausting time looking though many documents to flesh out what is going on. I recant some parts of my initial and second post on this topic.
Keeping the batteries in the fixture is paramount BUT employing the correct 12psi is also paramount to achieving the projected maximum potential cycle life. Forget the damn 300kgf, it's only pertinent to one battery models planar surface.

Next is understanding the forces at work within the batteries and how their expansion can and will effect improperly designed "fixtures."
I took the time to run the structural force calculations for a battery bank I'm building now and the materials I'd have to use to make it fit in the cabinet it will be placed in. I picked up 8" steel C channel with 1/4" webbing and 2" flanges. Will use 1/4" 20 tpi steel allthread to provide the axial clamping force. Some seriously expensive material to clamp this group without any chance for expansion.

This bank will be 5 parallel 8 series batteries, 40 batteries in one "fixture" by two fixtures total for an 80 battery bank of LF105, 51.2v 26,880 wh.

5 ea LF105 planar surfaces parallel within a single fixture comes to 5 x 40.5 in² = 202.5 in² by 12 psi = 2430 lbs axial force total. Using 4ea 1/4" 20 tpi steel allthread to provide the axial force, that's 607.5 lbs each axial force using 30.375 in-lbs torque each or 2.5313 ft-lbs each.

I've got 80 batteries to drain down between 30-40 SOC, they came with 3.293v each +/- .001v. Pretty impressive.

upnorthandpersonal · May 30, 2024

Tradewinds63 said:
most likely used in the referenced experimental setup (LF304)

the 300kgf was already present in the literature before the LF304 existed though - at least as early as the original LF280, but since the surface area of those two is pretty much identical, it's valid for both.

HRTKD · May 30, 2024

upnorthandpersonal said:
the 300kgf was already present in the literature before the LF304 existed though - at least as early as the original LF280, but since the surface area of those two is pretty much identical, it's valid for both.

That's what I was thinking too.

S Davis · May 30, 2024

HRTKD said:
That's what I was thinking too.

Yep that is what was being used for the LF280N, mine are just under 12 psi. at 640 lbs total across the face.

fafrd · May 30, 2024

S Davis said:
I was responding to post 242 talking about crushing the cells.

My apologies - my post was directed to post 242 by Tradewinds62 - I was surprised any of us were taking that concern of crushing Eve cells in a 300kgf clamping fixture seriously (at least any of us that have actually built one

).

fafrd · May 30, 2024

Tradewinds63 said:
That's what I thought also as mm² and cm² would be insane amounts of pressure. But the literature expresses within a single snippet mention, the area in which the force is applied to, referencing the plane perpendicular to the Y axis and providing that areas dimensions. So it isn't applied to any standardized unit, making it non-transferable upon batteries with different surface areas. Yet, they do just that throughout the many manufacturer specification docs.

Even companies such as CATL references the same experimental results using the informal non-standardized 300kgf for their lifepo4 prismatic batteries axial compression force. Which if used will result in fairly different results among the various battery models, in some cases, failure. For example the LF32ah with effectivly a 148x91 mm surface area or 20.875 in² almost half the LF105 and a mere 37% the surface area of the LF304. Providing a total 300kgf axial clamping force to the 20.875 in² surface would result in 14.37 k/in² or 31.68 psi! That would eventually likely result in cell damage as they say 9 kN will cause internal failure. On the LF304 that 300kgf comes to about 3 kN on the planar surface which on the LF32 is 2.7 times the force of 3 kN, at nearly 9 kN or more presicely 8.1kN and the literature clearly states that 9kN force will cause battery failure. So, 8.1kN force through repetitive charging would very likely eventually cause internal failure also.

What I don't like, I had to read through the literature of several models to realize what they had done, copy, paste, edit the same experimental literature. Then it took a while to determine which model was most likely used in the referenced experimental setup (LF304). Between that and the other independent experimental test results it became clear what EVE and others have done with respect to that single experiment they did. They dropped the ball and didn't convert the measurment to a standardized unit.

Unfortunately, that's going to result in many improper applications of the "fixture." It's pretty clear they don't care. LOL, Otherwise they'd issue a standardized unit update to clear the air on the matter.

So, after an exhausting time looking though many documents to flesh out what is going on. I recant some parts of my initial and second post on this topic.
Keeping the batteries in the fixture is paramount BUT employing the correct 12psi is also paramount to achieving the projected maximum potential cycle life. Forget the damn 300kgf, it's only pertinent to one battery models planar surface.

Next is understanding the forces at work within the batteries and how their expansion can and will effect improperly designed "fixtures."
I took the time to run the structural force calculations for a battery bank I'm building now and the materials I'd have to use to make it fit in the cabinet it will be placed in. I picked up 8" steel C channel with 1/4" webbing and 2" flanges. Will use 1/4" 20 tpi steel allthread to provide the axial clamping force. Some seriously expensive material to clamp this group without any chance for expansion.

This bank will be 5 parallel 8 series batteries, 40 batteries in one "fixture" by two fixtures total for an 80 battery bank of LF105, 51.2v 26,880 wh.

5 ea LF105 planar surfaces parallel within a single fixture comes to 5 x 40.5 in² = 202.5 in² by 12 psi = 2430 lbs axial force total. Using 4ea 1/4" 20 tpi steel allthread to provide the axial force, that's 607.5 lbs each axial force using 30.375 in-lbs torque each or 2.5313 ft-lbs each.

I've got 80 batteries to drain down between 30-40 SOC, they came with 3.293v each +/- .001v. Pretty impressive.

I appreciate your ‘ recant’ on portions of your earlier post.

For those of us with Eve 280Ah cells, if you read through this thread from the beginning, you’ll see that 12psi is what we’ve all bee aiming for (at 50% SOC - the thread also includes limits for minimum pressure @ ~10% SOC and maximum pressure @ ~99% SOC important to stay above/below (especially the staying under the max pressure) and recommended by Eve through back channels.

Best practice for 300kgf ‘fixture’ 280Ah cells

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HRTKD

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