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

[rickst29]

I tried to find it again... perhaps im confused

Within the MB30 ''datasheet' which justgary posted for you, Chapter 4 'Application Conditions' contains 12 sections (4.1, 4.2 4.3, etc) which the customer's usage must maintain for the product, used within a customer's "battery pack", during EVE's warranty lifespan.

These requirements are laid out for EVE's largest direct customers - builders of automotive energy packs, and a few builders of large storage facility units. Hardly anyone here is a direct customer of EVE, and the warranty does not apply to third-party customers like us (typically buying only a couple dozen cells from a secondary "reseller").

The compression requirement is section 4.6, showing the required values within Table 13. It specifies that a "pack" should be designed to apply 3000 Newtons minimum, 7000 Newtons maximum over the life of the pack. (justgary converted to PSI for you, correctly, at 12-28 PSI on large faces of these cells. That' minimum is a bit under 700 lbs of force at sea level.)

The datasheet includes that if excessive compression reaches 10,0000 Netwons for only an instant, will likely cause permanent damage to the terminal post assemblies (the anode "rod" and the cathodic plates), or blow out the cell (along the corner edges, or within the surfaces of the smaller "sides", or the "top" or "bottom".

In designing a pack to meet EVE's specification, the tendency for the cells to swell or shrink in thickness must be addressed. Gary's sheet for the MB30 provides only the rules for the test, without providing any "formula" for the pack designer to estimate the pressure changes which will occur within a proposed "pack assembly" under the varying SOC and temperature conditions which that pack is expected to experience during its working lffespan.

The total length of expansion/shrinkage in a "row" of cells, arranged face to face, varies according to the number of cells in the row. If you build a 16-cell "52 volt" battery pack as a single row, it the total length will vary twice as much as two rows of 8 cells. In previous calculations, I found that it was is impossible to build a 16-cell "row assembly" which could meet the requirements (12 PSI minimum, the previous maximum value was only 24 PSI) unless the end plates for the row were allowed to move - under the control of spring loaded bars. Maybe you could get by in a different way (less variance in SOC, tighter temperature control, or a "flexing" outer case. But I couldn't invent such as assembly at low cost involving only a few packs.

My last design is at ricks-recommended-compression-containment-for-eve-cells-LF280k-LF304), but that pack included only 4 cells (12.8 volts, for RV use). I am about to build a pair of 16 cell packs, supporting backup and grid-feed time-shifting via an All-In-One Inverter unit for my home solar system. I can use the exact same plates and crossbars when I build these, but Il will need much longer springs - tightened down to provide the the same minimum force (about 175 lbs each), but providing a lot more room for to handle cell expansion without "bottoming out", and also without adding too much more force from an excessive spring "rate" of additional force per inch.

- - -

Can you "get by" without creating compression in your pack assembly AND ALSO meeting EVE's other requirements for temperature and maximum charge/discharge rates? Yes, but your cell lifespan won't reach EVE's specifications. YMMV, depending on lots of factors. (Temperatures matter. And, At high and low SOC, EVE requires lower charge rates -- and I don't know of very many charge controllers which can respect those rules.)

 

[ScrotpusGobbleBottom]

Within the MB30 ''datasheet' which justgary posted for you, Chapter 4 'Application Conditions' contains 12 sections (4.1, 4.2 4.3, etc) which the customer's usage must maintain for the product, used within a customer's "battery pack", during EVE's warranty lifespan.

These requirements are laid out for EVE's largest direct customers - builders of automotive energy packs, and a few builders of large storage facility units. Hardly anyone here is a direct customer of EVE, and the warranty does not apply to third-party customers like us (typically buying only a couple dozen cells from a secondary "reseller").

The compression requirement is section 4.6, showing the required values within Table 13. It specifies that a "pack" should be designed to apply 3000 Newtons minimum, 7000 Newtons maximum over the life of the pack. (justgary converted to PSI for you, correctly, at 12-28 PSI on large faces of these cells. That' minimum is a bit under 700 lbs of force at sea level.)

The datasheet includes that if excessive compression reaches 10,0000 Netwons for only an instant, will likely cause permanent damage to the terminal post assemblies (the anode "rod" and the cathodic plates), or blow out the cell (along the corner edges, or within the surfaces of the smaller "sides", or the "top" or "bottom".

In designing a pack to meet EVE's specification, the tendency for the cells to swell or shrink in thickness must be addressed. Gary's sheet for the MB30 provides only the rules for the test, without providing any "formula" for the pack designer to estimate the pressure changes which will occur within a proposed "pack assembly" under the varying SOC and temperature conditions which that pack is expected to experience during its working lffespan.

The total length of expansion/shrinkage in a "row" of cells, arranged face to face, varies according to the number of cells in the row. If you build a 16-cell "52 volt" battery pack as a single row, it the total length will vary twice as much as two rows of 8 cells. In previous calculations, I found that it was is impossible to build a 16-cell "row assembly" which could meet the requirements (12 PSI minimum, the previous maximum value was only 24 PSI) unless the end plates for the row were allowed to move - under the control of spring loaded bars. Maybe you could get by in a different way (less variance in SOC, tighter temperature control, or a "flexing" outer case. But I couldn't invent such as assembly at low cost involving only a few packs.

My last design is at ricks-recommended-compression-containment-for-eve-cells-LF280k-LF304), but that pack included only 4 cells (12.8 volts, for RV use). I am about to build a pair of 16 cell packs, supporting backup and grid-feed time-shifting via an All-In-One Inverter unit for my home solar system. I can use the exact same plates and crossbars when I build these, but Il will need much longer springs - tightened down to provide the the same minimum force (about 175 lbs each), but providing a lot more room for to handle cell expansion without "bottoming out", and also without adding too much more force from an excessive spring "rate" of additional force per inch.

- - -

Can you "get by" without creating compression in your pack assembly AND ALSO meeting EVE's other requirements for temperature and maximum charge/discharge rates? Yes, but your cell lifespan won't reach EVE's specifications. YMMV, depending on lots of factors. (Temperatures matter. And, At high and low SOC, EVE requires lower charge rates -- and I don't know of very many charge controllers which can respect those rules.)

I have a Luyuan case with 2 rows of 8. Plate has three columns of three screws, Left, middle and right all with 3 screws top to bottom.
I have just tightened the screws fairly lightly with one hand and no exertion. It seems as when I tighten one side, the other loosens as expected.
I retightened and checked all screws for equal tightness and retighted until they all were snug equally. This was at shipping SOC, about 40%

I would think this would be "snug" but not overly compressed. Do you think I went to far? I do understand that you have no idea what I would consider snug.

 

[rickst29]

I have a Luyuan case with 2 rows of 8. Plate has three columns of three screws, Left, middle and right all with 3 screws top to bottom.
I have just tightened the screws fairly lightly with one hand and no exertion. It seems as when I tighten one side, the other loosens as expected.
I retightened and checked all screws for equal tightness and retighted until they all were snug equally. This was at shipping SOC, about 40%

I would think this would be "snug" but not overly compressed. Do you think I went to far? I do understand that you have no idea what I would consider snug.

If that case is "Luyan battery case" (photo from another Thread), the thin sheets of steel used to assemble the box are incapable of providing EVE-compatible compression. You have a "snug" fit, in which the cells don't slide around, - but virtually zero compression.

If you look at my EVE compressed assembly again, you will see that the "plates" are 1/4" rolled steel - heavy stuff. But even they can't support the required force without bending badly. The "ends," of the plates, where the bolts and springs apply the force, are pulled inwards along the rods, while the vertical "middle" of each plate bulges away from the face the cell (with hardly any force at all). That's why I needed to add the segments of 1" angle bar (also 1/4" thick), across those plates.

The bolts pull in the the angle bars AND the plates, with total thickness of 1/2" steel, and the fact that they're angle bars (not just aded segments of flat plate) creates enormous resistance against bending in the middle of the flat plates (where they otherwise wanted to bend up and out from the faces of the end cells.).

700 lbs total force is a LOT, and the requirement to be spreed evenly makes it hard to accomplish. It takes hours to drill those darn screw holes.

 

[ScrotpusGobbleBottom]

But even they can't support the required force without bending badly

Well then! I guess im no where near too much. AND no where near enough.
Bah....

My case is more like this one: It has a compression plate but Im sure your judgement still stands as is.

It takes hours to drill those darn screw holes.

I shudder to think of that experience.

 

[HRTKD]

I was skeptical that the Luyuan battery case would put enough compression on my 16 cells (same case as in the above post). When it came time to tighten the cell plate, I was surprised at how much compression it actually created. This is my second DIY battery. The first used the typical end plates and threaded rod. I don't have any numbers for how much compression the Luyuan case created. I just know that it was about the same as my prior build, and that was good enough.

 

[Daddy Tanuki]

I was skeptical that the Luyuan battery case would put enough compression on my 16 cells (same case as in the above post). When it came time to tighten the cell plate, I was surprised at how much compression it actually created. This is my second DIY battery. The first used the typical end plates and threaded rod. I don't have any numbers for how much compression the Luyuan case created. I just know that it was about the same as my prior build, and that was good enough.

yeah I went down that rabbit hole calculating load force and spring compression till my brain bled. then i ordered my plate, springs and rods...did i get it right? who knows, but i think any compression is better than no compression. one thing I will posit... i think we are all worrying about the upper end compression as they refer to it as a fixture which does not indicate any leeway for expansion. I just am not sure.

 

[off.the.grid]

OK, ive got some questions, how much torque is required for a box with 2 rows and 6 bolts?
Does the number of bolts affect the torque per bolt, how does force equate into torque? I see 10-15nm of torque thrown around in this thread.
It appears some expansion and contraction depending on SOC is normal, are EVE saying to clamp at around 40-50% SOC?
Ive top balanced some cells without compressing them and they have expanded a little but wouldnt go as far to say they are bulging, but apparently this is normal, i see some suggest foam over fibreglass to allow for this?

 

[rickst29]

OK, ive got some questions, how much torque is required for a box with 2 rows and 6 bolts?
Does the number of bolts affect the torque per bolt, how does force equate into torque? I see 10-15nm of torque thrown around in this thread.
It appears some expansion and contraction depending on SOC is normal, are EVE saying to clamp at around 40-50% SOC?
Ive top balanced some cells without compressing them and they have expanded a little but wouldnt go as far to say they are bulging, but apparently this is normal, i see some suggest foam over fibreglass to allow for this?

For the Luyan case (and similar cases with double rows of 8 cells for 16 total cells), the total force required in the specification is 6000 Newtons (about 1350 lbs of force at sea level). From the Luyan case Screenshot above, I'm guessing that the the 6 compression bolts are arranged 2 + 2 +, 2, each going through thin rectangles (maybe 6-10mm thick) with the bolts inside, screw holes provided at the ends. One such "rectangle"along each side, with another down the middle to separate the two rows.

The number of bolts is critical. With only 6 bolts, each pair of end nuts must be tightened enough to to create 225 lbs of 'Pull-In' force along each rod. But the use of only 2 vertical bolts (along those 3 vertical plates) will create another issue: The "end plates", on which the nuts are tightened, will bend. The end cell faces will feel much higher pressure pressure near those bolts and nuts, while the pressure near the the middle of the cells faces will be much lower. Near the very "top" and "bottom" of the cell faces, the the force will be further reduced by more distance from the bolts and nuts.

To control warping, my recent case for EVE-280 cells used 1/4" steel for the end plates - but even that tended to bend "in" at the edges near the bolts, and "away" along the vertical center. (Upon tightening, the edges had too compressive force, and the "center" area of the cells had too little - along a vertical line up the middle.) I added horizontal "L-Bar" segments to level out the plates press in the horizontal direction. (My EVE case
, with photos)

If I were to "adapt" the Luyan case for higher compression spread evenly, I would increasing the number compression rods from 6 to at least 9 (3 per vertical), and add horizontal L-bars between the top and bottom pairs. If the side and center "long plates" which now provide the hole pairs cannot be adapted to handle another rod - and preferrably move the current rods feruther up and down, I'd eliminate those plates and just use naked rods with insulators in the. L-bars do not need to be as heavy as the ones I used.

The torque measurement on the bolts varies according to the thread angle and, and it's not very reliable - scratching resistance between the threads of the rod and nut make it seem "already higher" whenever you start movement. More importantly, of course, fixing the length of the rods (without spring loading) has a high risk of creating pressure outside EVE's limits when the 8 cells swell or shrink (in thickness) with changes in SOC and temperature. The correct "working range" is only between 3000N minimum and 7000N maximum, less doesn't extend lifespan much, while more risks breakages between the containment and the terminals.

 

[justgary]

OK, ive got some questions, how much torque is required for a box with 2 rows and 6 bolts?
Does the number of bolts affect the torque per bolt, how does force equate into torque? I see 10-15nm of torque thrown around in this thread.
It appears some expansion and contraction depending on SOC is normal, are EVE saying to clamp at around 40-50% SOC?
Ive top balanced some cells without compressing them and they have expanded a little but wouldnt go as far to say they are bulging, but apparently this is normal, i see some suggest foam over fibreglass to allow for this?

Bolt torque is a poor method to use for predicting applied force due to the wide variation in thread finish and lubrication.

I would not add generic foam. Consider Poron foam applied at one 1/4" sheet per four cells.

 

[off.the.grid]

For the Luyan case (and similar cases with double rows of 8 cells for 16 total cells), the total force required in the specification is almost 1400 pounds. From the Luyan case Screenshot above, I'm guessing that the the 6 compression bolts are arranged 2 + 2 +, 2, each going through thin rectangles (maybe 6-10mm thick) with the bolts inside, screw holes provided at the ends. One such "rectangle"along each side, with another down the middle to separate the two rows.

1400 pounds sounds like a fair bit!
Does the number of cells have a bearing on how much force is applied?
I have 2 rows of 4 in my pack.
will have 2 bolts between the rows and another 2 on each end.

Bolt torque is a poor method to use for predicting applied force due to the wide variation in thread finish and lubrication.

I would not add generic foam. Consider Poron foam applied at one 1/4" sheet per four cells.

OK, so how do you measure this then? Is it something I should be too concerned about and just clamp enough to hold them in place without dropping out?

 

[rickst29]

1400 pounds sounds like a fair bit!
Does the number of cells have a bearing on how much force is applied?
I have 2 rows of 4 in my pack.
will have 2 bolts between the rows and another 2 on each end.

OK, so how do you measure this then? Is it something I should be too concerned about and just clamp enough to hold them in place without dropping out?

Each row needs 3000 Newtons pressing the end cell "faces" together. (About 680 lbs). So, in your 4-cell pack and the Luyan case, the total compressive force (on two faces) needs to be 1350 lbs minimum, spread evenly.

More cells in a row affect the amount of expansion which occurs with SOC changes, and a case or 8 cells per row might exceed 7000 Newton limit if the containment doesn't allow form some expansion (via springs). EVE provides a formula to calculate that expansion. A row of 8 cells will expand (in combined thickness) by twice as much as your 4 cell "rows".

Adding only enough pressure to keep them from sliding down when you pick up the pack (with the middle totally unsupported) is not anywhere near enough pressure - that's maybe 250 lbs per row, you need around 700 lbs per row. The thin "insulators" which you should add between each of pair of cells, and also the end cells versus container, may also be more or less "slippery" than the blue film on the cells themselves, changing the amount of pressure needed to hold them "tight" against gravity.

 

[off.the.grid]

Each row needs 3000 Newtons pressing the end cell "faces" together. (About 680 lbs). So, in your 4-cell pack and the Luyan case, the total compressive force (on two faces) needs to be 1350 lbs minimum, spread evenly.

More cells in a row affect the amount of expansion which occurs with SOC changes, and a case or 8 cells per row might exceed 7000 Newton limit if the containment doesn't allow form some expansion (via springs). EVE provides a formula to calculate that expansion. A row of 8 cells will expand (in combined thickness) by twice as much as your 4 cell "rows".

Adding only enough pressure to keep them from sliding down when you pick up the pack (with the middle totally unsupported) is not anywhere near enough pressure - that's maybe 250 lbs per row, you need around 700 lbs per row. The thin "insulators" which you should add between each of pair of cells, and also the end cells versus container, may also be more or less "slippery" than the blue film on the cells themselves, changing the amount of pressure needed to hold them "tight" against gravity.

OK, so that means I dont have to be too concerned about applying too much pressure, if I go with foam then that would certainly change things too. Looks like I wont need to worry about springs with the size of my pack but is there any formula to equate bolt torque into applied force?
Going by what justgary is saying, bolt torque is not a reliable way to measure this, I will be using M8 threaded rod.
From what I can tell the SOC is cruical when applying the force too, so shouldnt be done at a full SOC.

My cells in question are EVE MB30, the datasheet doesnt really suggest what material to place between the cells, but says expansion force is about 60000N and to reserve 2.0-2.5mm expansion space between cells, so not sure if thats implying to add foam or not.

It has some clamping examples for a single cell further down on the page and shows it using 6 bolts, which come to think about it is probably better than a bolt on each corner.

 

[justgary]

OK, so that means I dont have to be too concerned about applying too much pressure, if I go with foam then that would certainly change things too. Looks like I wont need to worry about springs with the size of my pack but is there any formula to equate bolt torque into applied force?
Going by what justgary is saying, bolt torque is not a reliable way to measure this, I will be using M8 threaded rod.
From what I can tell the SOC is cruical when applying the force too, so shouldnt be done at a full SOC.

My cells in question are EVE MB30, the datasheet doesnt really suggest what material to place between the cells, but says expansion force is about 60000N and to reserve 2.0-2.5mm expansion space between cells, so not sure if thats implying to add foam or not.

It has some clamping examples for a single cell further down on the page and shows it using 6 bolts, which come to think about it is probably better than a bolt on each corner.

You are asking questions that are answered in this thread... have you read it, or do you want someone else to give you the executive summary?

Putting six bolts in like you suggested means that the middle two need twice as much force since they have to provide the force for cells on either side of them. I got this wrong somewhere around page 21 or so and shouldn't have. As it turns out, I built two separate compressed stacks of cells so I could lift them, then connected them with a bus bar to complete the battery.

 

[justgary]

Don't forget these two threads while you are reading:

https://diysolarforum.com/threads/pack-cell-compression-optimized-by-using-springs.14751/

https://diysolarforum.com/threads/another-cell-compression-thread-this-time-about-foam.16537/

 

[off.the.grid]

Each row needs 3000 Newtons pressing the end cell "faces" together. (About 680 lbs). So, in your 4-cell pack and the Luyan case, the total compressive force (on two faces) needs to be 1350 lbs minimum, spread evenly.

More cells in a row affect the amount of expansion which occurs with SOC changes, and a case or 8 cells per row might exceed 7000 Newton limit if the containment doesn't allow form some expansion (via springs). EVE provides a formula to calculate that expansion. A row of 8 cells will expand (in combined thickness) by twice as much as your 4 cell "rows".

Adding only enough pressure to keep them from sliding down when you pick up the pack (with the middle totally unsupported) is not anywhere near enough pressure - that's maybe 250 lbs per row, you need around 700 lbs per row. The thin "insulators" which you should add between each of pair of cells, and also the end cells versus container, may also be more or less "slippery" than the blue film on the cells themselves, changing the amount of pressure needed to hold them "tight" against gravity.

Sorry mate, been reading so much stuff and dont have enough time to go through all 40 pages in here and one person says one thing and another says something completley different.
For example, you dont recommend using bolt torque and then others here say its fine, so still dont have an answer, but from my research, force is convertable to torque, but this is where im having trouble to work out the conversion, there are a bunch of calculators online, but im trying to find one for what im trying to do here.

From what I can tell, these are my calculations here, the cells measure 20.4cm x 17.5cm = 357cm square, convert to inches = 55.3 inches square.
EVE recommend 300KG of force applied, so converting that to pounds = 661.3 pounds

Now im assuming this is 300KG per the surface area of the cell, so i divide 661.3 pounds by 55.3 inches and i get 11.9 pounds per square inch
And this is where i get stuck, im trying to find a calculator to convert the force (300KG/661.3 Pounds) to newton metres or foot pounds, i then divide that figure by the number of bolts and set my torque wrench accordingly correct?

I was reading this thread earlier today, seems my numbers are not far off, however it suggests 8 INCH pounds per bolt for 4 bolts, how they are coming up with that number to convert 165 pounds per bolt into 8 inch pounds of torque im not exactly sure.

Post in thread 'Rick's recommended compression containment for EVE cells (LF280K, LF304)'

May 18, 2024

I’m soon going to be putting into service a new 8 pack of LF304K cells, so trying to get up to speed with compression mounts. It’s not so easy with so many threads and posts about cell compression and differing opinions!

I have in fact already made a mount using M8 threaded rod and fixed end plates, no springs. If I have interpreted things correctly the procedure should be to get the cells discharged to a low state of charge (under 3V per cell?) then apply a 300kg force by tightening nuts on the m8 threaded rods and after that just lock the nuts with threadlock compound or locknuts...

• Dynoman

• 

To make matters worse, ive bought all the materials I thought I would need and now am considering dropping my FR4 and using foam, you recommend poron which I will have trouble finding, or do i just get neoprene instead?
All I know is these commercially assembled units appear to be using FR4 sheets and no foam.

 

[fatjay]

OK, ive got some questions, how much torque is required for a box with 2 rows and 6 bolts?
Does the number of bolts affect the torque per bolt, how does force equate into torque? I see 10-15nm of torque thrown around in this thread.
It appears some expansion and contraction depending on SOC is normal, are EVE saying to clamp at around 40-50% SOC?
Ive top balanced some cells without compressing them and they have expanded a little but wouldnt go as far to say they are bulging, but apparently this is normal, i see some suggest foam over fibreglass to allow for this?

I did 18 inch pounds all around, ~240lb of clamping force for 6 bolts, 960lb in total. In theory, 2 rows side by side doubles the expansion/compression force. So each pack only has 4 sets of bolts exerting force on it, rather than 6. And the center 2 bolts, I think need 2x because they're getting force from both packs.

 

[off.the.grid]

I did 18 inch pounds all around, ~240lb of clamping force for 6 bolts, 960lb in total. In theory, 2 rows side by side doubles the expansion/compression force. So each pack only has 4 sets of bolts exerting force on it, rather than 6. And the center 2 bolts, I think need 2x because they're getting force from both packs.

OK, that is a bit more than the 660lb or so, but is probably OK, are your cells EVE or CATL? Your right about doubling the force for the centre bolts.
Im considering using 9 bolts to spread the load more evenly, any benefit in doing so?
What material have you placed between your cells?
I was told by my retailer that all the purpose of the FR4 was for is to insulate between the cells to prevent any shorts developing between the casings if the wrap gets worn.

 

[rhino]

I was told by my retailer that all the purpose of the FR4 was for is to insulate between the cells to prevent any shorts developing between the casings if the wrap gets worn.

Yes, this is true. You must prevent the cases from touching each other or anything metal they may be sitting in. Can use FR4 or even Poron which is a type of foam that doesn't lose isn't ability to expand after being compressed many times like almost all other foams. That is also easy way if you want to compress your cells since just compress the cells until the foam is about 25% compressed.. It also allows some expansion still..

 

[justgary]

From what I can tell, these are my calculations here, the cells measure 20.4cm x 17.5cm = 357cm square, convert to inches = 55.3 inches square.
EVE recommend 300KG of force applied, so converting that to pounds = 661.3 pounds

Now im assuming this is 300KG per the surface area of the cell, so i divide 661.3 pounds by 55.3 inches and i get 11.9 pounds per square inch

Yes, that is correct.

I was reading this thread earlier today, seems my numbers are not far off, however it suggests 8 INCH pounds per bolt for 4 bolts, how they are coming up with that number to convert 165 pounds per bolt into 8 inch pounds of torque im not exactly sure.

Force applied at a specific torque value depends on the thread pitch. The online calculators are estimates based on diameter and assume a common pitch. Like I mentioned before, it also depends on the surface finish of the threads and any available lubrication, so it is generally accepted that simply applying torque to a bolt will give you a very wide range of clamping force. The only way to get a narrow range of force for your application is to build a test jig and actually measure force per torque several times using your fasteners.

Yes, this is true. You must prevent the cases from touching each other or anything metal they may be sitting in. Can use FR4 or even Poron which is a type of foam that doesn't lose isn't ability to expand after being compressed many times like almost all other foams. That is also easy way if you want to compress your cells since just compress the cells until the foam is about 25% compressed.. It also allows some expansion still..

Yes, use FR4 between the cells and one 1/4" sheet of Poron 50-15250 per four cells. The obvious place to put the foam is at one end cap for four cells, or at each end cap for eight cells.

Compress the foam 25% for one layer (four cells) or 50% for two layers (eight cells). The reason for this is because the foam is a spring, and stacking springs changes the total force per distance compressed. I tested this by compressing one and two layers of 1"x1" squares of Poron foam and recording the results. Adding more layers of foam is *not* better.


Also don't forget to insulate the bottoms and sides of the cells. I used Kapton tape to make sure that the blue plastic wasn't the only thing providing insulation.

 

[fatjay]

OK, that is a bit more than the 660lb or so, but is probably OK, are your cells EVE or CATL? Your right about doubling the force for the centre bolts.
Im considering using 9 bolts to spread the load more evenly, any benefit in doing so?
What material have you placed between your cells?
I was told by my retailer that all the purpose of the FR4 was for is to insulate between the cells to prevent any shorts developing between the casings if the wrap gets worn.

EVE cells, the data sheet says 3000-7000 newtons, which is ~700-1500lbs. Since springs have a linear compression, they get stronger as they're compressed, as long as I don't hit the 1500lb mark I think i'm ok. I start at the low end of compression, as the cells expand up to 3%, or 0.65", the springs compress, increasing the force on the springs.

I cut strips of thin plastic from home depot normally used to line cabinets and drawers.

I think as long as you have a rigid plate, there's no need to increase the number of bolts.

 

[S Davis]

EVE cells, the data sheet says 3000-7000 newtons, which is ~700-1500lbs. Since springs have a linear compression, they get stronger as they're compressed, as long as I don't hit the 1500lb mark I think i'm ok. I start at the low end of compression, as the cells expand up to 3%, or 0.65", the springs compress, increasing the force on the springs.

I cut strips of thin plastic from home depot normally used to line cabinets and drawers.

I think as long as you have a rigid plate, there's no need to increase the number of bolts.

Make sure to adjust your springs to have enough travel for the number of cells expansion, longer the stack the more travel to not bottom out and spike the clamping force. My 4s packs all use 160# die springs and are 2.5 years old in a off road environment.

 

[fatjay]

Make sure to adjust your springs to have enough travel for the number of cells expansion, longer the stack the more travel to not bottom out and spike the clamping force. My 4s packs all use 160# die springs and are 2.5 years old in a off road environment.

My springs have a full 1.1" of travel, so since I have them in a 2x8" configuration, 3" of the total length is only 0.65", which is perfect. My springs are off a 2015 corvette LT4 engine that are new and never installed.

 

[fatjay]

Yes, this is true. You must prevent the cases from touching each other or anything metal they may be sitting in. Can use FR4 or even Poron which is a type of foam that doesn't lose isn't ability to expand after being compressed many times like almost all other foams. That is also easy way if you want to compress your cells since just compress the cells until the foam is about 25% compressed.. It also allows some expansion still..

But how much force does the foam expand with? Is it forcefully rebounding as the SOC on the cells drops?

 

[rhino]

But how much force does the foam expand with? Is it forcefully rebounding as the SOC on the cells drops?

"PORON polyurethane and BISCO silicone materials deliver consistent push back force to optimize battery cell life and performance."
Some info here: https://rogerscorp.com/Applications/Battery Pad and Cushion

 

[off.the.grid]

Yes, that is correct.

Force applied at a specific torque value depends on the thread pitch. The online calculators are estimates based on diameter and assume a common pitch. Like I mentioned before, it also depends on the surface finish of the threads and any available lubrication, so it is generally accepted that simply applying torque to a bolt will give you a very wide range of clamping force. The only way to get a narrow range of force for your application is to build a test jig and actually measure force per torque several times using your fasteners.


Yes, use FR4 between the cells and one 1/4" sheet of Poron 50-15250 per four cells. The obvious place to put the foam is at one end cap for four cells, or at each end cap for eight cells.

Compress the foam 25% for one layer (four cells) or 50% for two layers (eight cells). The reason for this is because the foam is a spring, and stacking springs changes the total force per distance compressed. I tested this by compressing one and two layers of 1"x1" squares of Poron foam and recording the results. Adding more layers of foam is *not* better.

Also don't forget to insulate the bottoms and sides of the cells. I used Kapton tape to make sure that the blue plastic wasn't the only thing providing insulation.

Yes, I was reading about how lubrication, thread pitch can have a bearing on this, I know this all too well when tightening cylinder heads, that being said, ive never really run into problems except for some older type gaskets that require a good amount of compression to seal properly, and that is a tremendous amount more force than a pack of these cells!

Interesting, so you are just putting FR4 between the cells and foam on one end only for a block of 4? I assumed people were putting this in between each and every cell instead of FR4, looks like you are using this to behave in much the same way as a spring?

I think I might be getting a bit too technical here, but Im sure that adding memory foam probably has a ton of advantages, but how many more years is it going to extend this battery pack? Its not like im trying to get 20 years out of it, I will have well and truly upgraded the whole system by then, but at the same time I want to make sure im assembling everything as good as I can.

Looking how my batteries have expanded after top balancing, they now have a high point in the centre, so foam would go a long way to fill all the ups and downs between the cells, but from what I have made out here, its best to discharge them to around 40% after connecting the BMS and then clamp up.

I dont anticipate myself using this battery past 10 years anyway, but if its still performing well by then I would be happy.

My biggest concern here is over clamping the cells, going by thread pitch as quoted by others, I will be using M8 bolts, will lubricate them and will clamp accordingly, but probably keep on the lower side to be safe, so assuming my figures are correct, I can probably be safe enough with 8 inch pounds, the bigger issue i see is if my wrench can even go that low, everything is in foot pounds on it from memory.