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

Gazoo

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I have a stupid question. I have six 5/16 threaded rods and six 5/16 stainless steel lock washers. If I torque all six lock washers down until they are flat, approximately how much PSI would that be? Enough? If too much I can use 4 lock washers. I have built a 8s square pack but still need to mount the plywood. I am not going to worry about springs and what not. Even with 2000 cycles the cells will most likely outlive me.
 

Luthj

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To further elaborate on the spring selection. You first determine how many rods you want/need. Lets say 4.

Then you determine the clamping load. If you have one cell stack, that's about 600lb 600/4 =150lb per spring.

Now we need a coil spring which can apply 150lb before it binds/goes solid. Additionally we want a spring that won't require use to tighten a nut down 3" to get that force. We also want a spring with a small enough OD/ID that it can be used with a threaded rod and thick washer.

For example a hypothetical .8" OD spring with a spring rate of 200lb/in. For ever inch its compressed, it requires 200lbs of force. This spring has a max load of 175lb, which is fine for this example.

To get 150lb we then need to compress the spring by 200/150=1.33".

From there its simply a matter of measuring the springs free length, and then tightening each nut down until the springs are 1.33" shorter.
 

Luthj

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I have a stupid question. I have six 5/16 threaded rods and six 5/16 stainless steel lock washers. If I torque all six lock washers down until they are flat, approximately how much PSI would that be? Enough? If too much I can use 4 lock washers. I have built a 8s square pack but still need to mount the plywood. I am not going to worry about springs and what not. Even with 2000 cycles the cells will most likely outlive me.

There is lots of variance, but it can take 15-40lbs to compress a spring washer flat. I don't really call them lockwashers, as the spring type don't really improve fastener retention in most cases.
 

Gazoo

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There is lots of variance, but it can take 15-40lbs to compress a spring washer flat. I don't really call them lockwashers, as the spring type don't really improve fastener retention in most cases.
Thanks. So considering EVE is calling for 12PSI it should be ok to use all 6.
 

Luthj

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Thanks. So considering EVE is calling for 12PSI it should be ok to use all 6.
Sure, you can always do a quick and dirty test by hand. Push on a washer with a deep socket, and guess how much weight you are putting on it.
 

ghostwriter66

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I strongly dislike mechanical engineering - and to me thats what these spring things are -- i can usually finally figure it out but always wonder if I actually got it right our not ...

So if you were me - and you were looking at the below PDF -- and I am assuming that it would be the RED section on page 14 .. and you had (4) 3/8 threaded bolts you were using to press aluminium plates against the battery -- which one of these things should I order to get the right 12PSI (600kPF) compression that I am looking for ?? ...


Thx
 

Dzl

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I strongly dislike mechanical engineering - and to me thats what these spring things are -- i can usually finally figure it out but always wonder if I actually got it right our not ...

So if you were me - and you were looking at the below PDF -- and I am assuming that it would be the RED section on page 14 .. and you had (4) 3/8 threaded bolts you were using to press aluminium plates against the battery -- which one of these things should I order to get the right 12PSI (600kPF) compression that I am looking for ?? ...


Thx
I share this sentiment.

A nice clear explainer/primer from someone that feels they know what they are doing would be great!
 
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Bob B

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I am a LONG ways from being a mechanical engineer, but maybe one will check in.

Here is my theory.
Assuming we are in agreement that the desired pressure is 12 PSI,
The first step would be to determine the square inches for the side of the battery. For my 7" x 8" battery that would be 56 Sq inches.
Second, Multiply that by 12 to get the desired total pressure. For mine that is 56 x 12 = 672 lbs force.
Third, divide 672 by the number of threaded rods to get the pressure / rod. 672 / 4 = 168 lbs per rod.
Fourth ... convert that to KG. So we need a spring with 76.2 KG at a reasonable compression rate.

So .... for my batteries it looks like the 93-2030 on page 15 might be a good fit. (They show how much to compress it for 3 different pressure on the right side of the chart. Every spring in that group has the same capability but with different lengths.
 
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Dzl

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I am a LONG ways from being a mechanical engineer, but maybe one will check in.

Here is my theory.
Assuming we are in agreement that the desired pressure is 12 PSI,
The first step would be to determine the square inches for the side of the battery. For my 7" x 8" battery that would be 56 Sq inches.
Second, Multiply that by 12 to get the desired total pressure. For mine that is 56 x 12 = 672 lbs force.
Third, divide 672 by the number of threaded rods to get the pressure / rod. 672 / 4 = 168 lbs per rod.
Fourth ... convert that to KG. So we need a spring with 76.2 KG at a reasonable compression rate.
I think most of the middle steps can be skipped because EVE has done the math for us already (300KGF),
I originally got the 12PSI by doing the math from KGF to PSI.
So we don't need to convert back, we can just start with the original spec.

But I think your overall explanation is correct in principle, divide KGF by number of rods. And then choose springs to match whatever you come up with.

Where I personally start feeling less sure of myself is how to choose the spring and how to implement the design / how to know if the pressure is in the right ballpark. Is it all about measuring the compression of the springs?

edit: i'll checkout the page you mentioned, that sounds like just what i was hoping for.
 

Bob B

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I think most of the middle steps can be skipped because EVE has done the math for us (300KGF),
I originally got the 12PSI by doing the math from KGF to PSI.
So we don't need to convert back, we can just start with the original spec.

But I think your overall explanation is correct in principle, divide KGF by number of rods. And then choose springs to match whatever you come up with.

Where I personally start feeling less sure of myself is how to choose the spring and how to implement the design / how to know if the pressure is in the right ballpark. Is it all about measuring the compression of the springs?
Yeah ... It's more of a universal compression calculation thing .... My cells aren't actually EVE cells. Sounds like 12 psi compression might be good for all of the prismatic cells. I don't know what those cells of Dacian's are.
 

Dzl

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Yeah ... It's more of a universal compression calculation thing .... My cells aren't actually EVE cells. Sounds like 12 psi compression might be good for all of the prismatic cells. I don't know what those cells of Dacian's are.
I believe Dacian's are 20 or 40 Ah A123 pouch cells and If memory serves, the ones from the video were also A123 pouch cells but I'm not positive.

I would guess if ~4-18 PSI (or ~6-17 PSI according to EVE) is applicable to both small pouch cells and large prismatic cells, it should apply to most any of the cells we are purchasing.
 

SherylinRM

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Not even sure if this will help but what I have done is put a piece of plywood on each end and put 6 threaded rods on it [3 on each side] then tightened them until I could lift the batteries without them falling out.
I figured that was strong enough and did not need to be any tighter.

It might not be exact science, but it works for me. :)
 

Dzl

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Not even sure if this will help but what I have done is put a piece of plywood on each end and put 6 threaded rods on it [3 on each side] then tightened them until I could lift the batteries without them falling out.
I figured that was strong enough and did not need to be any tighter.

It might not be exact science, but it works for me. :)
I 'spect its a more than good enough approach (at least for anyone who doesn't care to obsess over technical details to the extent I do ;))
 

SherylinRM

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I 'spect its a more than good enough approach (at least for anyone who doesn't care to obsess over technical details to the extent I do ;))
I have OCD but I do not have "oh my goodness I missed a specification to the tenth decimal point. I have to start over now" type of disorder thank you. :) :ROFLMAO: 🤪 ;) :cool::)
 

BiduleOhm

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BTW a threaded rod will act as a spring, it'll have more give the thinner it is.

So if you don't want to put springs because it's too complicated or other reasons then don't use M10 or M8 rod (3/8 or 5/16) but smaller ones like M6 or M5 (1/4 and whatever 5 mm is in SAE sizes...) so you'll have a lower spring constant (stiffness) ;)

For experimenting purposes I might put a load cell at the end of each pack to measure the force/pressure (I'll have a spare 16 bits analog input on each BMS so I might as well use them for something useful ^^). And why not put springs on two of the 4 packs and no springs on the other two to compare data between both solutions while I'm at it :)
 

ArthurEld

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BTW a threaded rod will act as a spring, it'll have more give the thinner it is.

So if you don't want to put springs because it's too complicated or other reasons then don't use M10 or M8 rod (3/8 or 5/16) but smaller ones like M6 or M5 (1/4 and whatever 5 mm is in SAE sizes...) so you'll have a lower spring constant (stiffness) ;)

For experimenting purposes I might put a load cell at the end of each pack to measure the force/pressure (I'll have a spare 16 bits analog input on each BMS so I might as well use them for something useful ^^). And why not put springs on two of the 4 packs and no springs on the other two to compare data between both solutions while I'm at it :)
Lock washers have a spring effect if you torque them to exactly 15 psi when the cells are at full capacity. 😁

There are a lot of variables, especially when you line up 16 cells. My damn work bench has a sag in the middle. There's another variable.

Thanks for contacting EVE ghostwriter
 

SherylinRM

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BTW a threaded rod will act as a spring, it'll have more give the thinner it is.

So if you don't want to put springs because it's too complicated or other reasons then don't use M10 or M8 rod (3/8 or 5/16) but smaller ones like M6 or M5 (1/4 and whatever 5 mm is in SAE sizes...) so you'll have a lower spring constant (stiffness) ;)

For experimenting purposes I might put a load cell at the end of each pack to measure the force/pressure (I'll have a spare 16 bits analog input on each BMS so I might as well use them for something useful ^^). And why not put springs on two of the 4 packs and no springs on the other two to compare data between both solutions while I'm at it :)
I used 1/4 inch threaded rods actually.
They do the job good enough for me. :)
Try'em, you'll like'em. LOL :)
 

toms

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Keep in mind that the spring pressure will not change unless it’s spring bound or loose. If you set the pressure to 12psi with compression springs and they stay in their range you will have 12psi across the SOC range.
 

SherylinRM

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Keep in mind that the spring pressure will not change unless it’s spring bound or loose. If you set the pressure to 12psi with compression springs and they stay in their range you will have 12psi across the SOC range.
As the cells expand the springs will apply more pressure and as they contract they will apply less pressure. Minuscule but it will be there.
Whereas just clamping them when at a low SOC and using a "hold" type of clamping, then when they expand then they will apply their own pressure to the sides themselves.
So the cells end up doing all of the work for you and you save money on springs, time on adjusting them PERFECTLY, and frustration in adjusting them PERFECTLY.
Just my take on it is all :)
 
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