Pack / Cell compression Optimized By Using Springs.

[Supervstech]

Just about to wrap this project up! I wound up buying some .75"OD x 1" OL 135lb. springs. Thanks for the help guys. I think this will work!

Holy smokes!
from the way this thread was going, I sure did not expect that work of art to be the final design... WOW they look sexy!
I want some.

 

[K8MEJ]

Fantastic work! I know I'm asking for more work on your part, but would you be willing to list the parts you used and where to find the harder to find items? In particular, I'm wondering where you got those flexible bus bars, plexiglass, corner brackets, and springs. Links would be great, of course, but I can dig them up. I'm most interested in the flexible bus bars.

FWIW, I'm going to blatantly copy your work.

Thank you

 

[Supervstech]

Flexible bussbars can be made easy with copper pipe, and some wire braid.
tin plating after the crunch makes them look great and last.

Tin Plating, Nickle Plating or Solder plating Bus bars - History

diysolarforum.com

 

[Electro Dan-O]

Thanks guys! I can't take too much credit though. I robbed and got help for most of these ideas from this awesome forum. I'll make a separate thread with a ton of pictures and info in the next few days. Fun stuff!

The only thing that was slightly difficult to come up with was the springs, but that was mostly me being a cheap a$$. I got over that, called W.B. Jones with my specs and talked to a guy named Sam. He was super helpful and got me taken care of, no problem. I did have to make a "minimum order" of $35+shipping, so that's something for you guys to think about. Group buy maybe?

That being said, if anyone needs a few 135lb springs, let me know. I have a lot of extras!

 

[K8MEJ]

Thanks guys! I can't take too much credit though. I robbed and got help for most of these ideas from this awesome forum. I'll make a separate thread with a ton of pictures and info in the next few days. Fun stuff!

The only thing that was slightly difficult to come up with was the springs, but that was mostly me being a cheap a$$. I got over that, called W.B. Jones with my specs and talked to a guy named Sam. He was super helpful and got me taken care of, no problem. I did have to make a "minimum order" of $35+shipping, so that's something for you guys to think about. Group buy maybe?

That being said, if anyone needs a few 135lb springs, let me know. I have a lot of extras!

I need 12. Do you have that many extra and would you be willing to sell and ship them to me in Ohio?

 

[fafrd]

I need 12. Do you have that many extra and would you be willing to sell and ship them to me in Ohio?

135 lbs may be enough for those thinner cells, but the 280Ah cells generally need more (at least 165 lbs).

You’re aiming for 300Kg or ~165lbs per spring (assuming 4 springs and EVE 280Ah cells).

With most springs, the rating given is at full compression (solid) and it goes down from there, so you’ll ideally want a spring that reaches 165 lbs + delta at solid (or slightly backed-off from solid) which is where the spring will be set at maximum compression / 100% SOC, reaches close to 165 lbs after about 1mm of relaxation from that starting point (for 8 cells) around 50% SOC, and reaches 165 lbs - delta after about another 1mm of relaxation from that midpoint (close to 0% SOC).

 

[Bob B]

@Electro Dan-O .... looking forward to you picture heavy thread. Hoping you're going to link to it here.

 

[noenegdod]

I thought Id stick this in here for some feedback. My original intention was to go with no springs but I always knew there was a decent chance Id end up using them.

The logic I used for the design is to try to eliminate strain on the terminals while using bus bars. I believe the usual arrangement of having the cells connected to the one above/below it (in the picture below) instead of left/right it will result in lateral load on the terminal. The spring is there to provide the compression but also allow a degree of movement as the cell expands and contracts over its charge cycle. This must result in load on the terminal if a solid bus bar is used.

Connecting the cells left/right will result in the string moving relative to the string above/below it but not in relation to the cell it is connected to. I concede that there is an amount of unavoidable movement and vibration (mobile application). Using a belleville washer on the terminal will mitigate any rotation at the terminal caused by uneven expansion and contraction of connected cells and twisting of the pack. Crossing fingers and hoping vibration due to mobile application will be tolerated. The vehicle its going in has 14" of plush suspension travel (6 up and 8 down) so hoping it is ok.

The build as it sits today:

• The series strings will run left/right and will be paralleled at the left/right ends.
• There is an air gap 360 degrees around the narrow side.

Future steps (no particular order):

• Springs
• Insulation
• Bottom heater, Induced draft fan for top and thermostat
• SBMS thermocouple install
• Epoxied, vented studs
• 1/16"X 1.5" Aluminum bus bars (max current from each string will be in the area of 125A)
• 3mm foam liner applied to plywood/cell broad side contact to help provide more even pressure to cell sides
• A small plywood block on top of the cells between terminals to fully capture the cell in jig.

Any constructive criticisms welcome!

 

[noenegdod]

If someone is willing, please check my logic and math:

The pack has 5 pairs of rods compressing 4 sets of cells. The outside pair of rods work on 1 set each but the other 3 inside pairs of rods work on 2 sets each.

With 650 LBS as the target for each cell set, if you were compressing just one set of cells you would want 162 lbs of tension on each of the 4 rods around that cell group. As the 3 inside pairs of rods are working on two sets of cells I believe those rods need to have twice the tension on them. Correct?

ATM I have decided on these springs:

• For the outside 4: https://www.leespring.com/compression-springs-hefty?search=LHL1000B09

Spring rate is 184lbs\inch, free length is 4" and solid length is 2.38 leaving a working stroke of 1.62". At 184lbs/inch if I want 162lbs then I want to compress the spring 0.88" and the target length of the spring should be 3.12"

• For the inside 6: https://www.leespring.com/compression-springs-hefty?search=LHL1000C09

Spring rate is 345lbs\inch, free length is 4" and solid length is 2.66 leaving a working stroke of 1.34". At 345lbs/inch if I want 324lbs then I want to compress the spring 0.94" and the target length of the spring should be 3.06"

Sound about right?

 

[Johncfii]

Hello all .....
At first, I was skeptical about this idea of using springs for pack compression, but after reading all of this discussion, and thinking about it a lot, I concluded that it was the best affordable means of compression for my three new 8S packs of 280 amp-hour cells.

I finished getting them put together a couple of days ago.

After a lot of searching, the springs I selected are Danly 9-1208-26 Red Die Springs The springs are 2” free length, 3/4” O.D., 3/8” I.D., with spring constant of 58 pounds per 1/10 inch of compression. I tested the spring constant with a DIY scale fixture, and found them to be amazingly linear. Here is the spring data sheet.

9-1208-26 - DieMax XL Die Spring 0.750 (20mm) x 2.00 (51mm) Heavy Load (Red) - ISO Standard | Danly

9-1208-26 is a Danly DieMax XL Die Spring 0.750 (20mm) x 2.00 (51mm) Heavy Load (Red) - ISO Standard

www.danly.com

The best place that I found to buy them was online at Sup-R-Die.com for $3.67 each, plus a very reasonable shipping cost ($5.95 shipping for 12 springs).

For my 8S pack, I chose to double the springs, putting one on each end of each threaded rod, in order to increase linearity and to preclude any chance of them bottoming out.

My compression fixture is 5/16” threaded rod (with each rod covered in HS tubing), 3/4“ x 3/4” steel square tubing, and 3/4” plywood.

Thanks for all of the great ideas here.

 

[fafrd]

Hello all .....
At first, I was skeptical about this idea of using springs for pack compression, but after reading all of this discussion, and thinking about it a lot, I concluded that it was the best affordable means of compression for my three new 8S packs of 280 amp-hour cells.

I finished getting them put together a couple of days ago.

After a lot of searching, the springs I selected are Danly 9-1208-26 Red Die Springs The springs are 2” free length, 3/4” O.D., 3/8” I.D., with spring constant of 58 pounds per 1/10 inch of compression. I tested the spring constant with a DIY scale fixture, and found them to be amazingly linear. Here is the spring data sheet.

9-1208-26 - DieMax XL Die Spring 0.750 (20mm) x 2.00 (51mm) Heavy Load (Red) - ISO Standard | Danly

9-1208-26 is a Danly DieMax XL Die Spring 0.750 (20mm) x 2.00 (51mm) Heavy Load (Red) - ISO Standard

www.danly.com

The best place that I found to buy them was online at Sup-R-Die.com for $3.67 each, plus a very reasonable shipping cost.

For my 8S pack, I chose to double the springs, putting one on each end of each threaded rod, in order to preclude any chance of them bottoming out.

My compression fixture is 5/16” threaded rod (with each rod covered in HS tubing), 3/4“ x 3/4” steel square tubing, and 3/4” plywood.

Thanks for all of the great ideas here.

Nice find!

I cannot find the solid length of that spring (or the load at solid) - does Danly supply one of those two specs?

If not, have you measured the length at solid?

Also, can you tell us what your shipping cost was?

Lee Springs charges ~6 shipping + a $20 ‘handling fee’ if the order is under $40...

 

[Johncfii]

Nice find!

I cannot find the solid length of that spring (or the load at solid) - does Danly supply one of those two specs?

If not, have you measured the length at solid?

Also, can you tell us what your shipping cost was?

Lee Springs charges ~6 shipping + a $20 ‘handling fee’ if the order is under $40...

In the Danly catalogue, it says that this spring has a maximum compression of 30%, or 0.6 inches.
From Sup-R-Die, the shipping cost for 12 springs was $5.95. There didn’t seem to be any minimum order size, and there was no handling fee.

 

[Johncfii]

If someone is willing, please check my logic and math:

The pack has 5 pairs of rods compressing 4 sets of cells. The outside pair of rods work on 1 set each but the other 3 inside pairs of rods work on 2 sets each.

With 650 LBS as the target for each cell set, if you were compressing just one set of cells you would want 162 lbs of tension on each of the 4 rods around that cell group. As the 3 inside pairs of rods are working on two sets of cells I believe those rods need to have twice the tension on them. Correct?

ATM I have decided on these springs:

• For the outside 4: https://www.leespring.com/compression-springs-hefty?search=LHL1000B09

Spring rate is 184lbs\inch, free length is 4" and solid length is 2.38 leaving a working stroke of 1.62". At 184lbs/inch if I want 162lbs then I want to compress the spring 0.88" and the target length of the spring should be 3.12"

• For the inside 6: https://www.leespring.com/compression-springs-hefty?search=LHL1000C09

Spring rate is 345lbs\inch, free length is 4" and solid length is 2.66 leaving a working stroke of 1.34". At 345lbs/inch if I want 324lbs then I want to compress the spring 0.94" and the target length of the spring should be 3.06"

Sound about right?

I might not understand your design fully, but there is likely to be a problem with a plan to ”share” rods and springs between two side-by-side battery packs. I am pretty sure of this because it was my first thought/plan also. The problem is that you will soon discover that the side by side packs are not exactly the same total length. I don’t know why, other than normal process variation, but my side-by-side 8S packs varied in combined “length” by 1/16“ to 1/8“. What this means is that spring pressure will not evenly divide between packs. The pack with greater length will be compressed more tightly. And even if the packs seem to be the same length in an uncompressed state, there is no assurance that each cell will expand indentically.

I then chose to assemble each pack with a dedicated compression fixture. I think this really is necessary to assure a consistent result.

Of course, there is a way to share rods by placing the springs between a “support plate” and a “pusher plate”, but that would probably be even more complex to build.

 

[fafrd]

In the Danly catalogue, it says that this spring has a maximum compression of 30%, or 0.6 inches.
From Sup-R-Die, the shipping cost for 12 springs was $5.95. There didn’t seem to be any minimum order size, and there was no handling fee.

Cool, thanks. Sounds like a much better deal than Lee Springs (because of the $20 ‘handling fee’ fit orders under $40$.

If 30% compression translates to 1.4” at solid that also means 348 lbs at solid. With 4 rods, that translates to 1392 lbs or 25.5psi (too much).

So you only want to compress your springs about 5/16” which delivers to 181.25 lbs per spring or 725lbs / 13.3psi (the springs on either end need to be compressed that amount.

Assuming that is the load at 100% SOC, each spring will decompress about 2mm to 0% SOC, meaning each spring will lose 45.67lbs to 135.6lbs at which point total pressure is 542.3lbs or 9.9psi.

It’s a nice solution (as long as you can get the springs set correctly at the beginning).

 

[Johncfii]

Assuming that is the load at 100% SOC, each spring will decompress about 2mm to 0% SOC, meaning each spring will lose 45.67lbs to 135.6lbs at which point total pressure is 542.3lbs or 9.9psi.

Sorry, I misread your comment. Your arithmetic looks pretty good. Yes, it would sure be nice to have a load cell between battery cells to confirm the actual pressure.

 

[Bob B]

Hmmmmm ..... why do you think that the recommended 300 kg of force is intended to apply to batteries at 100% SOC?
Others on this forum seem to have concluded that it should apply at 50% SOC, in order to assure that from 0% SOC to 100% SOC, we remain in the “acceptable range” of from about 6 psi to about 19 psi.

Is there any developed consensus about this?

In my opinion it could be either .... depending on how you adjust your springs.

If you adjust the spring at 100% for the max force you want to see .... that would work fine .... assuming the spring you choose have plenty of travel to have good pressure when the pack is full discharged.

I will probably adjust mine at 50% so that it is the recommended 12 PSI at that point and make sure that the pressure isn't too high when full charged.

I don't think there is a best way to do it.

 

[Johncfii]

In my opinion it could be either .... depending on how you adjust your springs.

If you adjust the spring at 100% for the max force you want to see .... that would work fine .... assuming the spring you choose have plenty of travel to have good pressure when the pack is full discharged.

I will probably adjust mine at 50% so that it is the recommended 12 PSI at that point and make sure that the pressure isn't too high when full charged.

I don't think there is a best way to do it.

My mistake. I misunderstood fafrd’s comment at first. We are not far apart in our thinking.
I agree that we probably don’t have enough information to be sure of one “best“ way.

 

[fafrd]

Hmmmmm ..... why do you think that the recommended 300 kg of force is intended to apply to batteries at 100% SOC?
Others on this forum seem to have concluded that it should apply at 50% SOC, in order to assure that from 0% SOC to 100% SOC, we remain in the “acceptable range” of from about 6 psi to about 19 psi.

Is there any developed consensus about thi#?

That’s correct, ideally you want 660lbs applied at 50% SOC at which point you will have exactly 12.1psi and from which you will expand ~2mm to 100% SOC and contract ~2mm to 0% SOC. The trick is finding the right process & point to calibrate.

Knowing that your pair of springs on each rod will be increasing by a total of 91.3lbs over the ~4mm total between 0% SOC and 100% SOC, you can try to compress your springs to 614 lbs at 0% SOC or you can try to compress your springs to 705.7 lbs at 100% SOC.

In general, most have reported that range of expansion starts at more than 0.5mm/cell over the first few cycles and then settled to ~0.5mm total between 0% and 100> SOC.

That means calibrating at 100% SOC is ‘safer’ (because you are unlikely to expand beyond that set point when charging).

But after a few cycles you are likely to discover that your 100% SOC expansion is no longer as great as it once was, meaning your springs will no longer be as compressed at 100% SOC as you initially set them, meaning you may want to tighten/recalibrate.

Thise springs you found seem to have plenty of range so you should find it pretty easy to stay within the ‘safe zone.’

 

[fafrd]

In the Danly catalogue, it says that this spring has a maximum compression of 30%, or 0.6 inches.
From Sup-R-Die, the shipping cost for 12 springs was $5.95. There didn’t seem to be any minimum order size, and there was no handling fee.

I’ve been searching through the catalog and cannot hind that reference to 30% maximum compression - is there a link you can share?

 

[noenegdod]

There is a problem with a plan to ”share” rods and springs between two side-by-side battery packs. I am pretty sure of this because it was my first thought/plan also. The problem is that you will soon discover that the side by side packs are not exactly the same total length. I dont know why, but my side-by-side packs varied in combined “length” by 1/16“ to 1/8“. What this means is that spring pressure will not evenly divide between packs. The pack with greater length will be compressed more tightly. And even if the packs seem to be the same length in an uncompressed state, there is no assurance that each cell will expand indentically.

I then chose to assemble each pack with a dedicated compression fixture. I think this really is necessary to assure consistent result.

Of course, there is a way to share rods by placing the springs between a “support plate” and a “pusher plate”, but that would probably be even more complex to build.

I had considered that as a concern but decided it was not a significant issue because:

• They are supposed to be under the load so assessing their physical dimensions not under load is not something that can really be evaluated.
• When stacking cells together, if the areas that have bulged are in different locations (one in the top half and the next one in the bottom half vs two cells where the swelling is both in the middle) the overall dimension is meaningless.

I am adding 3mm drawer liner to the sides of each and every cell to help mitigate. I appreciate the solution is not perfect but using springs is a far from perfect solution in the first place so as long as I am kind of in the ball park Im happy. The biggest reason I am using springs is so I dont severely over compress.

Hmmmmm ..... why do you think that the recommended 300 kg of force is intended to apply to batteries at 100% SOC?
Others on this forum seem to have concluded that it should apply at 50% SOC, in order to assure that from 0% SOC to 100% SOC, we remain in the “acceptable range” of from about 6 psi to about 19 psi.

Is there any developed consensus about thi#?

The pressure is not dependent on state of charge. It is a constant pressure regardless of SOC. It is not a pressure based on SOC. There are a bunch of theories floating around on where the target pressure should be set but they are all speculation and best guesses. The springs I chose have a large difference between free and solid dimension and a relatively low spring rate so as the cells expand and contract based on their state of charge there will be relatively little change in force on the cells so Im setting mine at 50% soc. They are 4 inch spring but I have room so I get that luxury.