Pack / Cell compression Optimized By Using Springs.

Sverige

A Brit in Sweden
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Oct 8, 2020
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Stiff end plates and threaded rods through the springs would seem like an easy way to go.

Just look at the "rate" parameter on Digikey: pounds per inch of compression.
Do you think M4 threaded rod would be up the job, or is something bigger warranted?
 

Hedges

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Do you think M4 threaded rod would be up the job, or is something bigger warranted?
If 12 psi is the goal and a battery is 6" x 8" = 50 square inches, need 600 pounds.
600/4 = 150 pounds per rod.
Mild steel is 30,000 psi strength (yield?)
150/30,000 = 0.005 square inch
sqrt(0.005) = 0.07 inches square (I'm too lazy to apply "pi")

Strength of threaded rod could be estimated as a round rod diameter the depth of thread.
I'll estimate as an inscribed square.
sqrt(0.07^2 + 0.07^2) = 0.10" diameter threaded rod might do the job.

(but consider a nut that isn't thin, at least several threads tall. Even a threaded standoff for extra threads.)

M4, 4mm diameter.
4/25.4 = 0.157" diameter.

Yes, looks like 4mm could do it.
But I would probably use 6mm.

I only use tiny 2.5 to 4mm screws on small enclosures or holding PCB, not for larger mechanical purposes.

By the time you've cranked nuts down half and inch or an inch to get desired spring force, the threaded rod will be sticking out. Maybe use that a both ends for mechanical mounting. A large and heavy pack would be better supported in the middle too.
 

zorlig

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I used 3/8 threaded rod. I used a scale between two 2x4 to see how much pressure was required, it got to 300lbs with a bit more than totally flattened lock washer. I then did that amount of force by hand on the cells with two sets to get close to 600lbs.

It seems really solid. I have 24 cells in series and if I move them they move as a group. I couldn't get anywhere near this pressure using straps and I'm not sure how springs would work.

The rods are like 6.50 each at home depot and they have set of hardware enough for three rods for a few dollars. Ended up being very cheap for something so effective.
 

Hedges

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it got to 300lbs with a bit more than totally flattened lock washer
I like compliance, more travel with relatively constant force.
I haven't worked with these batteries. But they are a sandwich of some materials, each having a particular TCE. They will experience a range of temperatures. If anything in the sandwich expands with heat, either the spring system takes up the travel and maintains steady pressure, or pressure increases and something inside tries to get extruded. Alternatively, under cold conditions, they shrink and the pack falls out. (or vice-versa, depending on relative TCE rate of steel rods. vs. battery sandwich)

1607012197110.png

How about doing just what you described, but with coil springs around each rod?
Suitably selected, they would be partially compressed when you reached the force desired.

One thing I really see in my yard is that PVC conduit expands, buckling into an "S" shape. Then contracts, pulling connections apart or breaking things. Much higher TCE than steel conduit and concrete wall it is mounted on.
 

Samsonite801

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When I used to be a mechanic, we would use valve spring pressure testers to help us determine what a cylinder head's valve spring 'installed height' should be set to.

They have cheaper ones where you can use a bench vise and put the tester on one end of the spring:

And they have more expensive ones dedicated for bench spring testing:

Say we wanted 100 lbs of pressure with the valve on the seat (closed), we would go use the tester and compress until we had 100 lbs pressure, and measure the height we were at (length of spring while holding this pressure), which was say: 1.70"... So then we could know to shim up our valve/spring/retainer package to achieve that installed height on the head.

Also to note, I remember my old 96 Chevy Blazer I used to own, had springs on the exhaust headpipe-to-manifold bolts, that seem like they might be about right for the range of using on our plates for 12 lbs (might make a good candidate), and they sell those at any auto parts store.



I've seen them in both straight and tapered:

1607019658687.png

Just throwing it out there, there might be better options out there, but it came to mind so thought I'd share it...
 

zorlig

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I like compliance, more travel with relatively constant force.
I haven't worked with these batteries. But they are a sandwich of some materials, each having a particular TCE. They will experience a range of temperatures. If anything in the sandwich expands with heat, either the spring system takes up the travel and maintains steady pressure, or pressure increases and something inside tries to get extruded. Alternatively, under cold conditions, they shrink and the pack falls out. (or vice-versa, depending on relative TCE rate of steel rods. vs. battery sandwich)

View attachment 29216

How about doing just what you described, but with coil springs around each rod?
Suitably selected, they would be partially compressed when you reached the force desired.

One thing I really see in my yard is that PVC conduit expands, buckling into an "S" shape. Then contracts, pulling connections apart or breaking things. Much higher TCE than steel conduit and concrete wall it is mounted on.
I am very curious to see if they actually expand at all under this pressure - and I plan to adjust the pressure once I get to a steady 3.4v
 

Electro Dan-O

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Let's do this! 5/16-24 SS rods and hardware are ordered and I started hacking on the wood plates tonight. Hopefully you guys can see from the picture what's going on in my head. Initial thought was to use four springs behind the smaller plate to evenly load the "face" of the cell vs. having the springs on the rods pushing on the outside edges. Now I'm wondering if I'm overthinking this(as usual). These wooden plates are 1/2" Douglas Fir that I plan on covering with 'glass and maybe carbon fiber on the end pieces. They will be flat and CRAZY strong. I doubt they will have much flex to them. What do you guys think? I'd rather keep it simple, but I also want to be nice to my brand new cells...

Also, I found some thin plexiglass that should make a reliable crush free insulator between the cells. I don't have any springs yet either. I don't mind to buy some, but I wonder what's laying around in my friend's small engine scrap pile. :unsure: Ahh, fun stuff!

mFUMQxN.jpg
 

Hedges

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I think the springs should still be on the rods. The smaller plate is too small to put the rods though.

Did you want to distribute springs across it, so any bowing is reduced? The board on the left doesn't get that treatment; it is only held by bolts.

Is this what you wanted to make them out of ?

1607407499259.png1607407471676.png
 

Electro Dan-O

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Gosh, I didn't even think about "the board on the left". You're 100% right...:ROFLMAO: Springs on the rods is the plan then. I like that better anyway!

No, I have some carbon fiber cloth left over from other projects that I think would be cool to use on the ends. Mostly for cosmetics, but added strength also.
 

Critter350

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I found some springs in my garage, which might be suitable for this purpose. What would be the most effective way to test their strength and figure out what compression I need to apply to get to my 12psi (once I’ve done the sums to figure out what force is needed per spring)?
If these are stiffer springs, and you have access to a drill press and bathroom scales... use the drill press to compress the springs a certain distance and read the load on bathroom scales?
 

Sverige

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338
If these are stiffer springs, and you have access to a drill press and bathroom scales... use the drill press to compress the springs a certain distance and read the load on bathroom scales?
Thanks. Good idea - I’ll give this a go
 

Hedges

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Gosh, I didn't even think about "the board on the left". You're 100% right...:ROFLMAO: Springs on the rods is the plan then. I like that better anyway!

No, I have some carbon fiber cloth left over from other projects that I think would be cool to use on the ends. Mostly for cosmetics, but added strength also.

I happen to have an ability to visualize things.

A Stanford PhD ME and professor would produce 3D drawings with Ansys, could generate exploded views.
I would look at his drawings and declare "You can't assemble those parts due to interference". "The Philips screw which holds that board to enclosure can't be installed due to order of assembly."

After he had enclosures fabricated, sure enough you could only put in two screws out of four even though they were Allen head.
I took it home, arc welded the seams, pulled out my trusty Milwaukee bandsaw and made new seams in a different place.
I then presented it to Mr. PhD and said, "Now it is practical."

PhD my @$$
 

Bob B

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I happen to have an ability to visualize things.

A Stanford PhD ME and professor would produce 3D drawings with Ansys, could generate exploded views.
I would look at his drawings and declare "You can't assemble those parts due to interference". "The Philips screw which holds that board to enclosure can't be installed due to order of assembly."

After he had enclosures fabricated, sure enough you could only put in two screws out of four even though they were Allen head.
I took it home, arc welded the seams, pulled out my trusty Milwaukee bandsaw and made new seams in a different place.
I then presented it to Mr. PhD and said, "Now it is practical."

PhD my @$$
I wish we could all find that one thing we are great at ... and be able to utilize it for our work.
I have a similar ability to scan and see the flaws in a building control specification. Can't tell you how many times I looked at a Spec for 2 minutes and announced ... That's not going to work.
Then, my project manager would have to start making phone calls and doing change orders. One engineer got smart and started sending me his specification first. The first time I met him, we almost had a fist fight .... 3 months later and we are good friends.
 

Luthj

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Greetings everyone. Would there be interest in a pre made kit for clamping/mounting the 280/272AH cells? This would consist of pre-bent metal and plates, rod hardware, springs, and rubber pads. Thinking about single and double row options. Below is some parts we had made up for a mobile project.image0_copy_1826x2591.jpegScreenshot_20201211-103006.png
 

noenegdod

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One thing to keep in mind and has been said many times before in this thread and others is that generally speaking, any given spring length will correspond with a specific load. From what I understand, these cells want to expand 1-2mm from discharged to charged. Applying the 12 psi will change this expansion of course but do we know if they still grow or not when the load is applied? Any way, the point of this post is to simply the following:

If you are able to apply the proper amount of force to 1 discharged battery, when fully charged if it grows by 1mm then the load on the cell exerted by the spring will have increased. This will vary depending on the spring used of course but 1mm most likely wont have a significant increase in load on the cell. If you have 4 cells in a pack and they all grow by 1mm now the spring has been compressed by 4mm and if the pack was 8, now it has grown by 8mm (well over 1/4") This can be very significant unless you are using a rather long spring. Keeping the number of cells the springs are acting on as low as possible will provide the most predictable loads on the cells.
 

Bob B

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One thing to keep in mind and has been said many times before in this thread and others is that generally speaking, any given spring length will correspond with a specific load. From what I understand, these cells want to expand 1-2mm from discharged to charged. Applying the 12 psi will change this expansion of course but do we know if they still grow or not when the load is applied? Any way, the point of this post is to simply the following:

If you are able to apply the proper amount of force to 1 discharged battery, when fully charged if it grows by 1mm then the load on the cell exerted by the spring will have increased. This will vary depending on the spring used of course but 1mm most likely wont have a significant increase in load on the cell. If you have 4 cells in a pack and they all grow by 1mm now the spring has been compressed by 4mm and if the pack was 8, now it has grown by 8mm (well over 1/4") This can be very significant unless you are using a rather long spring. Keeping the number of cells the springs are acting on as low as possible will provide the most predictable loads on the cells.

The whole point of the springs is the recognition that the pack is going to expand and contract with SOC, temperature, etc. ..... but, by using springs, the pressure changes will be much less than if there is just fixed compression.

While 12 PSI is the sweet spot .... if we assume that pressure variations of +-3 PSI or so would be acceptable.

If 4 rods are used on a 56 Sq In cell .... Would be 168 lbs per rod.
At 9 PSI, the pressure per rod would be 126 lbs.
at 15 PSI, it would be 210 Lbs

While 3 PSI is kinda random ... I think most would agree it is acceptable and the range of pressure per rod in the above example could vary between 210 and 126 lbs.

So, there can be significant change in the pressure / rod / spring and still be in the effective range to protect cycle life of the cell.
Maybe we can get to the point that someone is actually able to measure the pressure change across a wide range of temperature and thru the full SOC.
 

noenegdod

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The whole point of the springs is the recognition that the pack is going to expand and contract with SOC, temperature, etc. ..... but, by using springs, the pressure changes will be much less than if there is just fixed compression.

While 12 PSI is the sweet spot .... if we assume that pressure variations of +-3 PSI or so would be acceptable.

If 4 rods are used on a 56 Sq In cell .... Would be 168 lbs per rod.
At 9 PSI, the pressure per rod would be 126 lbs.
at 15 PSI, it would be 210 Lbs

While 3 PSI is kinda random ... I think most would agree it is acceptable and the range of pressure per rod in the above example could vary between 210 and 126 lbs.

So, there can be significant change in the pressure / rod / spring and still be in the effective range to protect cycle life of the cell.
Maybe we can get to the point that someone is actually able to measure the pressure change across a wide range of temperature and thru the full SOC.
I think you may have missed what I was trying to say so Ill say it another way.

If you put each individual cell in its own individual "clamp", springs will deliver extremely consistent pressure. Every cell you add after that reduces the consistency that a typical spring can provide. Thats it. Im not knocking springs. Im just saying that if you want the most out of your cells and if getting the most means keeping them as close to 12 psi as possible, than you should try to minimize the number of cells in each clamp as much as possible (if your using springs). If your happy with some degree of the "better than nothing" approach, then add as many cells as you are comfortable with. Im not trying to knock anything. just trying add value and a perspective I hadnt seen considered yet.
 

Bob B

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I think you may have missed what I was trying to say so Ill say it another way.

If you put each individual cell in its own individual "clamp", springs will deliver extremely consistent pressure. Every cell you add after that reduces the consistency that a typical spring can provide. Thats it. Im not knocking springs. Im just saying that if you want the most out of your cells and if getting the most means keeping them as close to 12 psi as possible, than you should try to minimize the number of cells in each clamp as much as possible (if your using springs). If your happy with some degree of the "better than nothing" approach, then add as many cells as you are comfortable with. Im not trying to knock anything. just trying add value and a perspective I hadnt seen considered yet.

We would have to have a specific spring to look at to know how much the deviation would be .... but I agree with your point that there will be greater deflection the more cells are in series.
 

noenegdod

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Just use a longer spring with a lower k constant; same result and far easier ;)
Absolutely. Using a long spring with a very low rate can substantially mitigate or almost eliminate any change in force over the distances we are talking about. Again, it was just a perspective I hadnt seen considered anywhere. Most of what I had read was just fixture + spring = good so was just trying to add a little value.
 
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