Pack / Cell compression Optimized By Using Springs.

[Bob B]

I am opening this thread to split it off from the general cell compression thread. That way those looking for info don't have to sort thru a thread that has many paths for pack compression.

If you are new to this idea, it stemmed from an EVE cell specification that indicated a significant increase in cell cycle life could be gained by keeping the cells under pressure.
Through the course of the discussion, it has been determined that the optimal pressure would be 12 PSI .... and any pressure above 17 psi would be counter productive.
@vtx1029 has gone to the trouble of putting together a spreadsheet to calculate variables depending on springs used and pack sidewall area. He will send it to you if you message him.

If you haven't seen the other thread, it can be found here. https://diysolarforum.com/threads/e...ed-tight-or-spaced-for-expansion.7892/page-23

 

[Dzl]

Good idea starting a new thread on this specific topic, that one is getting pretty lengthy and hard to wade through. For anyone who has not been following that thread, some highlights to give context to what is being discussed are posts: #4, #24, #195, #215, #216, #217, #233, #248, #340 (there is a lot of specific and useful information contained in that thread, specifically pages 1-3ish and 10-14ish, but its a lot to wade through)

 

[Bob B]

In general, to determine the spring capacity needed. .... I am using Sq In since the 12 psi number is what we have to work with .... but the same principle will work if converted to metric.

1. Determine the area of the side of the cell.
So, for a cell that is 7" x 8", the area will be 56 Sq Inches.
2. Multiply the area by 12 since we want 12 psi. (56 x 12 = 672 total lbs of force needed)
3. Divide the total force needed by the number of springs being used. (672 / 4 = 168 lbs per spring if 4 springs are being used.)

There are pictures of a pack done by Dacian (Electrodaucus Guy) on that other thread ... maybe it can later be added here along with pics from others who have done this.

 

[Bob B]

I should compliment @Dzl who has been the first one to post here .... his information gathering has been largely responsible for us being able to melt this down to 12 psi as the optimal pressure.

 

[Bob B]

Oh, and just to be clear up front .... I you are not interested in this idea .... or think it it a bad idea .... or think it is a waste of time or resources ... There are other threads where you can post that sentiment.
This thread is for those who are interested in or think it is a good idea .... or are already pursuing the idea.

This way, we don't have to police unproductive comments later on.

Thanks for indulging this somewhat rude post.

 

[vtx1029]

Thanks for the props Bob! Here's some screen shots for people interested.

 

[Sillyputty]

It looks like this info provided by @vtx1029 may be helpful in other areas as well.

I have a "spring" related issue I'm trying to solve. This is regarding vibration-proofing and isolating as much as possible a 280ah battery that will be in a camp trailer that travels some very bumpy roads.

Basically it involves a relatively rigid container to house the cells and keep them compressed, which (I think) will be further isolated from the trailers floor (or tongue) by relatively short compression springs beneath it, and perhaps some counter-acting extension springs above and/or laterally. If anyone has experience with this idea (good or bad) I'd like to hear it.

Very interesting stuff! Thanks to all the contributors.

 

[BiduleOhm]

Basically it involves a relatively rigid container to house the cells and keep them compressed, which (I think) will be further isolated from the trailers floor (or tongue) by relatively short compression springs beneath it, and perhaps some counter-acting extension springs above and/or laterally. If anyone has experience with this idea (good or bad) I'd like to hear it.

It's a good idea. You want a rigid pack on compliant mounts to attenuate the vibrations and shocks. Some additional info here

 

[Sonne]

I know it was the Eve 280Ah cells which sparked this discussion about compression at 12psi, but how likely is it that this also applies to other manufacturer’s cells? I mean are we safe to assume that this method is safe / worthwhile for prismatic cells whereever they come from?

 

[Sonne]

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)?

 

[Luthj]

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)?

Get a force gauge, such as a luggage scale. Then measure the springs coefficient. Its found by taking force divided by distance. So 50lb at 2" compression yields a K of 25lb/in.

 

[vtx1029]

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 you know of any automotive machine shops in your area its not uncommon for them to have spring testers that would work well for this.

 

[Sonne]

Get a force gauge, such as a luggage scale. Then measure the springs coefficient. Its found by taking force divided by distance. So 50lb at 2" compression yields a K of 25lb/in.

View attachment 28097

Thanks - i have one of those but it will only work in extension rather than compression. Is the K for a spring the same in both directions?

 

[Luthj]

Thanks - i have one of those but it will only work in extension rather than compression. Is the K for a spring the same in both directions?

Just run a bolt/rope though the spring with a washer on the other side. That way you can test compression with a pull gauge.

 

[Hedges]

For a compression spring with relatively square ends and short/squat aspect ratio like valve springs ...

Use four springs to support the corners of a square of plywood.
Measure height off floor.
Stack exercise weights, car batteries, or something else heavy and measure new height.
Increase weight and measure again.

Probably you'll find a linear displacement vs. force.

Could also make a platform above a bathroom scale, or support a bathroom scale off the floor with springs.

Any way you do it, best to keep force centered in platform and not tilting. Otherwise each spring gets different displacement, and assuming all linear you might then solve for spring constant K.

 

[Electro Dan-O]

Interesting stuff! I just got some new EVE cells that need to be put together for my boat project. I'm a decently crafty guy in the shop and feel like I could whip something up. I understand that these aluminum case cells can short out if not properly insulated from each other. That being said, I really liked the idea of using a thin layer of closed cell foam for insulation, but I wonder how that and also other materials would effect the constant pressure thing. Also, I was wondering if(instead of measuring spring height) it would be possible to figure out the bolt torque value on the "fixture" to pull up the spring pressure with a torque wrench. Just a thought. Might be getting into the "splitting hairs department" here...

 

[Cdkipp]

I have an RV also. Does anyone have an opinion on whether 4 spings or 5 with one in the center or a 6 arrangement would be better? I found this site which lists springs and their rating. Any engineers out there that could get someone in the ballpark on rating for each spring? Also would need some type of hardware to compress the spring and hold it in position. https://www.digikey.com/en/products/filter/springs-compression-tapered/1002 These also look interesting.. https://www.mcmaster.com/shock-absorbers/high-load-fastener-mount-compression-springs/

 

[Hedges]

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.

 

[Cdkipp]

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.

I was actually thinking of something for the under the base upon which the batteries rested to provide a buffer from the bumps. Maybe dense foam would be the best bet.

 

[Dzl]

I was actually thinking of something for the under the base upon which the batteries rested to provide a buffer from the bumps. Maybe dense foam would be the best bet.

I would think something like that would be a good idea, particularly for conditions like washboard I would imagine.

 

[Sonne]

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]

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]

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]

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)



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]

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:

Amazon.com: ProForm 66835 Mini Valve Spring Tester: Automotive

Buy ProForm 66835 Mini Valve Spring Tester: Compression Gauges - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

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

Amazon.com: Intercomp 110058 Arbor Digital Valve Spring Tester: Automotive

Buy Intercomp 110058 Arbor Digital Valve Spring Tester: Springs - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

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.

chevy blazer exhaust bolt and spring - Google Search

I've seen them in both straight and tapered:



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