How to deal with uneven placement of compression springs?

trougnouf

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Aug 15, 2021
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Compression springs do not offer an even 11 psi if they are not distributed evenly, as that is often the case (especially if there are none between battery packs which is often the case).

Springs should thus be adjusted to each offer a separate force.

I've posted about it on https://www.reddit.com/r/AskEngineers/comments/pvawdk and I don't have a very conclusive solution. The two takes are using a FEM tool to calculate the force/stress across, and/or putting an additional plate with springs sandwiched between two plates.

Neither of these solutions seem very straightforward; I couldn't get FEM to run in FreeCAD without error, and the additional plate seems a bit tricky since the springs could only be held in place from at most one of the plates, but I guess that's likely not much of an issue.

I think my example is a bit extreme as I have most of the springs on the bottom ("floor" support) and I avoid them on the top (conductors). Am I overthinking it and I should just use a few bigger springs on the sides and call it a day? Or the additional plate is a pretty good way to go about it? Or there's a best solution out there?

Screenshot from 2021-09-25 18-26-13.png
 

Short_Shot

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You resolve this with sufficiently stiff plate to effectively eliminate the deflect which causes reduced pressure in the middle.

Or a crossmember and springs place mid-cell. All of this adds length to the assembly. This would be your "additional plate" method.

You should not arrange them the way you have done there either. They should be evenly distributed.
 

trougnouf

New Member
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Aug 15, 2021
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You resolve this with sufficiently stiff plate to effectively eliminate the deflect which causes reduced pressure in the middle.

Or a crossmember and springs place mid-cell. All of this adds length to the assembly. This would be your "additional plate" method.

You should not arrange them the way you have done there either. They should be evenly distributed.

What is this crossmember you are talking of?

Thanks for the feedback :) I guess the distance between each rod would simply be cells_height / #rods, and I should avoid putting any springs on the bottom unless I put the same number and at the same distance as those on top?
 

Short_Shot

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What is this crossmember you are talking of?

Thanks for the feedback :) I guess the distance between each rod would simply be cells_height / #rods, and I should avoid putting any springs on the bottom unless I put the same number and at the same distance as those on top?
The crossmember would just be a second plate like you already mentioned to put springs in the middle.

Uneven distribution of springs will give uneven force from top to bottom like that.
 

100 Proof

"Please Lord, don't let me do something stupid."
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Why not use 4 rods, 2 on either side of the cells? That's what most folks, me included, are doing quite successfully.
 

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Bob B

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Sep 21, 2019
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Why not use 4 rods, 2 on either side of the cells? That's what most folks, me included, are doing quite successfully.
I agree .... I don't see why all those extra springs across the bottom are needed. With a single row of cells, if the springs are the right force, it shouldn't be necessary to have more than 4.
 

trougnouf

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Why not use 4 rods, 2 on either side of the cells? That's what most folks, me included, are doing quite successfully.
I think that's what I will do, thank you :)
Do you leave your battery exposed like that / not afraid of something falling on it / intruding and shorting it? My plan is to put acrylic on the sides that are not aluminum.
 

100 Proof

"Please Lord, don't let me do something stupid."
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Do you leave your battery exposed like that / not afraid of something falling on it / intruding and shorting it? My plan is to put acrylic on the sides that are not aluminum.
My bank is in an enclosed space with little chance of anything falling on it. I do have some covers I put directly over the terminals though.

I'll soon be adding an insulated enclosure to keep it warm over the winter.
 

Posplayr

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May 27, 2021
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My solution to the compression problem for 2P4S cell configuration. I used 1"x2"x11 GA steel rectangular tubing to create the strong back and side structures to hide the all-thread and 3 1/2" compression springs. The 12 psi load is distributed across 8 springs that have 0.75" deflection which is in the middle of the total compression range.

There is a fair amount of welding involved but the structure seems to be stiff enough (across the sides of the two cells) that I don't notice any deflection. I used a straight edge and a tape measure and there doesn't seem to be any bending of the stong back.

My plan is to build something similar for a large 2P16S battery. Whether this would be 32 cell battery would be arranged 8 wide by 4 deep or 4 deep by 8 wide is still TBD.

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