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Force on the terminals/studs of prismatic cells

merkatorix

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I saw many discussions about cell compression. When the cells swell, and the connections between the terminals/studs are rigid, some forces are transferred. So, if I understand it correctly, one reason for the compression is to lower those forces.
In the case of a setup of tight cells, which swell until the setup looks like a banana, I could imagine there are high forces on the studs. As shown in the attachment, I found here in the forum:

How robust are those studs? If a pack looks a little out of shape when assembled after top balancing at 3.65V with some foam between the cells and some duct tape, is that still OK? I fear some larger swelling later on, could cause damage. Suppose the connectors are already hard to assemble/disassemble. In that case, there is only very little tolerance left until high forces are transferred. If the cells are compressed by duct tape, some leverage might apply.
Are there flexible connectors for the studs? The connectors sent by LiitoKala are entirely straight.
 
Duct tape probably isn't going to cut it. Implement a rigid fixture to hold the cells snug so there is no movement. I use the standard bus bars to connect the cells and that's going to work just fine since I have a fixture to prevent the cells from moving. My batteries are in a bumper pull RV that sees plenty of rough roads.
 
How robust are those studs? If a pack looks a little out of shape when assembled after top balancing at 3.65V with some foam between the cells and some duct tape, is that still OK?
If you are talking about true prismatics, the stud is molded into the shell and the can take the stress. I had some Winston's swell and the cells fanned out with only the bus bars holding the tops.
The studs on the thin metal case are not as strong nor is the shell and that is why an apparatus for applying pressure is recommended. Duct tape and foam are probably not within the manufacturers definition of fixture or apparatus.
 
If you are talking about true prismatics, the stud is molded into the shell and the can take the stress. I had some Winston's swell and the cells fanned out with only the bus bars holding the tops.
The studs on the thin metal case are not as strong nor is the shell and that is why an apparatus for applying pressure is recommended. Duct tape and foam are probably not within the manufacturers definition of fixture or apparatus.
So, in summary, the trick is to overcome the pressure from the cells, so the studs don't see additional forces, so rigid busbars are fine. Examples are wooden boxes with some screws to apply pressure because they are relatively stiff, light, and non-conductive?
I thought compression would mean a constant force. Concerning the busbars, it would mean a constant position of the cells to save the busbars.

What are the consequences of a stud seeing too much force? Will there be damage before it is visible?

When doing the first test with the balanced cells, I thought there was a considerable force, and the cells were all at 3.65V. However, I only used the nut to tighten the screws. I didn't have a proper insolated wrench on hand and no ratchet (fun fact: The hardware store was only selling nuts, no ratchets).
Today, I did some tests at around 100A, and two studs became warmer than the others showing about 45° (maybe because of a screw not having contact, but I fear there might be other reasons. Also, the thermal cam might overreport the heat there due to reflections.
However, after recharging to 3.59V, the busbar had minimal tension, and I could lift it without any trouble by hand. Maybe I bent something by 0.03mm, which released the pressure. Perhaps the foam lost its tightness, or it wasn't as bad after all.
 

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I don't think foam is a good material to use for a compression fixture. I wouldn't use it between the cells and certainly not on the ends.

3/4" plywood plates on the ends and four threaded rods between the plates is all you need. If you want to put something between the cells, use something that doesn't compress.
 
It does. There are a lot of threads discussing the concept.
But the constant force would mean movement if the swelling goes down. In that case, the force should decrease, if the busbars should be free of any force, shouldn't it?
Some threads discuss the cell chemistry, that is a different reason for compression than supporting the studs, isn't it?
 
But the constant force would mean movement if the swelling goes down. In that case, the force should decrease, if the busbars should be free of any force, shouldn't it?
The concept is to prevent swelling so there is no movement. Or to compress them back to their non swollen state. That is how I have approached the issue. It is simple physics.
 
The concept is to prevent swelling so there is no movement.
I get it, and I should copy the version of HRTKD. I was just pedantic because no movement isn't a constant force. If there is no resistance from the cells, the force should be lower than if the fixture has to fight the swelling of the cells.

How likely is it to break the cells if they experience that force once? I am not sure if there was excessive force, but it could have been some kilos of force on the studs. However, maybe there wasn't; there were only several grams of force left today.
 
Given the pictures of cell strings that look like a banana, I would say that the terminals are fairly strong. However, strong terminals isn't the issue. Good, solid connections between the terminal and the bus bar is the issue. A cell that is allowed to flex over and over has the potential to loosen the connection between the terminal and the bus bar. That's when problems start to crop up.
 
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