Pack / Cell compression Optimized By Using Springs.

curiouscarbon

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this 12psi figure is derived from a 300kg force that is specified in a data sheet (think eve?) for big 280/300Ah cell. it’s repeatedly referenced around the forum so my source is here say and that i’ve seen the 300kg figure in the eve data sheet before

by dividing 300 kilogram by the surface area of the broad side of the cell, 12 pound per square inch (sorry units) was concluded from what i gather.

for these CATL 302Ah cells i will be best served to inquire with CATL data sheet for this model and or message them to ask what is the appropriate amount of force to apply to the face.

in setup with load cells, the force will be applied to two faces: cell end stop A, cell end stop B. four load cells will sense the sum of A and B. by summing the four load cell sensors there will be a kilogram/pound force value. by dividing this summed total force value by the surface area of both end stops, i believe a “force per unit area” measure can be derived
 

curiouscarbon

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this is the part that states 300kgf
screenshot_20200529_232014-png.14253

screenshot_20200530_175100-png.14297


This post goes over the calculations
https://diysolarforum.com/threads/e...ight-or-spaced-for-expansion.7892/post-161742

kgf is the same type of unit as lbf. Which for our purposes is the same as lb and kg.

300kgf is 660lbf. Cells are about 53.5 square inches on one side. So pressure is about 12psi. I can apply 12psi to 1 square inch with my thumb, so its not really a major load. In fact sufficient clamping force to prevent cell movement is probably at least 300lb unless some adhesive or rubber mat is involved.
 

Roswell Bob

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Yes. looks like 12psi is the number at 660 pounds-force.

So pick a spring that puts about 660/4 pounds of force with a reasonable compression would be a good way to go. The force could be calculated based on the spring deflection if load cells are not available. Yes?

Is the required force related to the charging C rate? I intend to use a 70A charge on the EVE 280Ah cells. I'm wondering if I should use less pressure.

thanks for thread to everybody involved - this is really good intel
 

Bob B

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Its not odd for the application. Its a valve spring. The only thing that matters is pressure when the valve is open (compressed) and pressure when the valve is closed (still compressed but not as much as when the valve is open).

Why would they be rated in PSI ..... for our use, we are going to have to figure out how to determine that actual spring pressure in Lbs or KG.
 

noenegdod

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Why would they be rated in PSI ..... for our use, we are going to have to figure out how to determine that actual spring pressure in Lbs or KG.
It is probably not PSI but in fact just straight up lbs. I missed the fact that it was in psi. Valve springs are application specific. The spring is made to go on a specific valve and the dimensions at closed are fixed so that spring on that valve has 125 lbs holding it closed. The open dimension can vary a bit based on the lift of the cam but not a whole lot so the open "pressure" is given for the stock cam. If you know the lift of the stock cam then you can calculate the spring rate and then the open "pressure" for a cam with more or less lift.
 

Bob B

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Maybe @curiouscarbon would consider doing a little testing with his strain gauge ..... Just not sure I would trust that they meant to say lbs of force instead of PSI .... and is their spec really trustworthy if they don't know the difference?
 

noenegdod

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Maybe @curiouscarbon would consider doing a little testing with his strain gauge ..... Just not sure I would trust that they meant to say lbs of force instead of PSI .... and is their spec really trustworthy if they don't know the difference?
It isnt Melling, it is Orilly that hires people that have no idea what the data is that they are entering.

Here is the exact same PN at Summit: https://www.summitracing.com/parts/mel-vs-360

and they rate the closed pressure (seat pressure) at 129 lbs and open pressure at 208 lbs and they give you the heights as well so you can figure out the spring rate for yourself.
 

curiouscarbon

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Maybe @curiouscarbon would consider doing a little testing with his strain gauge ..... Just not sure I would trust that they meant to say lbs of force instead of PSI .... and is their spec really trustworthy if they don't know the difference?
hi!! thank you for your many engineering contributions that keep me on track. given the 280/300Ah cells vary approximately 0.5mm per cell over total State Of Charge curve, monitoring the Cell Wall Displacement seems valuable..

nothing assembled yet. just “spare parts” for now.

the following is mandatory before pack assembly and commission with load cells!

individually calibrate each module of [load cell+amplifier] with “Reference Objects” that have been measured by a “Prevailing Reference” e.g. a kitchen scale sold in the usa or other locale reporting in e.g. oz grams or lbs, etc.

Mandatory Load Cell Calibration Procedure:
  • select Reference Object
  • place Reference Object on consumer scale
  • note grams equivalent value
  • place same Reference Object on LoadCellUnderTest
  • configure amplifier to Standard Observation Parameters
  • note Digital Number reported by ADC
  • remove Reference Object
  • calculate the Scale Factor between ADC Digital Number and Grams Equivalent Value
  • note this value
 

curiouscarbon

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by the way, for such a calibration procedure, if the Reference Object is hung off a threaded rod, it’s good to weigh the threaded rod in grams and subtract that from the equation.

with an S type load cell strain gauge, it’s important for measurement accuracy that the entire assembly is oriented vertically so that gravity’s vector and the axis of the threaded rod are as close to perfectly parallel as practical.
 

curiouscarbon

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during a test of a 1Kg rated load cell, with the same amplifier as to use with the 200Kg amplifier, the data signal resulted in a reliable discrimination of approximately 0.1 grams.

this was determined by comparing a reference object to a prevailing kitchen scale in grams, and by recording 30-60 seconds of samples at 320 samples per second, and taking the mean/average of the digital number reported each time, with that same reference object on the 1Kg load cell

this would linearly scale to a reliable discrimination margin of 20 grams with the 200Kg load cell
 
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Bob B

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during a test of a 1Kg rated load cell, with the same amplifier as to use with the 200Kg amplifier, the data signal resulted in a reliable discrimination of approximately 0.1 grams.

this was determined by comparing s reference object to a prevailing kitchen scale in grams, and by recording 30-60 seconds of samples at 320 samples per second, and taking the mean/average of the digital number reported each time, with that same reference object on the 1Kg load cell

this would linearly scale to a reliable discrimination margin of 20 grams with the 200Kg load cell
Wow .... glad your on this
 

100 Proof

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Those look interesting .... the pressure rating is a little odd though. They rate at 125 PSI closed and 200 open. Did you do any measurements to determine for sure what the pressure was?
Yes. I measured them and came up with approximately 165 lbs/inch. I say approximately because my measuring "apparatus" was not exactly lab quality.
 

100 Proof

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Is 12psi the pressure we are trying to keep?
12psi is the target for the area we are concerned with, but the spec is 6 - 18psi for that area. We're not building furniture here. With a +/- 50% range, my crude measuring apparatus was sufficient for me.
 

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curiouscarbon

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12psi is the target for the area we are concerned with, but the spec is 6 - 18psi for that area. We're not building furniture here. With a +/- 50% range, my crude measuring apparatus was sufficient for me.
OMG amazing characterization assembly!

The Device Under Test acts in effect as a pumb bob!!! That is a Class 1 reference!
 

Critter350

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12psi is the target for the area we are concerned with, but the spec is 6 - 18psi for that area. We're not building furniture here. With a +/- 50% range, my crude measuring apparatus was sufficient for me.
Great setup. Back in the day I’ve measured valve spring pressure using drill press and bathroom scales… Depth stop to measure open height etc.
 
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