diy solar

diy solar

EVE-280 cells should these be clamped tight or spaced for expansion?

I am going to use 4mm rubber sheets between the 280Ah cells and four slightly zig-zag crooked 4mm rods with threads at the end to compress them. The M4 nuts (not oiled) must be tighten by 0,77Nm to reach 100Kg per bolt.
Did somebody have any objections, or should it be ok like this?
 
I am going to use 4mm rubber sheets between the 280Ah cells and four slightly zig-zag crooked 4mm rods with threads at the end to compress them. The M4 nuts (not oiled) must be tighten by 0,77Nm to reach 100Kg per bolt.
Did somebody have any objections, or should it be ok like this?
You can't do it that way, you will not reach the 100kgf correctly, you need springs for that.

Believe me, I tried to get the calculation right (and it was not a problem) but then the physics's and engineering guys started hassling with me....
When you start the charge it will expand, and you are outside the clamping force.


But, it's better then nothing?


Not really. If you cycle them FULLY -more then- once a day, then clamp it.
If you need to fix them in a moving car, clamp it.


If not, pace them cause heat is the main killer.


Just my bucket of words..
Mine are in my car, clamped with springs.
 

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I am going to use 4mm rubber sheets between the 280Ah cells and four slightly zig-zag crooked 4mm rods with threads at the end to compress them. The M4 nuts (not oiled) must be tighten by 0,77Nm to reach 100Kg per bolt.
Did somebody have any objections, or should it be ok like this?

My fixture is using yellow pine wood 1 inch x 10 inch cut to length and four 1/4 inch course threaded rods with 1/4 inch nuts & washers on the 8s Lifepo4 battery banks.

This is Info I used and torqued a little less at 5 Inch Pounds with a torque wrench at about 3.2 to 3.3 volts charge in each cell. The cells are essentially just held in place.

The spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
Using 4 course 1/4 in threaded rods that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic.

Ran across this EVE LF280K spec sheet with diagrams dated April 2022. The Testing Cell Clamp diagrams are shown. It is interesting that it shows 10mm metal plates 6 M6 bolts on a single cell.
 

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Don't the forces of the individual cells add up when they are clamped together? Only one cell is ever considered in the specifications. If they were clamped with 300 kgf and each individual cell generated a counterforce, the counterforce of the clamping device would have to be n * 300 kgf, wouldn't it?
But that would destroy the cells. On the other hand, several cells together could exert a much higher force than the 300 kgf intended for one cell.
 
and each individual cell generated a counterforce

Keep in mind that the fixture is not there to counteract swelling. Swelling is abnormal behavior due to overcharging. The fixture is only there to prevent electrode expansion/delamination and prevent Lithium intercalation. In practice, we're talking very little if any counterforce by the cell.

Check out this paper in Nature and search for 'compression':

 
Don't the forces of the individual cells add up when they are clamped together? Only one cell is ever considered in the specifications. If they were clamped with 300 kgf and each individual cell generated a counterforce, the counterforce of the clamping device would have to be n * 300 kgf, wouldn't it?
But that would destroy the cells. On the other hand, several cells together could exert a much higher force than the 300 kgf intended for one cell.

No. There's a long discussion on this somewhere else (perhaps even this thread) about it. It can be a tricky concept to grasp, but that's not how physics works.
 
Don't the forces of the individual cells add up when they are clamped together? Only one cell is ever considered in the specifications. If they were clamped with 300 kgf and each individual cell generated a counterforce, the counterforce of the clamping device would have to be n * 300 kgf, wouldn't it?
But that would destroy the cells. On the other hand, several cells together could exert a much higher force than the 300 kgf intended for one cell.
No. There's a long discussion on this somewhere else (perhaps even this thread) about it. It can be a tricky concept to grasp, but that's not how physics works.
I will try to explain kin simple terms. If we consider a row of cells clamped together, large-face to large face (typically 4 8, or 16 cells, with a non-compressing additional insulator 'dividers' added between each one), the cells are all in a relatively fixed position, with only small amounts of expansion changing the total "length" of the structure. (That "length" is actually the sum of thickness for"n" individual cells, plus "n+1" dividers).

You are remembering that a force, if not balanced by a "counter force", will (in Newtonian theory, not modern theory) will cause the target to be accelerated, the amount of acceleration determined by the famous formula "f=ma".

But in this row of cells, after a tiny bit of compression has occurred, the force at the large face of each individual cell is not causing acceleration (increased movement over time, within the same frame of reference). The force is instead felt through the thickness of the cell, and opposed (or "countered" by that same amount of force being applied FROM the "back" face of this cell, through the divider, and into the "leading" face of the next cell. This transmission of force continues through the entire row, until the last cell presses its "back" face into the opposite side of the containing structure - again with equal force as the first cell "felt", while the cells are not being accelerated or collapsing/expanding in response to pressure changes.
- - -
Within a spring-loaded "single row" containment structure of the type discussed here, each cell feels the entire force compressive force being applied at the "face" of the first cell, ultimately countered by the "back" of the last cell pressing against the opposite end of the containment.

The "V3" specification sheet for EVE LF280K cells recommends that clamping be applied at a rate between 3000 Newtons and 7000 Newtons. The specification also indicates that they will collapse if more than 10,000 Newtons occurs. The very high maximum "recommended" clamping, a value of about 28 PSI, might allow for the design of non-sprung "fixed" containments (using the provided expansion rates in calculations). But the total expansion within a large "fixed" containment, e.g. 16 cells per row, would probably exceed the new and higher maximum limit. I haven't done those calculations, I continue to use spring-loaded clamping devices.
 
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My fixture is using yellow pine wood 1 inch x 10 inch cut to length and four 1/4 inch course threaded rods with 1/4 inch nuts & washers on the 8s Lifepo4 battery banks.

This is Info I used and torqued a little less at 5 Inch Pounds with a torque wrench at about 3.2 to 3.3 volts charge in each cell. The cells are essentially just held in place.

The spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
Using 4 course 1/4 in threaded rods that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic.

Ran across this EVE LF280K spec sheet with diagrams dated April 2022. The Testing Cell Clamp diagrams are shown. It is interesting that it shows 10mm metal plates 6 M6 bolts on a single cell.
Your battery pack build could provide the minimum total force of clamping on the cell faces, but it has two issues:

1. Your assumption that 8 inch-pounds at the nut will correspond to 165 lbs of force with each rod. In order to reach that desired rod tensile force value (165 lbs), the nut must be pushed a lot harder - because the thread surfaces of the nut and rod must be scraped against each other. The threads are badly formed, and their faces are not smooth.

2. Your Yellow Pine boards will bend a lot. You did notice that EVE's own test clamp used 10mm steel plates. Their diagrams also show some additional structure spreading the force from the pressure-applying device from a small area (the piston of the press) into a larger area of the plate.
In battery packs with smaller cells requring less total force, I have used 1/4" steel plates - but these plates did bend when I compressed the packs, (by noticeable amounts). At the highly-tensioned rods, outside from the cells, they bend inwards. In the middle of the cell bodies, they are correspondingly "popped out".

The pressure in my cells has been much higher than 12 PSI near the outside edges of the cell faces, and quite a bit lower near the middles. In my next pack, using EVE LF280K V3 cells, I am still using 1/4/" steel plates. But between horizontal pairs of bolts, I am adding 1/4" thick segments of 1" L-bar. The L-bar segments will help to hold the plates flat between their rod pairs, providing higher forces into the initial cell faces near the middle of the L-bar segments.
- - -
In your situation, if I still wanted to avoid the expense of springs, I would either upgrade from the yellow-pine end plates, or I would increase the force being applied along each rod. The newest EVE specificaion describes a "recommended" clamping force range up to about 28 PSI maximum. In order to get anywhere near 12 PSI across the 'Middle" of the faces of your cells, I'd be willing to push the force near the 'Edges' (closer to the the tension bars) quite a bit higher.

Anyway, here's the newer 'V3' specification document.
 

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Your battery pack build could provide the minimum total force of clamping on the cell faces, but it has two issues:

1. Your assumption that 8 inch-pounds at the nut will correspond to 165 lbs of force with each rod. In order to reach that desired rod tensile force value (165 lbs), the nut must be pushed a lot harder - because the thread surfaces of the nut and rod must be scraped against each other. The threads are badly formed, and their faces are not smooth.

2. Your Yellow Pine boards will bend a lot. You did notice that EVE's own test clamp used 10mm steel plates. Their diagrams also show some additional structure spreading the force from the pressure-applying device from a small area (the piston of the press) into a larger area of the plate.
In battery packs with smaller cells requring less total force, I have used 1/4" steel plates - but these plates did bend when I compressed the packs, (by noticeable amounts). At the highly-tensioned rods, outside from the cells, they bend inwards. In the middle of the cell bodies, they are correspondingly "popped out".

The pressure in my cells has been much higher than 12 PSI near the outside edges of the cell faces, and quite a bit lower near the middles. In my next pack, using EVE LF280K V3 cells, I am still using 1/4/" steel plates. But between horizontal pairs of bolts, I am adding 1/4" thick segments of 1" L-bar. The L-bar segments will help to hold the plates flat between their rod pairs, providing higher forces into the initial cell faces near the middle of the L-bar segments.
- - -
In your situation, if I still wanted to avoid the expense of springs, I would either upgrade from the yellow-pine end plates, or I would increase the force being applied along each rod. The newest EVE specificaion describes a "recommended" clamping force range up to about 28 PSI maximum. In order to get anywhere near 12 PSI across the 'Middle" of the faces of your cells, I'd be willing to push the force near the 'Edges' (closer to the the tension bars) quite a bit higher.

Anyway, here's the newer 'V3' specification document.

Agreed that the two 8s 271ah CATl wood battery boxes built are not optimal, but they do serve the purpose of holding the cells in place and best can tell are minimizing any loading on the cell terminals. The cells have been in this configuration for about 2 years with many cycles without any problems. Only maintenance is check the torque on the bus bar nuts with a torque wrench and scan with a FLIR camera for hot spots.

So far so good.

I am in the initial stages of updating the solar system and will look at some of the items pointed out when building new battery boxes. I am leaning toward steel cases.
 
My fixture is using yellow pine wood 1 inch x 10 inch cut to length and four 1/4 inch course threaded rods with 1/4 inch nuts & washers on the 8s Lifepo4 battery banks.

This is Info I used and torqued a little less at 5 Inch Pounds with a torque wrench at about 3.2 to 3.3 volts charge in each cell. The cells are essentially just held in place.

The spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
Using 4 course 1/4 in threaded rods that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic.

Ran across this EVE LF280K spec sheet with diagrams dated April 2022. The Testing Cell Clamp diagrams are shown. It is interesting that it shows 10mm metal plates 6 M6 bolts on a single cell.
Trying to achieve proper force with just a threaded connection is very complex, which means unless you have serious math ninja, you are really just guessing. Springs are so simple and they don't have to be expensive. I used these. I measured their K values to be ~165#/inch. less than $5 each. Easy to get.

https://www.oreillyauto.com/detail/c/melling/melling-valve-spring/mel0/vs360?q=Melling+VS-360&pos=0
 
Don't the forces of the individual cells add up when they are clamped together? Only one cell is ever considered in the specifications. If they were clamped with 300 kgf and each individual cell generated a counterforce, the counterforce of the clamping device would have to be n * 300 kgf, wouldn't it?
But that would destroy the cells. On the other hand, several cells together could exert a much higher force than the 300 kgf intended for one cell.
For solar power storage, Long story short,,if your going to compress, I would simply snug them up tight so the cells can’t be pulled out of the fixture. Nothing more. I used springs but are probably not needed. My cells do not expand at all (not even a 1/16) thru entire SOC. Good cells should not swell unless abused or used in very high demand applications. Good cells barely expand. I like compressing these cells to possibly prevent any delamination. I used isolators between each cell and 1/4” steel plates at the ends. Threaded rods with sleeves
I started a very very long thread about this. A lot of time wasted reading researching, then to see my cells don’t expand at all ??
 
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