diy solar

diy solar

Compress or not, flexible busbar or not

7075 has the highest tensile strength. I'll go with 6mm plates. If the cells manage to bend it then it is probably time for new cells. Or at least this is the impression I'm getting after going over the full thread.
7075 is the strongest, stiffest and machines very nice. It can’t be bent, if attempted it will fracture or be very compromised. Welding shouldn’t be attempted because it’s very likely to crack. It also has the highest corrosion potential because of the high zinc content which is also the thing that makes it weld terrible. I use it as a stiffener inside the bottom of plastic battery boxes with a few layers of Kapton tape so cells don’t sag and shift. Stiffer and cheaper than carbon fiber for dimensions, but heavier. On average I convert 30 pounds of the stuff into chips a day making parts from billet. I love how it machines, just sprays away, never gums up the tool.
 
Eve specifies 3/8" (9mm) for end plates. That said, I also used 1/4" (6mm).
The specification is for 10mm:

3.3. Testing Clamp Preparation
The single cell needs to be clamped with steel splints or aluminum alloy splints (thickness: 10 mm). The splints
need to cover the large surface of the cell. The splints are fixed with 6 M6 bolts. All sides of the splints need to be
covered with insulating film
 
And nobody complains from 3/4" plywood. But I suspect that it may sag with time.

I'll check on Monday the prices for 7075 sheets and if they are sane - I'll go with it. If not - with plywood. The reason why I'm expecting high a price is that I'll likely have to buy a 150cm x 21cm piece and the prices have gone crazy around since the war in Ukraine started.
 
And nobody complains from 3/4" plywood. But I suspect that it may sag with time.

Yes 3/4” Sande or Radiata pine plywood will bend in less than a month, however laminating an additional layer of 1/2” is enough to keep the bulge/flex less than .015”. Use Radiata pine from Loews because the veneer is thicker than the Sande pine.
 
I'm in east Europe. What I can get here is birch plywood. If I'm reading it correctly it is stronger than the radiata pine one.
 
I'm in east Europe. What I can get here is birch plywood. If I'm reading it correctly it is stronger than the radiata pine one.
Yes it’s very strong. Solid birch is what many modeler make their main wing spars from. It’s not cheap here. What the different internal plies are made from and quality of adhesives is always a concern of what ever you buy anywhere. Some years ago I bought marine grade plywood and I had drops left out in the rain over the weekend. When I was cleaning up, I noticed the glue had delaminated! Talking to other builders I found out that in order to make it more environmentally friendly they had gone to another adhesive and hadn’t tested it enough. Glad it wasn’t for a boat hull.
 
I'm in east Europe. What I can get here is birch plywood. If I'm reading it correctly it is stronger than the radiata pine one.
In East Europe alu prices are now insane!!
And I just found one local seller selling 10mm sheets... 1500x2000mm !!
Where are you located?
How are prices around your place?
 
In the UK I bought 200mm x 10mm Aluminium Flat Bar - 800mm length, and it was £48. Prices here have gotten ridiculous, my build has cost far more than I thought it would.
 
For those in Europe, I noticed this catalog:

pg. 54, spring # 61162
For those wanting to use M6 and 6 rods on a single LF280k row:
Size 51mm
Rate 128/mm
Compressed at 5,1mm -> 653N
Compressed at 6,1mm -> 783N

Seems fine to my inexperienced eye.
It will make
- 490N compressed at 3,8mm (around 12psi)
- 694N compressed at 5,4mm (around 17psi)

I'm considering it for an 8 cells row, though still searching for more options.

Opinions?

PS: Even so, I'm still a bit reluctant in using springs, given the datasheet does not show the use of any in their testing procedures!!! Just fixed rods compressing to 300kgf at 30% - 40% SOC
 
Where are you located?
How are prices around your place?
Bulgaria. They were insane in May. I'll get my quotes on Monday for the ALU 7075 plates. I may go for steel if they are too high or just go with the plywood.

For the springs - I built a 12V/105Ah in May this year. Back then I did research on where can I get springs from. I ended up with a cheap construction store selling many springs. No load ratings on them, but I got a few that were looking good and tests confirmed that they are good to go. Outer diameter is 15mm, inner is 10mm. The cheap ones are less than a dollar, and the expensive ones (certified, with load ratings) are 8-15 dollars (depending on what you choose).
 
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For those in Europe, I noticed this catalog:

pg. 54, spring # 61162
For those wanting to use M6 and 6 rods on a single LF280k row:
Size 51mm
Rate 128/mm
Compressed at 5,1mm -> 653N
Compressed at 6,1mm -> 783N

Seems fine to my inexperienced eye.
It will make
- 490N compressed at 3,8mm (around 12psi)
- 694N compressed at 5,4mm (around 17psi)

I'm considering it for an 8 cells row, though still searching for more options.

Opinions?

PS: Even so, I'm still a bit reluctant in using springs, given the datasheet does not show the use of any in their testing procedures!!! Just fixed rods compressing to 300kgf at 30% - 40% SOC

Not using springs in compression fixture. Just 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 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.
 
Bonjour @oliagri
Merci pour ce tableau.

je n'ai pas bien compris :
a) quelle est l'unité de mesure à l'intérieur des tableaux ? kgf ? N ?
b) Je ne comprends pas comment un ressort plus gros peut produire moins de force
c) quelle est la compression à laquelle il atteindra la force maximale spécifiée

Avez-vous plus d'infos sur ceux-ci ?

Merci d'avance
Bonjour,
a : l'unité utilisée est le kgf (comme indiqué en bas du tableau)
b : à longueur égale un gros ressort a toujours plus de force qu'un petit (plus un ressort est long moins il faut de force pour la pastille d'un mm)
c : les efforts indiqués correspondent à une compression d'un mm. la compression n'est pas complètement linéaire. Je n'ai pas d'informations supplémentaires mais après avoir testé la force augmente légèrement au fil de la compression
 

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This has got to be the most inane thread I have ever witnessed on this Forum.

Apologies to any poor newbies that have wandered into this morass this deep, but I’m out…
I'm still going to read it to the end, I've come so far.
 
I have watched all Andy's videos, but I wasn't convinced no compression was the correct way to go, so I compressed with foam and used flexible busbars.
 
I have watched all Andy's videos, but I wasn't convinced no compression was the correct way to go, so I compressed with foam and used flexible busbars.
Same.

I compressed with plastic sheets and made up flexible busbars from lugs and cable.
 
Compression will not stop bloating. Only causes bulge on sides and/or increased cell pressure. Popping the vent port is worst result.

Root cause is electrolyte break down. Overvoltage, high cell current, or high temp is most common cause of electrolyte breakdown. Less than 0.1v cell voltage will also cause bloating but that is not so common.

In Andy's case I would suspect high temp or high discharge current. The different age and condition of three banks can result in over current discharge on one of the strings. Needs to pay more attention to total current distribution between the three strings. Getting above 0.5 C(A) on a given string is a red flag.
 
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Compression will not stop bloating. Only causes bulge on sides and/or increased cell pressure. Popping the vent port is worst result.

Root cause is electrolyte break down. Overvoltage, high cell current, or high temp is cause of electrolyte breakdown.
I hope Andy gets it sorted out for himself
 
Back on page 1 of this thread, Andy from the Off Grid Garage YouTube channel was referenced as saying why he would not compress. I wonder if Andy is having 2nd thoughts?

I'm not saying, compression right or wrong. I don't know anything.

It was just interesting that a video at the top of this thread was referenced, and the same video creator came back a year + later with findings
 
Compression is good if you subject cell to high current. The object of compression is to minimized delamination of electrode material from their graphite negative electrode copper current colllector foil or LFP electrode to aluminum current collector foil.

Below 0.5 C(A) cell current, compression yields little benefit.

When electrolyte decomposes it causes dried out hydrocarbon tar gummy areas that increases stress on separator sticking to electrode causing pulling on electrode causing delamination. In the picture if there is fluid electrolyte permeating porous separator there would be no reason for graphite to stick to separator. This cell has been subjected to overcharging, overcurrent, or over temperature that resulted in electrolyte decomposing to dry, sticky, hydrocarbon tars that stuck graphite electrode to separator.

The electrolyte decompostion gas, mostly CO2 and CO, produces gas that bloat the metal container. It is not the most damaging affect, it is the other hydrocarbon tars created and left behind in the laminate.

Delamination of graphite from copper 2.png
 
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I think Andy’s cells will be fine as long as he gives them space to get the tension off of the terminals like he did on the top shelf. From what I’ve read, compression of swollen cells is a bad idea. His cells are mildly swollen and probably not an issue. I have and still will continue to compress with a centralized spring of a known spring rate. Time will be the true tell.
 
I think Andy’s cells will be fine as long as he gives them space to get the tension off of the terminals like he did on the top shelf. From what I’ve read, compression of swollen cells is a bad idea. His cells are mildly swollen and probably not an issue. I have and still will continue to compress with a centralized spring of a known spring rate. Time will be the true tell.
I agree, the bloating is not that bad.
 
Compression is good if you subject cell to high current. The object of compression is to minimized delamination of electrode material from their graphite negative electrode copper current colllector foil or LFP electrode to aluminum current collector foil.
This.. i compressed because mistakes happen. Bms malfunctions and because manufacturers said the cells will have more cycles so why not, it’s easy to compress them.
 
Compression will not stop bloating. Only causes bulge on sides and/or increased cell pressure
Compression is good if you subject cell to high current. The object of compression is to minimized delamination
This is kinda conflicting but I get what you mean. However since compression can minimize delamination, doesn’t compression do that by stopping expansion? bloating and expansion being different
 
My objection to compression is many DIY'er do a non compliant fixed dimensional compression that results in an exponential increase in pressure at full charge or higher ambient temperatures.

You can crack electrodes or crush separator causing a cell short if too much compression is applied. The spec list 300 kgf across cell surface which is 29 lbs per square inch which is high risk if not carefully controlled. The separator, preventing a cell short, is only a very thin, porous polypropylene plastic sheet.

If you want cell longevity don't run them above 0.5 C(A). Above 0.5 C(A) the cell internal temp cycling increases due to internal cell heating that stresses electrode lamination. At 0.5 C(A) maximum current there is low internal cell heating and cell compression has little benefit.

You have to weigh the risk versus benefit.
 
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My objection to compression is many DIY'er do a non compliant fixed dimensional compression that results in an exponential increase in pressure at full charge or higher ambient temperatures.
Do you differentiate between true cell compression and what would be more of a fixture frame that presses the cells together for mobile and RV applications. Or would those b essentially 1 in the same from a design, concept and execution point of view.
 
Do you differentiate between true cell compression and what would be more of a fixture frame that presses the cells together for mobile and RV applications. Or would those b essentially 1 in the same from a design, concept and execution point of view.
You need some sponge or spring compliance to keep pressure uniform over some dimensional expansion and contraction.

A few threaded metal rods to end plates is a hard fixed dimensional compression that can exponentially increase compression pressure when cells expand.

The surface of cells is not uniformly flat so a hard flat surface can create high pressure points. A compliant rubber pad between cells helps to even irregularities in cell surface.
 
My objection to compression is many DIY'er do a non compliant fixed dimensional compression that results in an exponential increase in pressure at full charge or higher ambient temperatures.
Exactly why I decided to use springs
 
The surface of cells is not uniformly flat so a hard flat surface can create high pressure points. A compliant rubber pad between cells helps to even irregularities in cell surface
This was my only concern with they way I did it. I have 2 rows of 8 cells , with 1mm PCB sheets between each cell. At either end of a row is 1/4 Poron foam, which is compressed using 10mm aluminium plates and studs/bolted tubes. I would have preferred foam between each cell but just couldn't buy the correct foam.

My second battery should be online at the end of the month, even with one battery I don't go near 0.5c even though I could. The most I've seen flowing in is around 100A with 280ah cells. Once the second battery is in, I'll charge at 140A, so 0.25C for each battery, solar charging will be less.
 
My biggest concern was when one cell was nearly flat and the next had a slight bulge(less than 1/16) and could violate the flat plane of the other. All the batteries I’ve seen have two vertical bulges where the “jelly rolls” reside so there will be two pressure points regardless. I fitted Formica sheets between each cell to help mitigate protrusion from another and provide insulation. I tried to find the fiberglass sheets like you see in the kits and they were too expensive. The ends are doubled up glued 3/4 ply. Even with 500lbs each row of eight have moved the pusher plate 3/16 and move very little through the cycle now. Basically, they have moved what they’re going to after 16 months.
I’d love to see what the close fitting rack cells look like after a year with welded busses and nowhere near an expansion force strength structure. Do really want to even look? It may give you an idea what the cells can actually take as far distortion and stress.
 
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