This thread title is about EVE LFP cells. I would be happy to discuss NMC chemistry on a new thread.I found an interesting paper involving Lithium Nickel-Manganese-Cobalt (NMC) batteries and force based capacity analysis. The researchers shared their test fixture data with pictures.
Those are already in a hard plastic enclosure. EVE LFP cells only have a thin aluminium enclosure. They're not really comparable.Winston LFP cells
Well, the Winston cells can also bulge, as is said in this topic. I think they do that much less than the EVE LFP cells, but they still do it.Those are already in a hard plastic enclosure. EVE LFP cells only have a thin aluminium enclosure. They're not really comparable.
Compression is for rapid charge/discharge multiple times a day.Compression is not used to prevent bulging - that's a sign of over discharge/charge (electrolyte decomposition). The reason for compression is to prevent the delamination of the electrodes.
Compression is for rapid charge/discharge multiple times a day.
I can agree to not starting that discussion again. ??No, the electrode will change size independent on C rate. It's more pronounced at higher C rates, and obviously will happen more pronounced with more cycles, but it does occur at lower C rates as well.
Whether or not compression is really useful at low C rates, well, I'm not getting into that discussion again and leave that decision up to the reader of this very long thread.
Good idea to keep them strapped, as that's they way they have been for years.I've got a bunch of yellow Winston LFP cells from 2015 (it is almost 40kWh).
They are coming out of an EV. 2 out of the 200 cells are dead, and they are barely bloated.
In the EV, they were there in pairs of 4, with the 4 hard plastic lines/straps around them.
There is another beam that just holds them in place loosely, it did not have any starting compression and it doesn't seem designed to compress them, even if they would bulge a bit.
I am thinking to reuse all of them, just keeping them in the packs of 4 with the plastic straps, and not do anything more in terms of compression.
So in the packs of 4, put them in some kind of wooden or metal boxes that I can find for little money. Cheap wooden boxes that I can close of well enough to avoid humidity. But it might be plastic or metal boxes, whatever is cheaper.
The original battery boxes aren't that convenient to use because of the size.
(and then to decide if I keep the high voltage setup, or to parallel them to something like a 16s system, for cheaper BMS and 48v convenience, but that is another topic)
Currently, they are all in series.
You can't do it that way, you will not reach the 100kgf correctly, you need springs for that.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?
and each individual cell generated a counterforce
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.
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.
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).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.
Your battery pack build could provide the minimum total force of clamping on the cell faces, but it has two issues: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.
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.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.
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 sleevesDon'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.