Is Cell Compression Necessary?

[Boondock Saint]

Ahhh, I see said the blind man. So the two specs are really the same, bingo.

I've read of using an insular material to prevent the possibility of shorting the outer jackets, but not so much a measure of sponge.

My plan was to use thin MDPE film from those black oven sheets you can buy on amazon and cut to just slightly smaller than the length and height and have one between each cell.

"I’ll be using 4/0 as opposed to bus bars." > What about sufficiently rated braided flexible busbars?

 

[curiouscarbon]

electrical insulation
physical reinforcement

both of these are things i’d do if i get 280Ah cells

overkill? prob

use a kitchen sponge? bad idea probs. each person ought to test whatever material by hand and feel personally comfortable with it. i think it’s worth deliberating over.. sponges get hard when squeezed

whether it’s spongy or not i want something with friction across the entire face to discourage lateral sliding of the cells under external vibration. and i want it to be pushed against the entire face as uniform as possible. who’s to say a 1mm sheet won’t result in the edges bowing out when the center bulges?

with fortune 100Ah cells there’s premade gap and the cell wall is strong enough to prevent any bowing in the first place, so i treat that like air ventilation space.

not trying to be rude. i respect whatever decision each person makes, they will be punished by the failure mode not for the failure mode ?

 

[Boondock Saint]

If I'm 1mm smaller on the sheet than the exact outer physical dimension, I don't think it really matters all that much. But I will add that 1mm back to the equation if it really matters.

Having the cells bound in a an .1 micron tolerance exact fitting box made from 2" thick steel and aluminum and titanium and grade 8 bolts and ratchet straps in a barometric chamber maintained at exact pressure and temperature sprayed with bedliner infused with beryllium and kryptonite . . . is that good enough? Jeeesh, I can't make everyone happy. (so much pressure!)

 

[ArthurEld]

lol

 

[curiouscarbon]

i might have ratcheted the straps a bit too tight…. *loosens* ?

 

[deeuubee]

Just a small update:
I emailed the manufacturer of CATL cells, and they said "yes" the cells should be compressed when put in a pack.
I'm waiting for actual compression data. She has to get that from an engineer.

PS- she just emailed back that they're closed until May 5th for the holiday. Waiting it is then.

 

[Gould]

Awesome, please do share when it comes your way.

 

[balachai]

I don't compress my 206ah cells, but I do constrain them ... made a frame to stop the spread.

That's what I'm planning on doing too - a strong battery box with the cells packed so they're not under constant pressure, but have no room to bulge. What do people think about that approach?

 

[lexio]

this is my take on it
3mm isolation sheets
top has a lit and box has a fuse
200a daly bms
testing hell tech 2* 5a balancing ( still not convinced, do hey more harm then good)
included cad drawing of the box
hopefully not to much pressure

 

[Gazoo]

That's what I'm planning on doing too - a strong battery box with the cells packed so they're not under constant pressure, but have no room to bulge. What do people think about that approach?

Personally, I am not getting into the weeds regarding compression. The key is to not apply too much compression as that can do more harm than good. I think some compression is mandatory to prevent stress on the cells terminals if using solid busbars. I decided to convert to braided busbars but have not installed them yet.

My EVE cells expand and contract. It was reported by another person his Lichen cells didn't. Maybe they could over time. Anyways the only data we have regarding compression is from EVE.

I also think it's a good idea to install an insulator between the cells as an added safety measure. I know the cells are wrapped in PVC wrap and the aluminum case may never be exposed but I wouldn't chance it.

 

[balachai]

I think some compression is mandatory to prevent stress on the cells terminals if using solid busbars.

Yeah that makes sense - wouldn't my approach also avoid stress on the cell terminals?

 

[svetz]

This has some links that you might be interested in: https://diysolarforum.com/ewr-carta/compression/

 

[ed1380]

so is the point of compression due to cell internals liking it more or to keep stress off the terminals.

because if it is only the terminals, then you can use longer busbars and have air gap between the cells and as they swell there will be no stress on terminals.

 

[Gazoo]

so is the point of compression due to cell internals liking it more or to keep stress off the terminals.

With the EVE 280ah cells the main reason to compress is to extend cycle life. I am not going to go there using springs and what not. And honestly I don't need to extend the cycle life of my cells because they are being used with a UPS. But I do have them mounted in a fixture and I lightly compressed them at a full SOC. I do not know if EVE's compression specification applies to other cells.

because if it is only the terminals, then you can use longer busbars and have air gap between the cells and as they swell there will be no stress on terminals.

Compression is also to keep the cells from expanding too much. I don't know how severe bloating would be after a period of time with properly used non compressed cells. IMO if the cells are used in a mobile environment then it's best to compress them.

There are many resources and suggestions on the forum. I am going by what I have noticed with my own cells. Even though they will never be in a mobile environment I know it's best to compress them.

 

[Gould]

Not to send this down a rat hole but I agree, some sort of reasonable compression. At what state of charge should you be applying the modest restriction?

 

[RobOfYork]

What has always confused me about this is if something expands, it takes the path of least resistance. So, if you compress the sides and the cells try to expand, it "seems" that the top or bottom of the cells would now be the path of least resistance.

 

[Johncfii]

Not to send this down a rat hole but I agree, some sort of reasonable compression. At what state of charge should you be applying the modest restriction?

About 50% SC is what is recommended by most forum posters. However, I have seen no laboratory data to support this. From what I have seen, there is one "application notes" paper, from one manufacturer, that suggests this, with no supporting data.

The real testing by the competent manufacturing engineers was to put the cells into a hydraulic fixture that was designed to maintain constant pressure, no matter whether the cells were expanded, or contracted, charged or discharged. Springs are an attempt to somewhat mimic this, at an affordable cost. So, we are attempting a reasonable approximation.

It makes most sense to adjust spring pressure to achieve desired compression force at about 50% state of charge. When the cells then expand or contract under spring pressure, the idea is that the springs will keep the clamping pressure within a much tighter range than if the cells were clamped with rigid rods. But the springs need to be carefully selected to have a compression force that is close to linear within the range of expected movement.

 

[Johncfii]

What has always confused me about this is if something expands, it takes the path of least resistance. So, if you compress the sides and the cells try to expand, it "seems" that the top or bottom of the cells would now be the path of least resistance.

There will still be internal pressure, but the sides are tops are much more constrained by the shape of the cells and their smaller surface area. Also, it is separation or delamination of the internal cell insulating layers, or damage to those layers, that is generally thought to be the mode of degradation of cells related to cell expansion. Those layers are parallel to the large cell face. So, it makes sense that this is the surface that it is most important to limit movement of.

 

[Johncfii]

Sooo... since I'm going to have a relatively low charge and discharge rating in relation to the capacity, do I really need to go to the trouble of binding and compressing the cells? I'll be removing the AGM batteries from the rack and the entire block of cells will fit on the top shelf.

If I understand your application, of course you do NOT need to go to the trouble of cell compression. You describe keeping those cells normally charged at 95% capacity, with them discharge cycling only a couple of times a year. I recall that Lishen, for one, claims 3000 charge/discharge cycles WITHOUT cell compression (and then still retaining 80% of original capacity at 3000:cycles). It sure sounds like you wouldn‘t get much benefit from cell compression.

If I were you, I would still clamp them lightly with rigid tie rods, or something simple like Kapton tape, but not fuss with springs. The cost of some simple rigid constraint is slight.

But I would be more comfortable with your plan if your power supply had SOME reserve capacity above your normal 15-25 amp load. I would want a 30-amp power supply. Why call upon the batteries for any more than momentary starting surge if that is avoidable at nominal cost? Of course, this depends on how much of the time the system is calling for the upper limits of that 15 to 25 amp load, which I can't tell.

Most of us here think in terms of applications that are much different than yours.

 

[Johncfii]

I know I am confused. Compress cells or not. Foam between cells or not. Hmmm...

LOL ... That’s the nature of any DIY community .... lots of different ideas about THE best way ... Different applications, different budgets, different priorities, different critical thinking skills. Read more posts and perhaps get more confused.

But there ARE a lot of knowledgeable folks around here.