Compression analysis for various configurations

[SolaroSsaurius]

> we're looking to keep the cells from expanding
That may be you goal, but not what is specified in the EVE spec sheet for the tests - that can be found in this forum (sorry, I'm in the phone right now so can't easily make the search).

 

[Tulex]

> we're looking to keep the cells from expanding
That may be you goal, but not what is specified in the EVE spec sheet for the tests - that can be found in this forum (sorry, I'm in the phone right now so can't easily make the search).

I think that's a terminology issue. I believe the goal is to restrict expansion under certain conditions. As in, they need to be able to expand a certain amount, but not in an uncontrolled manner.

 

[SolaroSsaurius]

I think that's a terminology issue. I believe the goal is to restrict expansion under certain conditions. As in, they need to be able to expand a certain amount, but not in an uncontrolled manner.

So why would they start at 30-40% with a given compression? Why bother with a given compression apparatus? (they have the schematics there)
I would believe that they make testing under optimal conditions so they can brag about the product and it's cycle / capacity results...

 

[Warpspeed]

As Tulex says, there is the Chinese/English translation problem to take into account.
Not saying that is the issue here, just something to consider.
Taken literally, some Chinese instructions on how to do things can be pretty surreal.

Sometimes common sense transcends Chinese instructions.

 

[Warpspeed]

I have read somewhere that we can expect better longevity (the difference between 4K and 6k cycles) by more optimal clamping arrangements.

I have read that too, and it puzzled me.
If the cells are tightly constrained, and the internal gas pressure goes up and down, and nothing is really lost, the gas just condenses back into liquid electrolyte in normal operation.

Now if a physically unconstrained cell has permanently bloated, what has changed that reduces the life cycles by so much ?
Still nothing is lost, and chemically nothing should be any different to a skinny clamped cell.

Then the thought occurred to me. A swollen cell will have an increased internal volume, and the level of the liquid electrolyte will be lowered.
That may be the reason for the reduction in life cycles. Its the only thing that I can see that has changed compared to a non swollen cell.

It may even be possible to force slightly swollen cells back into their original shape by clamping. If a cell case can be permanently deformed in the outward direction, it may very well be deformed back to the original shape without any subsequent damage or reduction of life cycles.

This is all just a hypothesis, but its an interesting idea, and the only reason I can see why unclamped cells may have a shorter life.
Over time, normal cycling gas pressure may tend to stretch the cell casing, which leads to a lower electrolyte level shortening its life.

So it may all come back to preventing natural long term swelling. The method is probably less important than achieving the desired result of maintaining the electrolyte level at its original design height, and that might be a lot more critical than we realise.

I have also read that installing cells in orientations other than straight up vertical is not recommended, as some of the pouches, or parts of a pouch can become high and dry, reducing both capacity and lifetime.

 

[HRTKD]

Then the thought occurred to me. A swollen cell will have an increased internal volume, and the level of the liquid electrolyte will be lowered.

That's exactly what I was thinking.

There is a post in the "Up in smoke" subforum where someone tried to compress bloated cells and it didn't end well. The cells became bloated by a certain level of (unintentional) abuse. However, the end result could have been due to other circumstances.

 

[Warpspeed]

There is probably a very big difference between a slow natural spreading over time, and massive sudden bloating due to one time serious abuse.

In the case of massive abuse, other changes may have permanently and irreparably destroyed the cell chemistry, and the cell is DEAD.

Gradual natural spreading over time, may possibly be corrected and even reversed with a bit of creative engineering.

 

[shinning]

I have read that too, and it puzzled me.
If the cells are tightly constrained, and the internal gas pressure goes up and down, and nothing is really lost, the gas just condenses back into liquid electrolyte in normal operation.

Now if a physically unconstrained cell has permanently bloated, what has changed that reduces the life cycles by so much ?
Still nothing is lost, and chemically nothing should be any different to a skinny clamped cell.

Then the thought occurred to me. A swollen cell will have an increased internal volume, and the level of the liquid electrolyte will be lowered.
That may be the reason for the reduction in life cycles. Its the only thing that I can see that has changed compared to a non swollen cell.

It may even be possible to force slightly swollen cells back into their original shape by clamping. If a cell case can be permanently deformed in the outward direction, it may very well be deformed back to the original shape without any subsequent damage or reduction of life cycles.

This is all just a hypothesis, but its an interesting idea, and the only reason I can see why unclamped cells may have a shorter life.
Over time, normal cycling gas pressure may tend to stretch the cell casing, which leads to a lower electrolyte level shortening its life.

So it may all come back to preventing natural long term swelling. The method is probably less important than achieving the desired result of maintaining the electrolyte level at its original design height, and that might be a lot more critical than we realise.

I have also read that installing cells in orientations other than straight up vertical is not recommended, as some of the pouches, or parts of a pouch can become high and dry, reducing both capacity and lifetime.

I thought it has to do with the cathode deformation/delamination.
I'm thinking if you receive some slightly bloated cells, compressing them with 300kgf (12psi) would be actually dangerous as the slightly bloated surface is like half or less than the cell side surface and as a result you could easily and unknowingly double that compression force. Did I miss anyone say anything about this thing?

 

[Warpspeed]

I thought it has to do with the cathode deformation/delamination.
I'm thinking if you receive some slightly bloated cells, compressing them with 300kgf (12psi) would be actually dangerous as the slightly bloated surface is like half or less than the cell side surface and as a result you could easily and unknowingly double that compression force. Did I miss anyone say anything about this thing?

You could be right, I really do not know.

But I have had success clamping bloated cells back to the original shape, which they then maintained.
Those cells are still working perfectly well years later.

 

[AlexBen]

I will try and delete this. Admin can do so for me.

 

[RCinFLA]

Just to disrupt your sleep at night:

What separates the positive and negative sides of an LFP cell is about 12-20 um thick layer of perforated plastic polyethylene sheet called a separator.

Apply too much compression pressure, crushing through the separator and exciting things happen.

 

[AlexBen]

Admin can delete. Sorry for WOB

 

[Checkthisout]

Doesn't preventing them from bloating keep the layers inside from separating?

If that's the case, what's the benefit to allowing expansion via springs?

Are we charging at rates high enough for it to matter?

I mean these cells are designed to capture energy in a hurry when the brake pedal is smashed right? In that case, maybe it's more likely for gassing to occur?

I did 3 rods with this reinforcement bar laid across 3/16 steel plate. Seems to apply even pressure. Have spacer material is some kind of 1/8 thick rubber.

It's ok so far but pack was just put into service a few months ago.

 

[Warpspeed]

It seems to me, that the battery manufacturer could build cells with a super thick cast aluminium battery casing to resist the sometimes very high internal gas pressures. That would make cells much larger, heavier, and a lot more expensive. Not really what any of us want.

So its left to the end user to provide the pressure resisting outer structure, which is going to be a lot more efficient.
What we don't want to do is crush new cells, which will very likely cause internal injury.
Apart from looking ugly, bloated cells lower the level of the internal electrolyte, which is the real problem we are trying to avoid.

Provided you react fairly quickly to a slight "bloating event" and press the cell back into its original shape fairly promptly, the cell will very likely not be permanently damaged. If its been badly bloated for a long time, chances are the electrolyte level has remained too low and the tops of the pouches completely dry out, and that cannot be reversed.

The picture above ^^^ is an excellent example of how to do it.
The sturdy sleeves along the length of the battery are cut to precise length so there can be no crushing, but those huge bolts will easily withstand massive expansive internal cell pressure.

 

[Skypower]

I run a centralized die spring on an 8 cell row (2 rows for 16S). The spring’s free length is 1-1/2”. With a deflection of .56” it has 635 lbs of force. I’m only compressing it only 1/2” which brings it into the 550/600 range. Why? This leaves a bit of room for cell expansion under high state of charge plus higher temperatures. I noted a total overall difference of length of 3/16” for the group of eight between the two conditions. This means that the cells are probably a tad over the ideal 660 lbs for short intervals of extreme conditions. But this begs the question, in an extreme condition, what kind of pressure are the cells exposed to in a lightly snugged up rigid fixture when the cell are known to expand? I haven’t got a clue.

 

[Warpspeed]

You don't want to allow cell expansion under ANY circumstances.
Electrolyte level is critical, and the manufacturer set the level to be very precise.
There needs to be some expansion space above the electrolyte for internal gas, but it must not grow in size to force the electrolyte level lower.

Allowing the cells to "breathe" in and out is almost certain to work harden and crack the thin aluminium case eventually.

 

[Lt.Dan]

Just look at the EV packs and see they are fixed with no chance of expansion.

Reason enough for me.

 

[RCinFLA]

At full SoC the graphite expansion will cause about 1-2 mm thickness expansion of a 280 AH cell. Doesn't sound significant until you consider the total summation of 16 cells arranged in a single row being over half an inch end to end.

 

[Skypower]

You don't want to allow cell expansion under ANY circumstances.
Electrolyte level is critical, and the manufacturer set the level to be very precise.
There needs to be some expansion space above the electrolyte for internal gas, but it must not grow in size to force the electrolyte level lower.

Allowing the cells to "breathe" in and out is almost certain to work harden and crack the thin aluminium case eventually.

You have a point. Like bending a paper clip enough times till it breaks. But there could be levels to this thought. How many cycles will it take? Of course nothing like the millions of a leaf spring or torsion bar. How gradual are the bend points or span of the flex. And considering the amount of observed full cycle flex.023” per cell or about .0115” on a side probably much less than a free standing cell would experience. So is the correct procedure to compress to a given value at particular state of charge and temperature and lock down the position of the end plates eliminating the free travel there on? Am I over thinking it?

 

[Warpspeed]

You are quite right, the metal fatigue threshold is very different for various metals, and there is a threshold point below which its not a problem.
There is a reason why springs are never made from aluminium.

The racing car guys would LOVE to run aluminium springs, but the fatigue life would be just about zero.
Aluminium works fine for stiff structures, such a wheels, pistons, engine blocks, flywheels and suspension arms, but NEVER the springs or torsion bars which are designed to continually flex.