Lifepo4 cell compression, for a stationary AND low capacity set up, useful or useless ?

[zorlig]

There is calendar aging data that one seen for really hot cells (40c+). But it's all guesswork as far as I know.

Until someone measures cells that have been sitting on a conditioned shelf after a decade we won't really know.

People should keep their cells indoors and conditioned, but that's just obvious.

 

[Daddy Tanuki]

The expanding cell pushes against the cells around it. The cells around it push back. The first level of pushback is the compression of the cells. All else being equal, they all see the force (300 psi?). However the cell in the middle sees the 300 psi, plus whatever force it takes to move the cell next to it to equalize the pressure. It has to overcome the friction of the cell sitting on the case to budge it. For a heavy battery, that is not trivial. Way more than 300psi can build up in the middle.

We are at a logerheads here then, its not the friction that causes the issue, it is the cell expanding.

 

[Skypower]

If the cells were loose with a flexible buss bar and in a mobile application there could be friction on the large faces of the cells….right up to failure.
Under compression there shouldn’t be friction on the face of the cells, just force. On the bottom and possibly the sides there can be friction. In my fixtures I’ve used polyurethane sheets (bag) or Formica so the cell bottoms didn’t stick to the header painted surface as they expand and contract.

 

[Checkthisout]

Do charge rates play a role in whatever it is that causes the cells to bulge?

Perhaps solar installations aren't hitting the same C rates as electric cars and so there is less likelihood of gassing or whatever it is that causes the internal pressure rise in the cell.

 

[zorlig]

The only issue with over pressure is crushing, a cell that is not being crushed when empty (which is when you will compress them) will not be crushed when it's full. Therefore there isn't really risk of over pressure on the high end of a charge cycle. Instead the battery simply doesn't provide as much capacity as an unrestrained cell.

With my steel rods and 24 cells in a line there is ZERO movement from empty to full.

I wonder if people using springs see any movement or not. If not, then springs aren't useful

What someone said above about the center cells having more pressure... That's just illiteracy of physics. It's not true. The pressure is always equal for cells set on a shelf then compressed.

 

[DIYrich]

Friction, force, crushing, whatever you want to call it. Friction prevents the release of the expansion force the cell generates. The outside cells push against the restraint. Cells in the middle press against all the cells between it and the restraint. Try it yourself. Push against an unrestrained cell. Then try to push against 8 unrestrained cells all lined up. If it were in space, it would be inertia. On the ground, gravity and friction add to the resistance to moving. That is what the middle cell pushes against when it expands. Add to that all the other cells expanding too.

 

[S Davis]

The only issue with over pressure is crushing, a cell that is not being crushed when empty (which is when you will compress them) will not be crushed when it's full. Therefore there isn't really risk of over pressure on the high end of a charge cycle. Instead the battery simply doesn't provide as much capacity as an unrestrained cell.

With my steel rods and 24 cells in a line there is ZERO movement from empty to full.

I wonder if people using springs see any movement or not. If not, then springs aren't useful

What someone said above about the center cells having more pressure... That's just illiteracy of physics. It's not true. The pressure is always equal for cells set on a shelf then compressed.

So how do you know you are not applying too much pressure at 100% SOC with the expansion of that many cells. And yes my cells expand with charge. If over pressure or compression was not an issue the manufacturer would not put it in the cell specifications, so you must know more than them?

 

[ScrotusGobbleBottom]

I love when we talk about compression/ no compression. That convo has never been settled, yet here we are talking about compression that isnt demonstratively perfectly equal.
Compression can kiss mine.... If I need to replace my batteries in 7 years vs 12 Im ok with that. Sure beats bashing my head into a brick wall.
Seems to me there is less risk of fire with uncompressed and separated. That is what I care about.

 

[Ampster]

Seems to me there is less risk of fire with uncompressed and separated.

I have never heard that compression increases the risk of fire, especially with LFP which already has very little risk of fire. . When I first starting using LFP cells I was using cylindrical cells. I over discharged one set and a few of them leaked some gas that smelled like ether. Because of their form factor cylindrical cells are always under compression once the contents expand. My first set of Prismattics were Winstons with the classic ribbed form factor which I assume was to limit expansion.

 

[AntronX]

he referenced a German engineer

Sure, one guy that goes against mountains of research that proper compression does increase cycle life.

 

[ScrotusGobbleBottom]

I have never heard that compression increases the risk of fire

Just my uneducated observation mixed with confirmation bias Im sure. It seems every time there was an angry LiFePo4 it was compressed. There are not too many occurrences yet, thankfully.

I understand that the cylindrical cells are always under compression and I wouldn't hesitate to use them.

 

[zorlig]

So how do you know you are not applying too much pressure at 100% SOC with the expansion of that many cells. And yes my cells expand with charge. If over pressure or compression was not an issue the manufacturer would not put it in the cell specifications, so you must know more than them?

Because threaded rods are elastic and they didn't expand so there was no incredible increase in pressure.

Also the "don't crush them" as the upper limit came from members of this board reaching out to the manufacturer to understand these exact questions. You could probably find the conversations posted if you look back a few years. It was a fairly wide range of pressure that was all much better than being uncompressed.

Crushing beans to compromise the integrity of the cell housing, which doesn't expand or contact.

 

[S Davis]

Because threaded rods are elastic and they didn't expand so there was no incredible increase in pressure.

Also the "don't crush them" as the upper limit came from members of this board reaching out to the manufacturer to understand these exact questions. You could probably find the conversations posted if you look back a few years. It was a fairly wide range of pressure that was all much better than being uncompressed.

Crushing beans to compromise the integrity of the cell housing, which doesn't expand or contact.

You need to read some cell specifications.

 

[zorlig]

You need to read some cell specifications.

Those cell specifications need to be compared with reality. Reality wins bud. Properly compressed cells don't expand like the specs indicate.

 

[kommando]

Just like an oil thread on a Classic car or bike forum these compression threads just go on and on and on .

 

[TimE]

It's all pretty academic for me. Since my cells are in a sailboat, they need to be restrained from flying around the battery box. They are 'compressed' as that allows them to be fixed in place. As a bonus, we don't get any stress at the cell terminals. My cells arrived with a data sheet that says compress them so that will do for me.

 

[Checkthisout]

Are the cells in server rack batteries compressed?

 

[ScrotusGobbleBottom]

Are the cells in server rack batteries compressed?

Some are compressed well, some are compressed with an afterthought.
My non DIY batteries are compressed with a thin metal shell with "locking tabs" and tape from the factory. Good enough?? Who the F knows.

 

[Checkthisout]

So compression prevents de-lamination when gasses form, yes?

So if we never operate the battery in a way that would cause it to gas then compression doesn't matter, yes?

What conditions create gassing in the battery?

Foe the record, my cells are compressed but scores of others run much many more cells non-compressed without issue.

 

[AntronX]

So compression prevents de-lamination when gasses form, yes?

No. Graphite also irreversibly swells and cracks over time. Compression keeps it in contact with current collector foil.

So if we never operate the battery in a way that would cause it to gas then compression doesn't matter, yes?

Good luck limiting your cycle depth to 10 - 20%. It will probably swell anyway.

but scores of others run much many more cells non-compressed without issue.

That they yet know of. Do they regularly test cell's remaining capacity? Will have to compare capacity after 10 years.