Compress or not, flexible busbar or not

[fafrd]

I’m saying no matter what the fixture is, flexible busbars are imo best to prevent the possibility of any bms issues.. I thought that would be easily agreed with. There is several 280ah prismatic cell sellers that now supply flexible busbars instead of solid busbars.. yes with a rigid fixture there is less expansion but it’s only less, so flexible busbars are still a good choice

Flexible busbars are never a mistake. If more costly, they may not be necessary within rigid fixtures.

 

[Cheap 4-life]

You’ve got to apply a calibrated 280kgf to 320 kgf to cells charged to 40% anyway, so the only question to me is will you get lower cycle life with a rigid fixture calibrated to 300kgf @ 40% SOC, or will you get at least the same cycle life (and possibly higher) maintaining 280kgf to 320kgf throughout the full charge / discharge cycle?

Indeed this is the question now.. is holding 300kgf better and possibly providing more cycles than rigid, rigid allows the pressure to rise higher than 300kgf as SOC rises and is that better to have more pressure than 300kgf as the SOC rises,.. Seems to me maybe something in the middle of both..
like less compression than rigid thru SOC but more than 300kgf thru the SOC that springs tries to achieve..

 

[fafrd]

Indeed this is the question now.. is holding 300kgf better and possibly providing more cycles than rigid, rigid allows the pressure to rise higher than 300kgf as SOC rises and is that better to have more pressure than 300kgf as the SOC rises,.. Seems to me maybe something in the middle of both..
like less compression than rigid thru SOC but more than 300kgf thru the SOC that springs tries to achieve..

We know high enough pressure cause internal damage and eventually leakage.

If it’s much less costly to implement a rigid structure calibrated to 300kgf @ 40% SOC, it’s a valid question.

But if it’s going to cost as much or more to implement a calibrated 300kgf rigid fixture as it is to implement a calibrated constant 280-320kgf fixture, I’ve seen nothing to suggest you’ll get less cycle life under a constant 280-330jgf than you will under a rigid fixture straying in to much higher pressure levels as cells degrade/age.

I’ll be more than happy if I can get 6000 cycles to 80% from my 280Ah cells - that would mean over 16 years of service live at more than the capacity I’m currently using.

I doubt higher pressure than 300kgf leads to increased lifetime and I’m pretty confident that a constant clamping force of 280-320kgf will deliver close to the full 6000 cycles specified (and possibly even more).

My guess is that a rigid clamping fixture is easier / cheaper / more practical in mass production, not superior to a constant 280kgf to 320gf fixture (which would add significant cost and complexity to EV batteries).

 

[Cheap 4-life]

We know high enough pressure cause internal damage and eventually leakage.

Right but doesn’t the data sheet imply pressure will not reach damage potential in a rigid fixture under normal usage..

If it’s much less costly to implement a rigid structure calibrated to 300kgf @ 40% SOC, it’s a valid question.

I don’t think the small extra cost either way should sway the decision of a DIYer. Doing what best however to get more cycles might matter. That’s if the user wants the best life for the cells instead of simply thinking calendar aging will happen first..

I doubt higher pressure than 300kgf leads to increased lifetime and I’m pretty confident that a constant clamping force of 280-320kgf will deliver close to the full 6000 cycles specified (and possibly even more).

Higher pressure thru the SOC might stop unwanted expansion that users are reporting when using springs and 300kgf. Remember your c-rates and charge cycles are low/not fast so you are not seeing as much expansion as two other users commenting on this thread that use springs.
.

We know high enough pressure cause internal damage and eventually leakage.

If it’s much less costly to implement a rigid structure calibrated to 300kgf @ 40% SOC, it’s a valid question.

But if it’s going to cost as much or more to implement a calibrated 300kgf rigid fixture as it is to implement a calibrated constant 280-320kgf fixture, I’ve seen nothing to suggest you’ll get less cycle life under a constant 280-330jgf than you will under a rigid fixture straying in to much higher pressure levels as cells degrade/age.

I’ll be more than happy if I can get 6000 cycles to 80% from my 280Ah cells - that would mean over 16 years of service live at more than the capacity I’m currently using.

I doubt higher pressure than 300kgf leads to increased lifetime and I’m pretty confident that a constant clamping force of 280-320kgf will deliver close to the full 6000 cycles specified (and possibly even more).

My guess is that a rigid clamping fixture is easier / cheaper / more practical in mass production, not superior to a constant 280kgf to 320gf fixture (which would add significant cost and complexity to EV batteries).

Yes cheaper. Have seen just strapping used. To allow some expansion instead of rigid fixture and still claiming 10k cycles..

 

[fafrd]

Right but doesn’t the data sheet imply pressure will not reach damage potential in a rigid fixture under normal usage..

No, it suggests you’ll get 6000 cycles to 80% at 25C if you clamp cells charged to 40% SOC to 280-320 kgf within a calibrated but non-flexible structure.

I don’t think the small extra cost either way should sway the decision of a DIYer. Doing what best however to get more cycles might matter.

We’re pretty much on the same page there, but my view is that it will cost more to get a rigid structure calibrated to 300kgf and leave it at that that it would be to leave the sprigs you used to calibrate 300kgf in place so that pressure remains constant in a clamping fixture that can accommodate a modest amount of expansion.

If I was building a rigid fixture, I’d sure want to include a pressure sensor as part of the design, that’s for sure…

That’s if the user wants the best life for the cells instead of simply thinking calendar aging will happen first..

Calendar aging is a whole different question, but the key point to me is that after seeing that expansion pressure increases as cells are exposed to more cycles, and after seeing that sufficiently high pressure can cause ‘interval defects’ and eventually leakage, how could you build a rigid structure without a pressure sensor?

Higher pressure thru the SOC might stop unwanted expansion that users are reporting when using springs and 300kgf. Remember your c-rates and charge cycles are low/not fast so you are not seeing as much expansion as two other users commenting on this thread that use springs.

You are much more focused on expansion than you are on pressure. From everything I read in the new datasheet, I suspect that is a mistake.

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Yes cheaper. Have seen just strapping used. To allow some expansion instead of rigid fixture and still claiming 10k cycles..

Would love to understand how you plan to get strapping calibrated to 280kgf to 320kgf of force…

 

[brum]

I've done an expansion test on EVE LF105 cells. Just once. Clamped with springs at ~200kgf (~12 psi) from 40% to 100% SOC. There was zero expansion. Based on the result I decided to use solid busbars. I'm using the cells at a 0.1C rate at most. The result shows that cell expansion is likely conditional.

My next build is a 15kWh battery pack. Two 8-cell packs. I'll use springs and flexible (braided) busbars this time. The cost of cells doesn't justify the risk of going with a solid one.

 

[1234enough]

This is my mock up of planned compression / drawing, will have 18 x springs in total ( 3S6P).
So far has cost £10.77 for the springshttps://www.ebay.co.uk/itm/Compress...a-e7d6-46cc-9b0e-f90b5ac460ca&redirect=mobile + couple of £ for the 12mm OD plastic pipe(sleeve on inside of spring to allow threaded bar to slide, plus on threaded bar along it's lenght to ensure isolation against cells) - all other bits are sourced from work for free? - hence why 8mm threaded bar - 6mm would be ok but none at work ?.
Spring rates chart is as tested under various compressed lengths, also compressed spring for a week to check for long term deformation and seems good.
So as far costs go- negligible, just a bit of time in man cave.

Note - this is my take on a solution for compliance with requirment, you make your own minds up how you wish to build.

 

[justgary]

No, it suggests you’ll get 6000 cycles to 80% at 25C if you clamp cells charged to 40% SOC to 280-320 kgf within a calibrated but non-flexible structure.

To clarify, it says that you will have at least 80% capacity left after you cycle from 3.65v to 2.5v at 0.5C 6,000 times. It also says to put the cells in the fixture at between 30% and 40% SOC, not exactly at 40%.

You are much more focused on expansion than you are on pressure.

I suppose that the true question we all have is, which is worse? Expansion or pressure? Has anyone here reported on the effects of pressure on cells? I mean the internal physics, not just that too much will crush a cell. It is entirely possible that pressure alone will not harm the cells as long as the aluminum enclosure is not crushed, since that would clearly cause the plates and electrolyte to move. My take is that movement of the components inside the pouch is what is the most detrimental. Pressure is simply a byproduct of those components trying to move within a constrained space, and is probably not the Boogey Man.

The specification for pressure build up under use is different from the destructive test of applying pressure externally until the cell leaks or ruptures. The chances of my cells getting external pressure applied (e.g. from a car accident) is close to zero.

 

[justgary]

Note - this is my take on a solution for compliance with requirment, you make your own minds up how you wish to build.

That's the spirit! I really like that you read and interpreted the specification, then made a plan to create a compliant structure for your batteries.

When all is said and done, we are all mostly crossing our fingers that we have done everything as correctly as we could. Then we sit back and prove to ourselves that we were right as our cycle count builds and builds, hopefully with minimal loss of capacity.

 

[justgary]

Would love to understand how you plan to get strapping calibrated to 280kgf to 320kgf of force…

In thinking about it for a few minutes, strapping may be fairly easy to calibrate. Drive belt force is typically checked with an offset (perpendicular) force test looking for a specific force over a specific distance needed to offset the belt. Strapping could be calibrated in the same manner by hanging a 300 kg mass from a length of the material, then testing the offset with a spring scale.

 

[Cheap 4-life]

No, it suggests you’ll get 6000 cycles to 80% at 25C if you clamp cells charged to 40% SOC to 280-320 kgf within a calibrated but non-flexible structure.

Data sheet suggests, Within that non-flexible structure there was no damage from high pressure at high SOC also with very high c-rates and beat to hell.
.

Calendar aging is a whole different question, but the key point to me is that after seeing that expansion pressure increases as cells are exposed to more cycles, and after seeing that sufficiently high pressure can cause ‘interval defects’ and eventually leakage, how could you build a rigid structure without a pressure sensor?

I agree, I would use springs or foam to leave some room for possible excess expansion

You are much more focused on expansion than you are on pressure. From everything I read in the new datasheet, I suspect that is a mistake.

I’m saying the data sheet might suggest the higher pressure of a rigid fixture might be better or fine or no problem or no reduces cycles for the cells thru SOC. May be better than keeping the cells at 300kgf constant via springs..

Would love to understand how you plan to get strapping calibrated to 280kgf to 320kgf of force

I don’t, simply saying that’s how it’s done with most premade packs using prismatic cells, some still claiming 10k cycles. Some even simply use tapes..

 

[Cheap 4-life]

In thinking about it for a few minutes, strapping may be fairly easy to calibrate. Drive belt force is typically checked with an offset (perpendicular) force test looking for a specific force over a specific distance needed to offset the belt. Strapping could be calibrated in the same manner by hanging a 300 kg mass from a length of the material, then testing the offset with a spring scale.

I’m thinking in the premade packs using straps they simply snug up the cells tight with the strapping.. and are still claiming 6k to 10k cycles

 

[justgary]

I’m thinking in the premade packs using straps they simply snug up the cells tight with the strapping.. and are still claiming 6k to 10k cycles

I have no information about premade packs. The question was about calibrating straps if one decided that straps were the thing to do.

The point that @fafrd made is valid. Calibrating the force on a rigid fixture may be somewhat more pesky than a flexible fixture. At least with springs or foam one can indirectly measure force by measuring deflection of the compressible medium.

 

[Cheap 4-life]

I have no information about premade packs. The question was about calibrating straps if one decided that straps were the thing to do.

The point that @fafrd made is valid. Calibrating the force on a rigid fixture may be somewhat more pesky than a flexible fixture. At least with springs or foam one can indirectly measure force by measuring deflection of the compressible medium.

Agreed, but wouldn’t simply snugging up the cells at 40% (or so) SOC in a rigid structure with threaded rod be close enough.. I mean the data sheet cell stayed well under damage pressure didn’t it? And we most likely will not be putting our packs thru anything close to that amount of abuse.. With straps there’s room for expansion anyways..

 

[justgary]

Agreed, but wouldn’t simply snugging up the cells at 40% (or so) SOC in a rigid structure with threaded rod be close enough.. I mean the data sheet cell stayed well under damage pressure didn’t it? And we most likely will not be putting our packs thru anything close to that amount of abuse.. With straps there’s room for expansion anyways..

What is snugging up in a quantitative sense? 660 pounds of force is a lot. I would agree with applying 660 pounds of force and then snugging up nuts on a threaded rod, then releasing the compression force.

 

[Cheap 4-life]

What is snugging up in a quantitative sense? 660 pounds of force is a lot. I would agree with applying 660 pounds of force and then snugging up nuts on a threaded rod, then releasing the compression force.

With 16 or 19 cells in a row, how would you apply 660lbs of force before tightening up the nuts for a rigid fixture. Would need some very long clamps and somehow incorporate a weight scale?
I’m planning on using springs and slowly tightening each one till they are at 50% deflection which is supposed to be 160lbs per spring.
I guess if a rigid fixture was wanted then the springs could be used and then measurements to the walls recorded then remove springs and retighten walls to that measurement..

 

[fafrd]

This is my mock up of planned compression / drawing, will have 18 x springs in total ( 3S6P).
So far has cost £10.77 for the springshttps://www.ebay.co.uk/itm/Compress...a-e7d6-46cc-9b0e-f90b5ac460ca&redirect=mobile + couple of £ for the 12mm OD plastic pipe(sleeve on inside of spring to allow threaded bar to slide, plus on threaded bar along it's lenght to ensure isolation against cells) - all other bits are sourced from work for free? - hence why 8mm threaded bar - 6mm would be ok but none at work ?.
Spring rates chart is as tested under various compressed lengths, also compressed spring for a week to check for long term deformation and seems good.
So as far costs go- negligible, just a bit of time in man cave.View attachment 113409
View attachment 113410
Note - this is my take on a solution for compliance with requirment, you make your own minds up how you wish to build.

Almost identical to the designed I used (but I went with only two Sid’s per side rather than 3).

But my cells are a single row of 16 clamped by 4 rods while it appears you are clamping a double-row of 8 cells in a single fixture…

Given the datasheet, one clamp for two rows will probably work fine but others not having space for a single row of cells have left space between the two rows for additional threaded rods and essentially firmed two seoerate 8-cell single-row clamping fixtures abutting each other.

But knowing what I know now and seeing the new datasheet, I’d have no worries about your double-row design.

 

[justgary]

You could use springs to calibrate your bar clamps by putting the springs between two plates and then compressing them with the bar clamps. Record the torque required on the bar clamp screws in order to get you the desired clamping force (calculated from the spring constant). You could then clamp your cells in the fixture using the same torque on the clamps, then snug the nuts and release the clamps.

 

[fafrd]

With 16 or 19 cells in a row, how would you apply 660lbs of force before tightening up the nuts for a rigid fixture. Would need some very long clamps and somehow incorporate a weight scale?
I’m planning on using springs and slowly tightening each one till they are at 50% deflection which is supposed to be 160lbs per spring.
I guess if a rigid fixture was wanted then the springs could be used and then measurements to the walls recorded then remove springs and retighten walls to that measurement..

The way I would build a rigid fixture clamped to 300kgf would be to use half the treaded rod with springs so I could tighten exactly the correct deflection to apply 660kgf, then I would snugly tighten nuts into the other threaded rods without springs. Whether you want to loosen the nuts and remove the springs from the first set of rods or not is largely immaterial.

 

[Cheap 4-life]

@justgary
Right but that would require really long clamps. Like 6ft long