Best practice for 300kgf ‘fixture’ 280Ah cells

[Dzl]

This piece was cut in the "X" direction to show how well squished the braid was.
Now cut it in the "Y" direction on both edges, removing the copper sheetmetal (flattened tube edges) that hold top and bottom together.
That way, only any bond of sheet metal to braid, braid to braid, and braid to copper holds the sandwich together.

Does it fall apart? Does it resist light or heavy tugging?

Thanks that makes sense now, I was misunderstanding what you meant. Now I see.

On that point though, how significant of a factor would that be?
Referring back to the CALB busbars (closeup), they are just thin stacked copper strips, and seem to have stood the test of time, with no more compression than heat shrink + bolt fastening them to the cell terminal.

 

[Bob B]

It will add length but if using a large enough awg it shouldn't matter?

I'm trying to visualize how I would do it with my 2P4S pack. I have them in a string right now, but want to re-configure them into a square form. I think I would need to have multiple connections on some of the terminals ... and don't like that either.

 

[Mark44]

So here is the thing I visualize. Others may disagree with me but I have found disagreements are good because I learn something new.

The cells do expand and contract when charged and discharged. I have sat here and watched it for myself. I like to use the word expand as opposed to swelling. I associate swelling with over charged cells.

Since they expand and contract, my own belief is by using something flexible between the cells for interconnects is the only way to prevent stress to the terminals whether they are compressed or not. For the near term personally I am not concerned. Long term usage is what I am concerned about.

preach it brother!

 

[cinergi]

Oh, interesting .. I never thought to rotate the cells such that you could use wire to parallel them. I'm using wires to put mine in series now



But you can still see solid bus bar for the parallel. Maybe there's a way to use wire for both ... because THEN I might go for the spring-based fixture route.

 

[Hedges]

Thanks that makes sense now, I was misunderstanding what you meant. Now I see.

On that point though, how significant of a factor would that be?
Referring back to the CALB busbars (closeup), they are just thin stacked copper strips, and seem to have stood the test of time, with no more compression than heat shrink + bolt fastening them to the cell terminal.

Question might be whether all strands of the braid make good contact. I figured it wasn't likely to pass the pull test used for a crimp terminal.
But probably with screw compressing middle and flattened tube holding the edges, it will be pretty good. Not that much wider than nut.

The copper strip laminations are all pressed together under the screw so they all have good contact.

 

[fafrd]

So we’ve now strayed into all this concern about stress on terminals when a set of cells ‘breaths’ within a fixture.

I believe the point is that if you have a fixture limiting the extent to which cells can expand (using springs or whatever), that 12psi or 17psi of pressure (at the extreme) will push the cells closer together as they discharge and place (possibly damaging) stress on rigid busbars, correct?

There enough folks far enough song on their fixture designs that I’m wondering whether any can provide some measurements of the levels of deformation we are talking about.

What I’m thinking is:

1/ Charge your cells in your fixture applying ~17psi at the charge extreme and measure both long dimension of the fixture as well as clamping force

2/ discharge to 50% SOC and repeat measurements of fixture dimension as well as clamping force

3/ continue to discharge to the low charge extreme and repeat the measurements of fixture dimension and clamping force

It would just be very helpful to understand what numbers we are talking about as far as mm of ‘stretch’ or deformation as cells charge within one of these spring-loaded fixtures.

Rigid busbars themselves represent a counteracting force against shrinkage / movement at the top surface of the cells (which is exactly the stress we are stressing out about), so if rigid busbars are re-seated when the cells are full, the full pack may retain ‘full’ dimensions at the top of the e pack and shrink primarily along the lower ends (like an accordion).

Would just be immensely helpful to have some idea what dimensions we are talking about.

Didn’t someone somewhere see that their threaded rods actually acted like a spring and ‘stretched’ under the force of full-charged cells? Do we have any idea of the magnitude in mm or whatever of that stretch? To the extent that threaded rods themselves represent some amount of spring force, do we know what psi of force we are talking about?

 

[fafrd]

A refinement of my ‘accordion fixture’ idea is to design an assymetric fixture with infinite pressure at the top (or whatever pressure is determined to be enough to prevent unacceptable stress on rigid busbars during full charge cycle) and 12psi or less of pressure with the bottom threaded rods.

The top surface will bend slightly as the cells expand but will remain rigid while all of the expansion the cells achieve will be in the form of tiny long triangles reaching their maximum extent along the bottom surface of the cells.

So no dimensional expansion at the top surface of the cells at the cost of twice the expansion in terms of dimension at the bottom of the cells...

 

[HRTKD]

A threaded rod can stretch. Will it stretch enough that you can measure it? Probably not. Maybe in a 16s fixture, but not in a 4s fixture. The 10' length of 1/4" threaded rod I used is rated for 900 lbs. We're not close to that amount.

If the fixture is well designed, uses proper materials and is sufficiently "snug", I think you're going down a rabbit hole thinking that there will be a quantifiable amount of stress on your cell terminals.

 

[fafrd]

A threaded rod can stretch. Will it stretch enough that you can measure it? Probably not. Maybe in a 16s fixture, but not in a 4s fixture. The 10' length of 1/4" threaded rod I used is rated for 900 lbs. We're not close to that amount.

If the fixture is well designed, uses proper materials and is sufficiently "snug", I think you're going down a rabbit hole thinking that there will be a quantifiable amount of stress on your cell terminals.

Now that’s exactly the sort of language I’m hoping to hear!

1mm of stretch on a Busbar ~75mm long sounds like a recipe for a problem, but I’m hoping there is enough ‘give’ in the overall system that the top surface where the terminals are can remain rigid in 2D and just bend/curve very slightly while the top surface can still expand to accommodate whatever swelling results from the recommended 17psi max pressure...

And I suppose that another source of ‘give’ to consider is that to the cells are connected in different directions on their two terminals/ends. This should mean that another way the upper-half of the cells can swell by ~1mm without stretching rigid busbars is to ‘accordion’ across the top, meaning as a pair of cells expand, a small triangle grows at the side opposite the rigid busbar spanning them...

 

[HRTKD]

I'm no expert on LiFePO4 cell construction. I've seen exactly ONE video of a cell taken apart. So with that in mind...

The jelly roll that makes up the guts of these cells is going to expand outward (horizontally) much more than it would up/down (vertically). If your fixture is solid, the cells aren't going anywhere and neither are your rigid bus bars. Don't make this any more complicated that you have to.

Why do these cells bloat/expand along with long side? Because they can! It's the weakest side of the battery. Compress the cells in a fixture and they can't expand along the long side. Maybe the magic inside the cells will make better use of the space inside the cell walls now instead of pushing out the long side.

 

[fafrd]

I'm no expert on LiFePO4 cell construction. I've seen exactly ONE video of a cell taken apart. So with that in mind...

The jelly roll that makes up the guts of these cells is going to expand outward (horizontally) much more than it would up/down (vertically). If your fixture is solid, the cells aren't going anywhere and neither are your rigid bus bars. Don't make this any more complicated that you have to.

Why do these cells bloat/expand along with long side? Because they can! It's the weakest side of the battery. Compress the cells in a fixture and they can't expand along the long side. Maybe the magic inside the cells will make better use of the space inside the cell walls now instead of pushing out the long side.

I’ve started clamping my cells in a simple 1S fixture for charge & discharge cycles during characterization and what I’ve experienced is that once the ‘easy’ sides have been constrained with a fixture, there is noticeably more expansion out of the ‘short’ sides (ends).

Not enough to be a concern (at least to me), but while I never noticed any expansion/bloating of the short sides when the long sides were unconstrained to expand/bloat freely, with the long sides constrained, there is now noticable expansion of the short sides and they are no longer perfectly flat after charging to 3.65V.

What a hairball EVE has thrown our way...

 

[HRTKD]

It's not just EVE. Many of the common LiFePO4 cells are constructed the same way.

The short sides can't expand that much. And if they do, it's not going to affect the bus bars unless you have the cells arranged short-side to short-side. Most people don't use that arrangement as it makes the compression fixture a lot more involved.

 

[fafrd]

It's not just EVE. Many of the common LiFePO4 cells are constructed the same way.

The short sides can't expand that much. And if they do, it's not going to affect the bus bars unless you have the cells arranged short-side to short-side. Most people don't use that arrangement as it makes the compression fixture a lot more involved.

I wasn’t referring to the construction, I was referring to the new specification they released last Spring...

(which is the first time any of us heard this ‘longer cycle life from 300kgf fixture hairball)

 

[HRTKD]

I think the compression issue would have still come up once people saw that the cells were expanding a bit. In my case, I needed something to hold my cells together that took up minimal room in my RV compartment. I may have arrived at the same fixture, but for different reasons.

 

[fafrd]

I think the compression issue would have still come up once people saw that the cells were expanding a bit. In my case, I needed something to hold my cells together that took up minimal room in my RV compartment. I may have arrived at the same fixture, but for different reasons.

For sure, installation in an RV is going to raise the bar and pretty much require a busbar solution that can accommodate some flexibility - what did you go with?

 

[HRTKD]

For sure, installation in an RV is going to raise the bar and pretty much require a busbar solution that can accommodate some flexibility - what did you go with?

Currently I'm using the cheap rigid bus bars that came with my cells. I have aluminum bar stock that I'll be using to make my own bars.

 

[Dzl]

I wasn’t referring to the construction, I was referring to the new specification they released last Spring...

Small (maybe inconsequential) correction, the last revision to the datasheet was spring 2019. Though from what I recall we didn't really pick up on it and start discussing it until May or so.

 

[Thersom1948]

using 4 standard unlubricatef 1/4” bolts, what level of torque (inch pounds) should be applied to get about 12psi of clamping force (and about 142% that much torque for 17psi, correct?)?

It would be really awesome if the thread called "best practice of clamping force" actually answered the question.

Remind me never to paint a shed with you all.....

 

[upnorthandpersonal]

It would be really awesome if the thread called "best practice of clamping force" actually answered the question.

Remind me never to paint a shed with you all.....

If it would be an easy question to answer, it would have been answered. There is no easy answer because of the many unanswered questions that need to be clarified before the answer to this particular question becomes clear.

 

[Marinepower]

Thanks that makes sense now, I was misunderstanding what you meant. Now I see.

On that point though, how significant of a factor would that be?
Referring back to the CALB busbars (closeup), they are just thin stacked copper strips, and seem to have stood the test of time, with no more compression than heat shrink + bolt fastening them to the cell terminal.

Dzi,

I have been looking for a flexible battery bus bar solution and you have provided some great ideas.

The 71mm CALB bus bars made for the CALB CA180 cells look like the best solution to me and they appear to be able fit the EVE 280 cells (holes are a bit oversized however for M6). However, no one seems to sell the CALB bus bars separately from the CALB cells. Anyone know where you can get these?