diy solar

diy solar

Compress or not, flexible busbar or not

My mistake if that so. I’m new to this compress or not stuff. Never even seen old data sheets.. although I did read members here saying that they thought the 300kgf was kept by EVEs testing rig throughtbthe entire test but I was unsure if that was confirmed
So you are saying the cycle life data graphs comparing unclamped and cells clamped with 300kgf has been removed from EVE’s latest datasheet?

Real pity if so.
 
I don’t think he’s seeing more now, but rather the same amount he’s always seen and has mitigated issues with flexible busbars
We’ll have to get him to weigh in on this thread. I can probably go back to find the post where I asked him specifically how much expansion he was experiencing after his pack had settled in from a few cycles in his 300kgf fixture and he replied ‘essentially none’
 
If you think you know better than EVE how to get even greater cycle life out of their 280Ah LiFePO4 cells than they do, go for it.
I’m just thinking out loud. I’m not saying I know better.. if 300kgf does actually prevent expansion if kept throughout the entire SOC then great, it’s best to do as you say the data sheet says.. maybe the members here that are still seeing noticeable expansion are not compressing with springs to the right amount and that’s why they are still seeing expansion and having opening busbars.. however if expansion (or accordion whatever) is still occurring at 300kgf at high SOC then I can’t see why it wouldn’t be even better (more cycles) to compress the cells even more at higher SOC to all but completely prevent expanding from occurring.. as long as the pressures the cells experience do not go over the new recommended pressures
 
We’ll have to get him to weigh in on this thread. I can probably go back to find the post where I asked him specifically how much expansion he was experiencing after his pack had settled in from a few cycles in his 300kgf fixture and he replied ‘essentially none’

He has been on here. He commented on this thread that he had nothing but issues with solid busbars and recommended the busbars that I purchased. And if you get time, read through his thread you can read that expansion was causing the busbar loosening
 
I think you underestimate how rigid metals are. Say I wrap my battery in a 10mm steel box, similar to what EVE seem to be doing when welding. Having 250mm height and 130mm width I get 2 x 250 mm x 10 mm + 2 x130mm x 10 mm = 5000mm² + 2600mm² = 7600 mm² cross-section area.

Or, let's use aluminium instead of steel, since it's not even half as rigid at 69 GPa.

If we apply 50kN over 7600mm², that gives 6.579 MPa. That creates a stress of 6.679 MPa / 69 GPa = 0.0000967. Over the length of 8 of my 36.35mm cells, a total of 290.8 mm, that gives 0.0000967 * 290.8 mm = 0.02mm of elongation if I'm not messing up my calculations. Or, 0.0035mm per cell.

Using 6 M16 rods of 200GPa steel we get 6 * 125mm²= 750mm². Applying 50 kN we get 66.67 MPa. Stress = 66.67MPa / 200 GPa = .00033. Over 290.8 mm we get 0.097mm elongation. Or 0.012 mm per cell. We can continue calculating how much this would decrease the pressure, but I don't think it would be that much..

For the LF280K it doesn't seem to be necessary, no. I'm not sure whether you have LF280K or LF280 though.

Yeah, It's partly an exercise in welding too. Might be useful to learn.

Indeed. In my case I have the width of a 19" rack to work with, and I can't fit 16 cells from side to side, so it becomes 2 rows of 8 and thus have plenty of room for springs and bolts. I considered using LF280K and placing them from front to back instead, but 16 cells in a rack would become impossible to handle.

I might get sidetracked with other projects before adding this, but it seems like fairly easy data points to add to indicate a possibly severe issue with the battery. The cost of adding it is nothing compared to the cost of the possible consequences of an issue.


In my case I'm trying to add this to a lead UPS where much of the battery handling is a black box I have to work around.

In my case I don't trust the BMS fully, especially since it can't communicate with the UPS.

100 000 N, or 10 000 kgf. Looking at a data table this is within what two M12 (½") rods can hold before starting to deform permanently.
We’re pretty much on the same page.

I was referring to the thin steel typical battery boxes are made fromand not steel 10mm thick. Are you sure EV batteries use steel enclosures that are that thick / heavy?

And on the force sensor, I’d nailed down a flat pressure sensor on Amazon that would have perfectly fit within the area of my rigid endcaps.

I get eyeballs on my battery / cells at l ast once a week and if I had to tuck them away out of sight for daily use over a year or more, I’d probably be more motivated to add some additional sensors as well…
 
So you are saying the cycle life data graphs comparing unclamped and cells clamped with 300kgf has been removed from EVE’s latest datasheet?

Real pity if so.
No I’m not saying that at all. I said I never seen that data sheet myself and simply heard people say they think the testing rig kept a constant 300kgf throughout the test.. that kgf was always kept the same they thought. But I never heard if that was confirmed
 
We made 20 pages!! Just lightly tighten in a rigid structure and use your cells. Temperature and voltage issues will damage them long before you reach cycle life.
 
We made 20 pages!! Just lightly tighten in a rigid structure and use your cells. Temperature and voltage issues will damage them long before you reach cycle life.
Lol.. at what SOC do you recommend lightly tightening them in a rigid structure? Or is this also a joke
 
I’m just thinking out loud. I’m not saying I know better.. if 300kgf does actually prevent expansion if kept throughout the entire SOC then great, it’s best to do as you say the data sheet says.. maybe the members here that are still seeing noticeable expansion are not compressing with springs to the right amount and that’s why they are still seeing expansion and having opening busbars.. however if expansion (or accordion whatever) is still occurring at 300kgf at high SOC then I can’t see why it wouldn’t be even better (more cycles) to compress the cells even more at higher SOC to all but completely prevent expanding from occurring.. as long as the pressures the cells experience do not go over the new recommended pressures
Among other things, getting a 300kgf fixture designed and calibrated so it is truly applying 309kgf of force was much more complicated than I would have imagined.

I’m just saying, I wouldn’t necessarily take any posters statement about having a 300kgf clamping fixture at face value…

And on the other point, I think I’m basically agreeing with you.

While the ‘old guideline’ was to calibrate the 300kgf at 50% SOC (which was a PITA), I’d be comfortable calibrating 300kgf at the minimum SOC (which is far easier) and not worrying about the fact that there is >300kgf applied at the maximum SOC…

But I’ve seen nothing ti suggest that that change might result in increased cycle life - it’s just much simpler / easier to manage.
 
Among other things, getting a 300kgf fixture designed and calibrated so it is truly applying 309kgf of force was much more complicated than I would have imagined.

I’m just saying, I wouldn’t necessarily take any posters statement about having a 300kgf clamping fixture at face value…

And on the other point, I think I’m basically agreeing with you.

While the ‘old guideline’ was to calibrate the 300kgf at 50% SOC (which was a PITA), I’d be comfortable calibrating 300kgf at the minimum SOC (which is far easier) and not worrying about the fact that there is >300kgf applied at the maximum SOC…

But I’ve seen nothing ti suggest that that change might result in increased cycle life - it’s just much simpler / easier to manage.
Well compression in general increases cycle life..which means preventing expansion is what’s actually resulting in more cycles. So as long as there is not to much compression at high SOC then I’d say it’s actually better to stop all expansion in a rigid fixture set at 300kgf at low SOC..
I’m also now more and more comfortable setting a rigid fixture at 300kgf at low SOC.. I’m just hoping it’s not to much at high SOC but I’m thinking poron foam as @justgary uses would stop to much pressure from happening at high SOC..
Although I don’t agreed thatbthe data sheets are saying donthis for longest life of the cells, I do think them showing these max pressures and stuff make me feel more comfortable about a fixed/rigid fixture with foam
 
I’ll have to go back and check up on Cinergi - he was the one who originally told me he got no compression after the first few cycles (which is what I’ve experienced).

If he has had to double his clamping force to prevent excessive expansion and stress in his rigid busbars as his cells have aged, I may need to keep an eye on my cells behavior.

On the one hand, because I’ve gone with flexible busbars, I’m certain I’ll have nothing to worry about in any case.

And on the other hand, should I ever see increased expansion or should new data emerge suggesting cycle life will greatly improve under higher clamping force of 600kgf (or whatever), it just means I’ll have to invest in new springs…

I don't think I intended to convey this. The cells flattened out after a few cycles but they continued to expand and contract even in my spring-based compression rig. I've not altered this since I deployed the battery 18 months ago.
 
Wow, so he’s gone back to flexible? Looks like I made the right call…

Yes, I did. The bus bars were sliding across the terminals even while clamped! This caused resistance issues between the bus bars and the cell terminals.
 
We’ll have to get him to weigh in on this thread. I can probably go back to find the post where I asked him specifically how much expansion he was experiencing after his pack had settled in from a few cycles in his 300kgf fixture and he replied ‘essentially none’

I think the inter-cell terminal was miniscule. The overall pack of 16 was still measuring enough change. But the inter-cell movement (like .5mm IIRC) was still enough to cause issues with electrical contact.
 
I don't think I intended to convey this. The cells flattened out after a few cycles but they continued to expand and contract even in my spring-based compression rig.

Do you think it’s possible that your springs were not applying enough pressure, or not as much as you thought or not as much as the 300kgf needed and that’s the reason for the access expansion?
 
Last edited:
I don't think I intended to convey this. The cells flattened out after a few cycles but they continued to expand and contract even in my spring-based compression rig. I've not altered this since I deployed the battery 18 months ago.
I came away with a slightly different memory but I’m getting old and it’s water under the bridge.

Good to have you on the thread.

As two of the pioneers pulling together some of the first 300kgf fixtures, how about if we both characterize the expansion / compression we are getting and share that data with the community?

I’m only charging my 560Ah pack of 16 cells with about 150-200Ah these days, so if I’m getting less expansion than you are experiencing, my small change cycle might be a big part of why…
 
I think the inter-cell terminal was miniscule. The overall pack of 16 was still measuring enough change. But the inter-cell movement (like .5mm IIRC) was still enough to cause issues with electrical contact.
OK, so 0.5mm per cell would translate to 8mm for my 16-cell linear pack. Pretty positive I’m getting nowhere near that (but I’ll check / measure).

As I just stated, my current daily charge generation is modest (27% to 36% of 100% of pack rating) so I can can do a check / measurement at that level quickly but getting up close an 80% charge / discharge cycle will be a hassle and will take a ~week…
 
Back
Top