There was a German video that had a battery engineer on there and he also said what @Tradewinds63 said. That is why the cells are vacuum sealed to try and get all the oxygen(?) out but since they can't get it all out that is what causes it to bloat slightly during the first few full charges. Clamping during that initial few charges is what they claimed increased the cycle count, not the continuous clamping.
Best practice for 300kgf ‘fixture’ 280Ah cells
- Thread starter fafrd
- Start date
Tradewinds63
New Member
- Joined
- Jul 4, 2021
- Messages
- 8
kgf is an informal or casual engineering term and relates to a unit of mass (the kilogram, hence kg. The remainder is f for force. Kilograms force) and should also be accompanied by some other unit of measure such as area, time, etc to provide a specified relationship of work.
Since we are discussing an applied mass force upon a surface area of the battery, the second unit of measure is typically expressed in either mm², cm² or m², etc. If it represented mm², the cells would be crushed if the force applied was equal to 300 kgf/mm² and the cells would also be crushed by 300 kgf/cm². it would be like you standing on a tin can and it buckling under your body wiegth.
300 kgf is the expressed force for the EVE LF line of prismatic batteries and all have far different surface areas, some more than double the smaller LF batteries. But all use the same 300 kgf term without ever expressing the second unit of relational measure. Therefore the reader is forced to engage in some form of rationalized consideration.
The applied force need not be great, these aren't balloons and the gasses will escape through the valve at the top of the battery as they form during the initial charge cycles. Some of the gasses are hydrogen and so forth. It depends on the chemical reaction that's occuring. But all that's needed, is to keep the walls of the batteries flat as possible during the intial charge cycles. Adding too much force can cause the electrolytic compounds to be crushed into thinner layers at the center and becoming thicker about the perimeter of the electrodes, not adding enough force will result in the formation of bubbles within both the region between the electrodes and the electrolytic compounds and the electrolytic compounds themselves. Therefore causing thicker gass infused electrolytic compound layers nearer the center of the electrodes.
You can argue this all you like and swear up and down otherwise but in the end, it is what it is.
This proccedure has nothing to do with restraining batteries in an automobile. This is a wholly seperate matter and is a procedure to degass new lifepo4 prismatic batteries. There's nothing more to it than that. Whether your application is home energy storage, automotive energy storage or you're using these batteries to power your aircraft... Whether you decide to degass them when new or not is up to you and you're free to believe whatever you like, to the contrary or not.
And again, all the EVE manufacturer literature shares the same 300 kgf unit for the entire LF prismatic lineup regardless of amp hrs/size.
Since we are discussing an applied mass force upon a surface area of the battery, the second unit of measure is typically expressed in either mm², cm² or m², etc. If it represented mm², the cells would be crushed if the force applied was equal to 300 kgf/mm² and the cells would also be crushed by 300 kgf/cm². it would be like you standing on a tin can and it buckling under your body wiegth.
300 kgf is the expressed force for the EVE LF line of prismatic batteries and all have far different surface areas, some more than double the smaller LF batteries. But all use the same 300 kgf term without ever expressing the second unit of relational measure. Therefore the reader is forced to engage in some form of rationalized consideration.
The applied force need not be great, these aren't balloons and the gasses will escape through the valve at the top of the battery as they form during the initial charge cycles. Some of the gasses are hydrogen and so forth. It depends on the chemical reaction that's occuring. But all that's needed, is to keep the walls of the batteries flat as possible during the intial charge cycles. Adding too much force can cause the electrolytic compounds to be crushed into thinner layers at the center and becoming thicker about the perimeter of the electrodes, not adding enough force will result in the formation of bubbles within both the region between the electrodes and the electrolytic compounds and the electrolytic compounds themselves. Therefore causing thicker gass infused electrolytic compound layers nearer the center of the electrodes.
You can argue this all you like and swear up and down otherwise but in the end, it is what it is.
This proccedure has nothing to do with restraining batteries in an automobile. This is a wholly seperate matter and is a procedure to degass new lifepo4 prismatic batteries. There's nothing more to it than that. Whether your application is home energy storage, automotive energy storage or you're using these batteries to power your aircraft... Whether you decide to degass them when new or not is up to you and you're free to believe whatever you like, to the contrary or not.
And again, all the EVE manufacturer literature shares the same 300 kgf unit for the entire LF prismatic lineup regardless of amp hrs/size.
I have built batteries with and without compression. Based on what I’ve seen with my own eyes, most cells do swell in time without fixturing/compression. This usually happens after enough cycles, elevated temperatures + high state of charge. I can also tell you that you can run a bunch of cycles compressed, take it out of compression and one trip to a full charge on a warm day and it’ll likely swell to the point of where I’d be hesitant to fixture it again. Swelling can happen the very first time you top balance, so if you’re going to compress, do it from the beginning. If you absolutely must take cell out of compression to make a change and recompress, do so at a low state of charge and cool cells. If you choose not to fixture, you should allow as much room between the cells as your buss bars can accommodate. My thinking is once you are committed to one method, fixtured or not stick with it. Calendar/chemistry life could be the ultimate tell….in time, we obviously don’t know yet. As for compression, I like 10psi on the wide face. This works out to be about 550 lbs of force on a 280/305 Ah prismatic cell. Anything less, don’t even bother because the cells are just going to push it out of the way WHEN they grow.
My biggest problem with what you are describing is if clamping were only needed ‘during the initial charge cycles’, why wouldn’t that be done by the manufacturer / EVE before shipping (as the final step of manufacturing)?
It’s a huge PITA for customers and far easier for the manufacturer to take care of it with one set of clamping fixtures rather than having many/most customers take care of it with their own clamping fixtures at incoming quality control (larger commercial customers) or assembly (DIY end-customers). Even if it modesty increases cell cost (I’d happily have paid a bit more for ‘pre-broken-in’ cells that avoided the need to assemble my own clamping fixture).
So I just can’t but what you are saying.
And right or wrong, if you only have one LiFePO battery and go to the trouble of building a 300kgf clamping fixture for it as specified, it’s easier to just leave the battery in the fixture forever (EOL) than to remove the fixture after ‘the initial charge cycles’…
Tradewinds63
New Member
- Joined
- Jul 4, 2021
- Messages
- 8
Is the graph you shared something you put together or something someone else did?
I'm currios to know if the test was performed on a single battery or was it performed on a series of batteries?
Next question, how long have you had your compression device in service and have you taken out any cells and tested their capacity to see if they are in line with the Eve cycle graphs for their fixture results? The graphs indicate battery capacity in relation to cycles and should be good enough to compare to after a couple years regular use if used for household power. Obviously there will be calander life effecting the results as well but calander life graphs are available as well.
After looking more closely at the EVE test procedure amongst the several different LF models it would appear they reference the same experiment for all their models and simply alter the compression area dimensions for the specific battery within the literature.
I'm currios to know if the test was performed on a single battery or was it performed on a series of batteries?
Next question, how long have you had your compression device in service and have you taken out any cells and tested their capacity to see if they are in line with the Eve cycle graphs for their fixture results? The graphs indicate battery capacity in relation to cycles and should be good enough to compare to after a couple years regular use if used for household power. Obviously there will be calander life effecting the results as well but calander life graphs are available as well.
After looking more closely at the EVE test procedure amongst the several different LF models it would appear they reference the same experiment for all their models and simply alter the compression area dimensions for the specific battery within the literature.
Bob B
Emperor Of Solar
- Joined
- Sep 21, 2019
- Messages
- 9,120
Probably a typo ..... instead of PSI .... Lbs?
S Davis
Solar Addict
- Joined
- Sep 25, 2021
- Messages
- 777
Yes thanks, typo 640lbs total across the face.
Is that even a serious question here?
My 16S 280Ah cells have been in a 300Kgf clamping fixture for over 3 years now with absolutely no signs of crushing…
S Davis
Solar Addict
- Joined
- Sep 25, 2021
- Messages
- 777
I was responding to post 242 talking about crushing the cells.
Tradewinds63
New Member
- Joined
- Jul 4, 2021
- Messages
- 8
After reading the EVE test data amongst the several LF model batteries and comparing it to this chart...
It's clear that the EVE experimental test subject was the EVE LF 304. If you compare the surface areas of the various batteries, only one, the LF 304 comes to approximately 660 lbs (300Kg) total force upon its surface (55.86 in²) when using the optimal 12 PSI as the clamping force/axial force applied to the battery.
So, should someone use the 300kgf for the LF105 (surface area 40.5 in²) as an example. They would be applying 16.3 psi, which is still in the peak zone but does reduce it by a projected potential 5k cycles, from nearly 20k cycles to about 15k cycles, when compared to 12 psi. An LF 105 would produce best results not with 300 kgf but rather 220 kgf.
It's too bad EVE didn't express their unit as 2.874 N/cm² Y axial force, which is 12 psi Y axial force. Then, they'd have avoided all the confusion with respect to the applied force and been far more accurate.
It's clear that the EVE experimental test subject was the EVE LF 304. If you compare the surface areas of the various batteries, only one, the LF 304 comes to approximately 660 lbs (300Kg) total force upon its surface (55.86 in²) when using the optimal 12 PSI as the clamping force/axial force applied to the battery.
So, should someone use the 300kgf for the LF105 (surface area 40.5 in²) as an example. They would be applying 16.3 psi, which is still in the peak zone but does reduce it by a projected potential 5k cycles, from nearly 20k cycles to about 15k cycles, when compared to 12 psi. An LF 105 would produce best results not with 300 kgf but rather 220 kgf.
It's too bad EVE didn't express their unit as 2.874 N/cm² Y axial force, which is 12 psi Y axial force. Then, they'd have avoided all the confusion with respect to the applied force and been far more accurate.
The fact that they use an outdated, non standard unit like the kgf has always puzzled me...
Tradewinds63
New Member
- Joined
- Jul 4, 2021
- Messages
- 8
I'm going to guess they contracted the test out to some old geezer in China who provided them with that?
What's sad is that they just slapped it into the literature for all their LF line-up, altering only the surface applied to data and battery model info. Not realizing it became inaccurate information when they applied it to batteries other than the LF304, solely because the unit expressed wasn't a standardized unit but rather a custom unit for the LF304, only.
While I haven't done the math, wouldn't 300 kg/m² be the more likely measure?
Tradewinds63
New Member
- Joined
- Jul 4, 2021
- Messages
- 8
That's what I thought also as mm² and cm² would be insane amounts of pressure. But the literature expresses within a single snippet mention, the area in which the force is applied to, referencing the plane perpendicular to the Y axis and providing that areas dimensions. So it isn't applied to any standardized unit, making it non-transferable upon batteries with different surface areas. Yet, they do just that throughout the many manufacturer specification docs.
Even companies such as CATL references the same experimental results using the informal non-standardized 300kgf for their lifepo4 prismatic batteries axial compression force. Which if used will result in fairly different results among the various battery models, in some cases, failure. For example the LF32ah with effectivly a 148x91 mm surface area or 20.875 in² almost half the LF105 and a mere 37% the surface area of the LF304. Providing a total 300kgf axial clamping force to the 20.875 in² surface would result in 14.37 k/in² or 31.68 psi! That would eventually likely result in cell damage as they say 9 kN will cause internal failure. On the LF304 that 300kgf comes to about 3 kN on the planar surface which on the LF32 is 2.7 times the force of 3 kN, at nearly 9 kN or more presicely 8.1kN and the literature clearly states that 9kN force will cause battery failure. So, 8.1kN force through repetitive charging would very likely eventually cause internal failure also.
What I don't like, I had to read through the literature of several models to realize what they had done, copy, paste, edit the same experimental literature. Then it took a while to determine which model was most likely used in the referenced experimental setup (LF304). Between that and the other independent experimental test results it became clear what EVE and others have done with respect to that single experiment they did. They dropped the ball and didn't convert the measurment to a standardized unit.
Unfortunately, that's going to result in many improper applications of the "fixture." It's pretty clear they don't care. LOL, Otherwise they'd issue a standardized unit update to clear the air on the matter.
So, after an exhausting time looking though many documents to flesh out what is going on. I recant some parts of my initial and second post on this topic.
Keeping the batteries in the fixture is paramount BUT employing the correct 12psi is also paramount to achieving the projected maximum potential cycle life. Forget the damn 300kgf, it's only pertinent to one battery models planar surface.
Next is understanding the forces at work within the batteries and how their expansion can and will effect improperly designed "fixtures."
I took the time to run the structural force calculations for a battery bank I'm building now and the materials I'd have to use to make it fit in the cabinet it will be placed in. I picked up 8" steel C channel with 1/4" webbing and 2" flanges. Will use 1/4" 20 tpi steel allthread to provide the axial clamping force. Some seriously expensive material to clamp this group without any chance for expansion.
This bank will be 5 parallel 8 series batteries, 40 batteries in one "fixture" by two fixtures total for an 80 battery bank of LF105, 51.2v 26,880 wh.
5 ea LF105 planar surfaces parallel within a single fixture comes to 5 x 40.5 in² = 202.5 in² by 12 psi = 2430 lbs axial force total. Using 4ea 1/4" 20 tpi steel allthread to provide the axial force, that's 607.5 lbs each axial force using 30.375 in-lbs torque each or 2.5313 ft-lbs each.
I've got 80 batteries to drain down between 30-40 SOC, they came with 3.293v each +/- .001v. Pretty impressive.
the 300kgf was already present in the literature before the LF304 existed though - at least as early as the original LF280, but since the surface area of those two is pretty much identical, it's valid for both.
That's what I was thinking too.
S Davis
Solar Addict
- Joined
- Sep 25, 2021
- Messages
- 777
Yep that is what was being used for the LF280N, mine are just under 12 psi. at 640 lbs total across the face.
My apologies - my post was directed to post 242 by Tradewinds62 - I was surprised any of us were taking that concern of crushing Eve cells in a 300kgf clamping fixture seriously (at least any of us that have actually built one
).I appreciate your ‘ recant’ on portions of your earlier post.
For those of us with Eve 280Ah cells, if you read through this thread from the beginning, you’ll see that 12psi is what we’ve all bee aiming for (at 50% SOC - the thread also includes limits for minimum pressure @ ~10% SOC and maximum pressure @ ~99% SOC important to stay above/below (especially the staying under the max pressure) and recommended by Eve through back channels.
Similar threads
