LiFePO4 Battery Compression, Fixture, and Assembly - What's the Deal?
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- เผยแพร่เมื่อ 8 มิ.ย. 2024
- Today, we're going to talk about battery pack assembly. What is compression and is it actually required? If fixture is so important, why don't many commercially-made battery packs seem to have it? What about stress on the terminals caused by normal expansion and contraction?
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00:00 Introduction
01:51 Battery Specifications
04:09 Commercial Batteries
06:23 Terminal Stress
09:30 Snake-Like Idea
12:05 Conclusions
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Ahh man such a bummer that the bus bars don't fit your cells in that configuration!! Fantastic discussion for the first half of the video. I agree with everything you said. Can't wait to see how you rebuild the pack when you find a way to connect those cells. Custom bus bars might be the easiest solution. I am tired of this issue as well. I've rebuilt my 15kWh five times now, and I hope I never have to do it again. Hopefully we found the solution now.
That is pretty funny that we are working on this same issue at the same exact time 😂
@@WillProwse you guys rock ! I've got everything set to my off grid setup besides eve battery from aliexpress battery that I bought in July lol can't wait no more 🙃
Yes
Fortune cells are rated for 3000 cycles and are designed to be bolted together with threaded rod on top and bottom with space in between . They maybe able cycle more with some rubber type material between the cells.
Glad to see you guys are discussing this! QUESTION: With all the connections you guys have to the sellers and manufacturers (and hopefully their engineers), I'd expect there would be some direct clear instruction from these experience experts (the ones that design and test these batteries) to weigh in on EXACTLY what should and shouldn't be done, no?
The fixture vs compress terminology is mostly a translation issue I believe. The basic idea with "fixture" is to prevent the expansion at high SoC. In other words, the cells shouldn't be "compressed" at low SoC, but the fixture should prevent expansion so that as they try to expand they exert pressure (Around 12 PSI as you noted) against the fixture, and the fixture just needs to be strong enough to maintain it's shape at up to 12 PSI.
Regarding whether you need to compress the smaller 230 AH cells: It all comes down to QA and "support". They probably haven't tested the 230 AH cells outside of a fixture so they don't have the numbers to list in the spec sheet. It's safe to assume their life cycle will be affected about the same as the 280s would be.
Regarding stress on the terminal cells: All you need to do to avoid this is to fully charge the cells before assembly and then try to leave as much of a gap as you can. That way when you place the cells together they are already at their maximum thickness. When they discharge they will contract a bit and create an air gap, and when they recharge they will just close that air gap without placing any additional stress on your terminals.
However I agree that longer bus bars are ideal. I like to have air space between my cells so that air flow can help keep them cooler. My feeling on the subject is that keeping the cells cool will extend their life much longer than compressing them would. You can also avoid the expansion of the cells by charging to a lower voltage such as 3.4v instead o 3.6v. Most of the expansion occurs at voltages >3.4v.
Thanks!!
I think Benny and hubertnnn,s comments are going in the right direction. The question is why are we interested in preventing expansion? With out a doubt one is the bus bar issue. You have to stop any movement between those two points to prevent stressing, and potential failure. I don’t think buss bar design is enough.
Second is, what is happening inside the cell. A non bloated cell is going to have less internal space then a bloated cell. The electrolyte on a higher soc bloated cell will settle to the bottom of the cell leaving the top electrode and cathode no longer immersed in the electrolyte. I have turned my 310ah single cell fully charged upside down and I can hear the electrolyte inside moving from top to bottom. When it was at a low soc I could not hear electrolyte movement. I have seen videos of autopsy of bloated cells that failed and there was evidence of dry areas within the cell that had also delaminated.
My opinion is that it is adventurous to prevent the cells from swelling. At all. None. If there is an issue, that little over pressure valve on the top of the cell will blow open releasing any pressure that will be required to be vented. The cell will be destroyed if that happens but I have not heard on this happening to a compressed cell. I have heard it happening to a none compressed cell that was allowed to bloat.
You can see what compression does to a pouch cell that has a dead short put to it. Nothing happens. An uncompressed cell bloats almost right away and bursts.
I have tried to look at the mechanics and science of what goes on inside and out on these cells and I have come to the conclusion that preventing expansion seems to be the most ideal situation for the health of the cells and to prevent mechanical damage of the cell terminal posts while secured by a buss bar.
I have chosen this method on four 310ah batteries I have made for my personal use on my boat and one for a solar system at work.
The debate will go on but I would say, try to look at the mechanics and science of what is going on. I unfortunately don’t think we will hear much from manufacturers because they just don’t care enough. It’s call engineered failure. You don’t want to make something so good you can’t sell it to someone again because it lasts forever. Good luck.
I solved the bus bar issue by making termanal ended jumpers between all the batteries. The wire allows all the flex you want. Takes more time but is easy.
The main question is "why do you need those batteries compressed".
I never seen anyone answer that question, and I believe the reason is to prevent movement of electrolite.
If you let the battery pulsing, it will over time move its internals (catode, anode and electrolite) around reducing the capacity.
Keeping it compressed will prevent that, by forcing the battery to bulge evenly and forcing the elctrolite to move back where it came from
rather than slowly moving to lower parts of the battery.
That is just a theory, but if thats the reason, then its less important how strongly you compress it, and more about how evenly its done.
Also, maybe someone knows other reasons why we should compross those batteries, that I am missing.
It's not to prevent movement of the electrolite. It's to prevent de-lamination of the cathode roll.
It's answered in articles such as this: apps.dtic.mil/sti/pdfs/ADA575499.pdf Or as other have noted the issue is that the expansion and contraction creates micro-fractures in the electrodes (There is one of which attached to both the anode and cathode) which will reduce capacity over time.
But you're otherwise correct that preventing movement of the internals is exactly the point. The more stuff moves, the more it can damage the electrodes and reduce capacity.
@@upnorthandpersonal hello fellow Klein
I had the same discussion on my channel when I build my first battery pack 6 months ago. As you correctly said and calculated you will never use one full cycle on the battery per day. And even further, you will never charge the battery with 0.5C and discharge the battery with 1C at 25°C. Exactly these conditions apply for their test of getting 1000 additional cycles. If you charge/discharge slower, as you would in a stationary solar environment, the benefits of fixture/compression are far less, maybe even zero.
If you really have close to 100% DoD on your battery each day, your design was completely wrong (or you have a very good reason for such a setup 🤷♂️).
I'm a non-compressing guy. My cells are sitting in a bit like a zickzack connection, slightly on an angle next to each other so the normal busbar still fit. If they contract/expand (which they may do under very high amps), they have all the room to do that without adding any stress on the terminals.
However, I would probably fixture them with reinforced tape for a mobile application.
Thanks for the ramble. ;-) I came to the conclusion that home-made heavy gauge multi-strand battery cables with self-crimped lugs (using an inexpensive hammer crimping tool) were the answer to both relieving any concerns over stress on the terminals from movement and allowing for flexibility for configuring the battery pack. My two 280AH 4S battery packs are then held in a battery box with stiff plastic sheets between the cells and 1/4" acrylic at each end. The battery box is narrower at the bottom than the top, so I simply wedged the cells in snugly with dense foam at the bottom and used one 1/4" threaded rod across the top to snug them up just enough to eliminate individual cell slippage while in use in my camper. After one summer of many miles on very bumpy gravel roads they've held up well.
Regarding the fixture in the datasheet, I believe it went something like this: EVE comes with a new cell (2017), they do their tests but perhaps use a smaller version of the cell since cycling 280Ah takes very long. This is what they base their life cycle on. In most applications, such as vehicles at the time, the cells are in a fixture, so no issues. Later on, they come to the conclusion (after more testing) that cell life is impacted by the cells not being in a fixture: test methods got better, analysis got better, and they realize that the cathode roll becomes de-laminated and this is the root cause of the issue (some comment asked why a fixture would be needed in the first place; this is why).
So now to make sure they cover some of this potential liability/correct a mistake they adjust the datasheet. As you noticed, your other datasheet (from 2020) specifies the fixture, and if you look at the new EVE LF280K cell datasheet, they specify it there as well and don't mention using the cell without fixture anymore.
In other words, they might not even have assumed the cell would be used outside a fixture, noticed if one did cell life decreased, so they fixed it with a datasheet update and from then on assumed a fixture in all use cases going forward.
I've been using 32 of these LF280 cells for over a year now. I put them in a fixture (threaded rods), use my DIY braided bus bars (no terminal stress) and put them in an insulated box (cold climate). No issues what so ever...
I was coming to post exactly this. Spec sheets are often among the last things to be updated/revised.
I’ve read this explanation somewhere before and it seems the most logical to me. And the braided connectors always seemed to be a better solution. Not to mention you get better continuity through fine copper strands (like welding cable) than a solid buss bar. Without laminating the buss bar, you introduce eddy currents into the conductor under load which increases resistance. Using stranded wire (the finer the better) greatly reduces eddy currents.
@@Do_the_Dishes stranded is best, but YIKES the cost of making them all. Even at 4 gauge, (undersized imho, I prefer zero gauge) with all the crimp connections it adds up fast.
@@Do_the_Dishes Thats good then, i bought 2nd hand (ex Lead Acid) Heavy duty bus bars, which are stranded between 2 copper plates at the terminal ends. Needed longer bus bars to bridge over bloated 310Ah cells. the 304/310/320Ah cells seem more likely to bulge, or be bulged. Eve must have a lot of these reject newer higher capacity cells being lower grade cells. Often re sold under different brand names. The pressed in studs with 10mm diameter base, i dont like. Were often used on the lower capacity cells, around 200Ah. Where fitted to 304 plus Ah cells. seems to be a sure sign of lower grade cell. it is not a good idea to to try to compress already bloated cells.
@@michaelbouckley4455 So you think that the 12 PSI compression spec is too much if the cell is bloated 2-3mm? I'm looking at compression springs that can handle as much as say 5m (double at the cell level) without exceeding 15 psi and optimally without bottoming out at an even larger bloating level. The arrange meant I'm ; looking at does not compress more than 4 cells under a single spring stroke.
Great video as always.
LiFePo4 : if layers are not compressed, then dendrite can grow more easily in the electrolyte. Primary effect is that dendrite will mecanicaly separate layers, thus decreasing capacity. You can see it : cell if inflating. When you compress it, dendrite will still grow, but way less easily. And because layers are compressed together, they will be less likely to separate around dendrites. Thus keeping your capacity longuer. A difference of 30% lifetime is comon. THIS is the reason why some manufacturers are still using cylindrical cells, layers are naturaly compressed inside.
attach buss bars while batteries are at 100%soc (Fully bloated), then when they are discharged the batteries will create a small gap between them. no stress on terminals
Good thinking but that assumes bloating remains constant.
First : You did a Great 👍 Presentation!
I purchased (20 + 1 for a backup) 120AH LiFePo4 batteries for my motor home. I was somewhat concerned about cell expansion. The Specs stated 0.3 mm of expansion at max. What I did was fastened them together with VHB tape on all 4 corners. I did have to enlarge the buss bar holes slightly to make them fit. Then I used the same reenforced tape that you used to hold them together to ensure the vibrations of the motor home when on-the-road didn’t loosen each 12vdc battery. Yes, I have 5 separate batteries with a BMS on each (limited storage space in storage compartment). Have been using this configuration for 2 years now with only one BMS failure. Never charged them fully 13.9 vdc… and so far have never discharged them below 85%!
Okay, my system is “Over-Kill-Design” but I’m a Retired Electronic Engineer and wanted them to last a lifetime 😜
❤ this video. I've been grappling with the same issue: To compress or not to compress. The pack will be two rows of 16 cells each for a 3P16S config, so the cumulative effect of swelling across such a long row has been a concen. There are plastic caps and feet on each of my 150 Ah prismatics that leave a few milimeters between each cell. What this video and comments tell me is that is about perfect, that if there is more swelling than that the caps will keep the terminals from moving. I'd give this video two 👍👍 if I could. Whew!
As already mentioned, internal cracking of cathode roll makes sense. Also, under catastrophic failure, the pressure relief might fail to work if the cell is unconstrained and allowed to balloon a lot, which obviously could cause a host of issues if wires/busbars/terminals are forced out of position and/or pinched, broken.
Very thoughtful video. I think you are asking the right questions. The complexity of the problem should not be underestimated. I'm very tempted to give suggestions but there are too many unknown parameters to this problem. If the batteries were kept under lab like conditions, instead of real world it would be a little easier.
High tech lab has a link for custom busbars. You are obsolutly right about the expansion and contraction of these batteries. I have to make jumper cable to use as busbars because some of the batteries had expanded enough to not be able to use the regular bus bars. One thing I noticed is the elongated holes of the busbars to give space for the expansion and contraction. I agree these cells need space for expansion. Silicon gaskets between them will help and the experts go along with the majority of opinions since their interest is to sell them.
Nice construction....I pondered for months on how to do this. The problem I was having was designing a fixture to keep a constant pressure on the battery through charge and discharge cycles. Eventually, I settled on the cheapest (springiest/stretchiest) ratchet strap I could find. The strap is completely wrapped around 2 thick plywood ends that cover the batteries at the ends and distribute the force. The straps are cinched as tight as I could get them. I'm guessing I have around 300lbs of force that stretch to follow the charge and discharge cycles.
Hey, nice video and I feel your pain re: cell movement and bussbar stresses. What I am going to do is arrange my 4S cells with the Positives in a line and the Negatives in a line on the other side. My 3 x bussbars will then run diagonally across the pack. This should heavily reduce the chance of bussbar stress as any cell swelling will be lengthwise but the bussbars are at an angle. Hope this makes sense and gives you an idea to try. keep up the videos. cheers
I would just take some 2 gauge or 4 gauge wire
with lugs and connect the cells . This way they can expand all they want and not pull on the terminals. I had to make a set of 2 so I could top balance my 280 amp hour cells cause it only came with 4 bars.
exactly, also you need to have the cables in a slanted fashion so you can maximize the flexibility of the cables, I've seen companies use cables that connect the cells using a slanted pattern between cells like this ///// , if you try to just connect them straight across than cables do not like to flex via compression, they like to move sideways
This
I was going to comment exactly this but you beat me to it. If you are really that concerned about terminal stress, just don't use bus bars. It's not rocket surgery.
@@eksine this is also what I'm doing. 2/0 with lugs overdone but it is what I have on the shelf.
Though I haven’t built it yet, my design is to use the threaded rods with springs. Between each cell, a thin rubber sheet 1/16” thick. Prevents wear of the blue shrink wrap and will allow for slight expansion of the cells. As for connections, I’m using appropriately sized welding cable and crimped lugs in small horseshoe shape. Completely eliminates stress on the terminals. Yes, it’s a lot of wire and lugs, but you only do it once. This way, you have the recommended compression, the ability for expansion without over compressing the cells and zero stress on the terminals. That’s the best design that I have come up with so far but I’m open to suggestions for improvement.
How long are your wires?
Very interesting topic, Will uses the reinforced tape, where Dexter uses VPH tape… I’ve also been torn between compression vs banning vs VHP tape vs reinforced tape… some companies use banning to compress the cells, but most companies use reinforced tape. But I must note that the companies that uses the reinforced tape, are using smaller aH cells. Like 100aH to maybe 120aH. These cells we are using or trying to use are the 280aH to 410aH, which has a lot more energy stored, so thus far the chemical inside is heating up ever so slightly. Just from the atoms flowing back and forth. Where the smaller cells storage isn’t no where near the amount, so the cells aren’t heating up hardly any. Now in saying heating, I’m not stating the cells are even getting hot or even warm, just the chemical is heating up ever so slightly. With my rambling… the larger cells with some compression will hopefully stop the bloating a little. I was thinking on mine, to use some plywood and plastic banning to allow it to move slightly. I have also been experimenting with thin sheets of copper and layering them like a large wire, so it can be flexible. In all, I agree with Will and yourself. The topic is hard to discuss but you have to also look at safety of the cell. Okay I’ll stop rambling now.
I was considering aluminum sheets as spacers however part of the issue would be the self sorting out if there was an issue, So I think I’m going to go with an EDPM rubber used on Rv roofs….
I’m going to put sheet style packing foam in between each cell and then tape them with fiberglass reinforced packing tape. Also I am making my bus bars out of 6AWG multi stranded welding cable with crimped lugs on each end they will flex. Make them in to a u shape before crimping.make them as long or short as needed.
I don't build packs but fascinating discussion regarding compression and its benefits. Thanks for going through in detail this unknown structural factor that can affect lipo4 life.
It is definitely preferred!
Braided busbars or wire+ring terminal. Great video, great topic! Always wondered why solid busbars are such a thing...
I use wire+ring terminals on my batteries, I would prefer solid busbars but only when I'm moving them about.
They are a thing because they are really cheap - and you dont need to crimp large (expensive) connectors.
thanks for the video on this topic, I an in same situation with 16 eve 304ah cells. got 16 more on the way but still thinking about compression...
If you can solve the bus bar issue and still want to put it into a box and have some steady pressure that will provide some give for expansion the Great Stuff Pro Gaps and Cracks insulating foam sealant is 11.9psi foam. It isn't made for high expansion and is just soft enough to not exceed the 12psi rating.
I assembled a small battery pack of the Top Brand Prismatic cells. I put VHB tape on the top and bottom to space cells then added the busbars to studs. Prismatic cells expand the most near the center of the largest flat surface. By spacing out cells it negates (completely or partially) the stress on studs and possible creep failures caused by cell expansion.
Earlier on I used assembled my packs with 26650 and 32700 cylindrical cells. If you don't like dealing with compression you can try the 70Ah SAB 60280 LiFePO4 cylindrical cells, also known as the S168 cells.
Personally, I am a fan of either the copper braid interconnects, or the curved bus bars to avoid stress on the terminals. The copper braid interconnects are definitely the best, but more expensive typically, and harder to find. I have actually resolved that I'll likely be making my own for the next big bank I build.
For fixture, which I agree is a better term, I do like the threaded rod method with plywood backed by metal (when practical, for long series especially) to ensure even application of force over the entire cell bank. I think that spring loading the nuts on one side of the assembly to allow expansion is ideal, but not necessary, as ensuring a suitable amount of expansion permissible in the fixture can be done so that at 100% SoC, the batteries experience a force of approximately what is desired by the manufacturer. Especially if you want to get the absolute maximum life out of the cells, the additional engineering effort is likely warranted.
Low cost, low C packs, unless forbidden by the manufacturer specifically, are likely okay with little to no fixture force, so reinforced tape is totally fine if you ask me. Honestly I see the risk of adding too much force as WAY more dangerous than not enough.
As far as spacers between the cells, personally I feel that really depends on duty cycle and charge/discharge rate. A low C rate (1C or less) is typically fine with NO separation (shrink wrap on the cell can is still necessary of course for electrical isolation), and you can actually just put the cells against one another inside a fixture. Again, because they will expand, this does mean the fixture must expand as well by the permissable amount.
At higher C rates, space is ideal for heat dissipation, and ideally you would have a few sections of compressible material between them, leaving space for air to flow between them. The idea is to maintain pressure, as well as allowing air flow. Tricky business, but as is to be expected, high C rate packs warrant a bit more care and thought.
2 strips of VHB tape height wise between cells and 3 wraps of fiber reinforced tape around the outside. The vhb gives slight room for expansion and the fiber tape minimizes stress on the terminals when expanded at high charge.
The bent bus bars is just how some people make them shorter. (i.e. the "correct" length.) The force necessary to bend / stretch one is orders of magnitude more than those terminals can take. What happens is the module accordions, fanning out at the bottom because the top is bolted together. (broken zip ties prove it.)
Great one sir, thank you.
Instead of bus bar, why not copper wire? They will flex etc. or thinking of the shape of the bloat, the long side is curved most in the center. What if combine your old method with the new snake method? Meaning instead of each battery touching on the smaller side, let them touch on the longer side but only for the 2” to connect bus bar.
Love the thought process. Thanks for sharing
Thank you for your Education
From all my observations it seems that the expansion of the battery is most predominant in the center of the cell. Therefore if you use your expanding terminal connectors and put small 1-2mm rubber spacers on all four corners between the cells, and possibly a small patch in the middle of the cell, t would accommodate the expansion as well as cooling. Any thoughts would be appreciated as I am about to fix 32 of the eve 304.
Part 4:
So anyway, and I apologize for going on into so much detail - but I wanted to try to give you a complete picture of what I’m planning on doing and why. In your case, I have not seen any indication from watching your videos that you are concerned about making any sort of provision for heating these batteries so that they may be charged. BUT - I think if I were in your situation - I would add a minimum consider the possibility of just perhaps even continuing to organize the batteries big flat side by big flat side and provide some sort of compressible spacer between the cells as I try to hopefully outline clearly above, and consider using homemade connectors using lugs and electrical cable between the terminals as necessary.
Then, around the bottom and top perimeter of the battery/the individual cells collectively, I would consider trying to build some sort of frame, perhaps constructed of ‘angle aluminum’ or something like that again with some underlying weatherstripping to protect the cells wherever they might come in contact with the aluminum AND devise some way - perhaps with more angle aluminum top to bottom on corner and/or simply using some sort of woodframe, build some sort of ‘crib’ or ‘scaffolding’ (or ‘frame?’) so that you can pick up and handle the batteries and BMS and whatever as a single unit.
Personally, and again - I just don’t think that trying to worry about COMPRESSING the cells to try to eke out a few more cycles is worth it AND then if you do that things tend to get complicated you have to worry about how accurate is the pressure and how consistent would it be over time and/or what happens if the cells decide they want to expand side to side and they cannot and then start to have to worry about them popping open the pressure relief ‘valves’ or whatever… Too much complexity and too many worries…
As far as I’ve been able to determine, the fact that a single may swell a little bit when it is charged or might even arrived with a fair amount of swelling when an order is received MAY OR MAY NOT MEAN THAT THE BATTERY IS IN SOME WAY SUBSTANDARD OR DEFECTIVE. As far as I have been able to determine (and I did a fair amount of checking around), the fact that a cell might be swollen even when one receives it does not necessarily mean that it is bad or unsafe. That is not to say that it might NOT be a grade B cell BUT it is also quite possible that it is still a Grade A cell and in any event, does not appear to represent any kind of safety hazard and the cells can - as long as they’re not leaking - be used safely.
In closing, I will just finally mentioned that when my cells arrived in I expected them, ‘to the eye’ they were perfectly flat when one inspected them visually looking down across the big flat sides.
However, if I laid a steel ruler/straight edge across those sides, you could actually see some ‘waviness’ across the flat surfaces - steel straight edge revealed that the cells were bulging ever so slightly ROUGHLY vertically down each side where one might imagine a pole or electrode or something might be running (even if it might be a wrapped pole or something) ACCOMPANIED BY a slight depression between the two ever so slightly bulging surfaces/waves.
When I stacked the battery side-by-side out-of-the-box, they fit together perfectly/flatly for all practical purposes.
However, after they had been fully charged up, I would say that the sides of some of the cells might have expanded by roughly a millimeter or so in total (adding up the bulging on both sides) AND SO that if you happen to put two of the cells together, side-by-side, you could wind up with a 2 mm or so gap at each vertical end of the cell and/or if you sort of rotated the cells around bulge, in order to get no gap on one vertical side, you could wind up with a 4 mm gap at the other extreme side!
So, in the end, I simply decided I’m not to worry about this stuff. There is an old expression that one can become ‘too anal’ about something… Worry about things that don’t really matter much at all in the grand scheme of things, if in fact anything at all.
Anyway, hope I’ve given you some ideas. I would personally just go with the “Boxer Shorts instead of Bikini Briefs for the cells; give them room (restrained room) to boogie and dangle/swing free however they might want to and just possibly consider making up busbar substitutes out of wire and lugs like I am.
Hope my thoughts help.
Cheers.
7:19 As you are talking about 1 mm movement I think there is plenty of space for the rubber padding to take up the movement.
If there is 1mm expansion and the rubber takes up 0,5 mm there is only 0,25mm movement left for the connectors to take. Using washers the way you do and the fact it is only on 1 side of the battery, I would expect this to be fine.
You do have to assemble the batteries when empty so the rubber goes into compression.
But I agree "bend bars will take on some of the force"
Fascinating Thank you
Is there a reason you heavy gauge copper welding cable like most flooded lead acid batteries use? Definitely carry the amperage and they have flexibility.
Great video
EVE seems to have updated their spec sheets to include more information about compression force in detail. General consensus regarding compression is that it reduces delamination specifically in high current applications, and as stated increases the life cycle of the cells.
I heard lock washers tightened down to suck the washers flat might be the 12 psi thanks for you’re input ! We
When I built my pack I made my own bus bars out of flat copper bar, and I used double sided 4mm thick rubber squares in the corners then tightly wrapped in heat resistant tape. That allows the cells to expand without pushing the cells apart , the terminals don't move that way
My 280ah ones I really didn't compress them although they are in an aluminum tray with top piece that is pretty tight fit (kind of like the battery you had there with the plastic piece top and bottom) and this is with some foam in between them. I've don't some short videos of my use for them I just don't see the worry about full compression.
Nice video with blue bricks ;) love it.
good video ty for doing the work
Could you use the appropriate sized wire with a ring terminal that is long enough to be flexible instead of a bus bar that would stress the terminal mounts when the cells expand?
This too was my immediate thought.
I think so. I am going to make flexible links from copper braid to replace my solid aluminium bus bars.
I mean just a thought, but if they know they expand, and they put them together at a low state of charge, once charged wouldnt they expand exerting force? any idea how much force they exert? would be something nice to figure out
The thing to do is just use a padding with the same compression strength as the tolerance stated 12 psi surrounding the cells giving it some room for expansion then going back to the 12 psi as it discharges.
So what you’re saying is go to zero SOC put in a rubber pad with a 12 psi compression material between the cells, , then take the pack and Compress at 12 psi ?? and the way you go?
I want the most out of my bats. Allowing them to expand w/o compression causes internal separation of the anode and cathode from the electrolite. Thus a reduction of 1k cycles. I like that reinforced tape idea. Also, the expandable buss bar. Oh, reducing the SOC is also smart.
Thank you for making this video it was much needed! I watched wills video and didn't find it clear on what was going on or why. This was great!
I would use sticky back neoprene. Add it at the corners or strip along the top edge to create the gap.. The batteries only expand in the middle. That will create the correct size gap avoiding stress on the terminals.. Then you can compress the full pack pack to spec… but then you have the problem of having no compression at a low state of charge.. does it say that the compression has to be constant .. ?
VHB tape strip on the top and bottom of each cell. Gives them room to expand / contract without stressing the bus bars.
Great video, I wish you would do a video on "tips to buy GOOD prismatic cells on Ali and ALi Express"
Car audio and hydraulic trick for buss bars, soft copper tubing.
Hammered flat, cut to length, then drilled for stud clearance
I have the same issue, I’m thinking of cross connecting the cells without rotation in. This would have an accordion style connection so expansion and contraction would present to the terminals as rotation, not linear expansion.
For a simple RV setup of a 12v 304a EVE Setuo i might make cable and terminal flexible bus bars - only nee 3 - and use reinforce tape and flat nylon package strap with corner edge protection? Not really to compress but to hold together hopefully in a battery box - Plus wont be over stressing either 👌
Thanks LS. Great conversation.
It seems to me we are conflating two aspects of the purpose of the fixture.
I understand that in general, we are attempting to restrict the state of charge expansion of the cells in order to stop cell bulging.
Why are we attempting to stop this bulging? Is it to stop lateral terminal stresses by the bus bars or to somehow influence the chemistry/internal structure of the cell, or both in order to increase cell life cycles?
I think the primary reason is to limit the relative movement between cells to reduce the stress on the terminals from tightened bus bars. I can easily imagine stress on the terminals reducing the cell life.
Perhaps there is an added advantage for the life of the cells by restricting this expansion due to SOC, but I am not aware of this being a factor, nor can I imagine a mechanism. (Not to say there isn't a mechanism - I just don't know of one). It is possible too that such restriction of SOC expansion/contraction may be detrimental to the life of the cells. I just don't know.
In any case I think we can agree that lateral stresses on the cell terminals due to bus bars should be reduced as much as possible.
The only solutions I can think of is running looped, suitable gauge cable from one cell to the other, or (the option I will adopt) is to make bus bars in two pieces - bolted together in the shape of a flattened "V". This will allow relative movement between cells and allow cell expansion to occur with minimal stress on the terminals.
For me personally, I am additionally compressing the cells in a fixture because my application is mobile. I have to protect the cells against vibration and violent movement, so I have to be able to secure them well to the vehicle.
By the engineering definition of fixture, compression is not the goal. A “fixed” position is the purpose. Prevent both gradual and sudden movements. Designs that don’t compress are engineered correctly.
The fixturing (compression) is to keep the "stack of cards" properly stacked. Over time, if the plates separate as they do during cycling, the capacity, and entire functioning of the cell begins to fail. If you've ever seen pouch batteries (RC, phone, other cheap crap) slowly becoming a balloon, you've witnessed this effect. The more they're cycled, and/or the higher they're loaded, the faster this happens. These types of cells are basically pouches in a weak "beer can" shell. The more expensive CALB cells have stronger cases, so they don't need the same containment.
@@jfbeam Good to know - thanks.
@@jfbeam Containment is much better description than compression when talking of fixtures in the world of fabrication and engineering. "Stack of cards" is a good analogue to the term fixture, in that the stack is efficient and reproducible. The object or workpiece being "located" in a position to draw on the formal definition. This term fixture in the data sheets accompanying the cells means located in a position. Compression on the other hand, is a term that has come from the DYI language found the media world such as TH-cam.
Sorry for so many comments, but How would you make this setup work in a mobile application? The movement of the cells would be worse bouncing down the road not being fixtured.
I'm thinking best to fixture with end cap type spacers. Strap down and surround w foam. Everlanders yt channel shows this. I would like to find an elastic spacer material w 12psi compression strength. Need material data
Hi. You'r right they must have space in between otherwise studs mite break out. Good luck with that.
I've been racking my brain over the same issue and after reading all these comments and also the DIY solar forum; I still don't know what the answer is for sure. Where I'm at now is: 280ah cell to compress to 300 KG force which 660lbs. Use 3/4 plywood at end of the battery with 4 3/8" rods torqued to 12 in-lbs each will give you 165 lbs clamping force/rod. To deal with the terminal issue, the busbars that came with cells are about 20 mm wide and 2 mm thick = 40mm2 which I figure is about a 1 AWG wire so I will use 1 AWG wire with lugs as "busbars" . I'm not trained in electronics but reading and asking to learn.
@Keyzer Soze Thanks so much for your response this is why I'm asking. Could you be specific on which numbers don't add up? Is it the force that I'm applying of 12 in-lbs to give 165 lbs? Is it the 1 AWG wire as an alternative to 20x2mm busbars? Again I appreciate your feedback and learning as I go.
I think MOST of us are stuck with this issue. I suppose another option, which would be expensive is to actually use cable and terminals instead of bus bars? That way they could have some flexibility if long enough. Otherwise where can we get custom bus bars? I also am kicking around the idea of taking a $ store silicone hot pad ( for kitchen ) and cutting in strips to put along the top and bottom between the batteries, may take some layering, but cheap and more heat resistant.
make a video measuring diff cells ar diff soc please
Thank you I have 3.2v 280A from Aliexpress !
I am thinking similar to how your store packs were…
Expansion is always in the wide side, and always from the center of the cell.
So, putting a strip of VHB tape, on the top edge, and the bottom edge, leaves 1mm space in the center.
Then placing reinforced tape around the pack should protect the terminals, and protect the cells.
If you look at bloated cells, you will note the bloat peaks half way down, not at the tops and bottoms. That makes sense, as they are boxes, so of course the tops and bottoms are fixed, until they might literally burst. I would put some fairly thick (maybe 1/4"-1/2") rubber or heatproof but compressible material in a strip at the top and bottom inch of the cell. The snake idea is actually used in some pre-made batteries, but they often use a purpose made plastic grid to space them. Many of them also use aluminum welded busbars. Aluminum is cheaper than copper (but is not so easy to treat against oxidization, so simply wrap it in heat shrink tubing), but has a higher resistance. Resistance falls with thickness, so you use a thicker bus bar if using aluminum. Let us say you have a 10cm x 2cm x 2mm copper bus bar. It has a 0.0004195 ohm resistance for DC current. The same shape aluminum bus bar would need to be 3.2mm thick to have the same resistance. Curiously, that is the thinnest stock bar aluminum available near me! Not a bad idea, but do remember to use aluminum nuts and washers. Then, at the sides, you would want thicker matting, on all four sides. I am building a battery right now with 16 REPT 280Ah cells, in to a 45 gallon plastic truck under bed storage container. It has a lockable hinged lid, the sides are square (and not tapered like a Lowes Tote box would be), and the sides are castellated in profile, making them very strong. I would use 1" Kapton tape to wrap it up.
You make perfect sense.
Use flattened copper pipe flattened and bent at 90 degrees vertically and flatten and drilled horizontally for the terminals. Cheap and includes strain relief at the 90 degree bend. should still be round between vertical and horizontal flats.
As for compression/fixture, use mass on a 90 degree lever to push in the centre of a 25-32mm ply wood board. You can adjust the mass to equal 12psi and match EVE's recommended pressure. Do it at each end.
You can order longer busbars that are 12mm longer than the standard ones. They fit when connecting the cells top/top, and not sideways.
I did threaded rods with two 3/4" boards at the end of my 12 volt packs. I just used the split lock washers to put constant pressure on my packs. Just don't wind them all the way down. Also I will only charge to just above my BMS balancing current to minimize expansion. If four cells of the 280AH will expand basically two mm, that really isn't a lot. Two split lock washers on each end that have 1 mm of clearance each should prevent catastrophic expansion, will apply some pressure, yet allow the nuts to stay engaged. I'm sure not charging them to their limits will prevent expansion to around 1mm or less, just a guess without actually doing experimentation. That expansion distance shouldn't put too much pressure on the terminals, with probably only .25mm expansion each. Likely there is enough bend in the terminals and busbars to take up enough slack to not damage something.
I tried to solve all your concerns cause I had the same thoughts.I took 4 pcs 4mm steel rods, bend them in to a slight zig zag shape and threatend them at the ends. I tighten the M4 nuts on the end plates with a torque of 0,565 Nm what results in a total force of 300Kg. Previous I glued vertically many 4x8mm natural black rubber stripes (I cut them from a sheet) 16mm from one each other. The busbars I have made by my self with cable and two cable lugs. I make each connection twice and took 16mm2 marine grade copper cable.
The fixture makes that tiny little air bubbles, which are left from production, can escape at the first few cycles.
Just cut your existing bus bars with angle grinder extending holes to the end of the bar, so it becomes like a two pronged fork.
Get Neoprene Foam 1/8 sheet put in between cells, use 4 threaded rods with HDPE board 1/2. At the end of each(8 total) rod place big rubber washer/bushing> big metal washer> nut. Hand tight everything, charge batter to full. Measure sq inch of battery face, multiply by 12lb torque ( if that’s what it’s called for) divide by 4 and tighten your rods with torque wrench.
Great video, and I feel your frustration as I am wrangling with the same thing in the design of my battery. My primary concern is stress on the terminals, and the solution that seems clear to me is probably the most expensive, heavy and annoying to implement. Bascially, each cell requires it's own enclosure that is fixed in place within a larger enclosure. So each cell has a compartment on all sides but the top, and is sized to be within the spec of the expansion of the cell. This would allow the terminals to remain centered within the cell enclosure at any state of charge (or with far less travel than in a conventional compressed system). If that is acheivable then your bus bars can be attached to the terminals and nothing should move, and the only physical stress would be on the individual cell enclosures. The downside is a big one though; the battery case would be a lot heavier and a little bit larger. Also much more expensive. This would also be pretty difficult to DIY since it would need to be spec'd to the cell your using, and molded plastic or steel constuction would probably be best. I expect we may see things like this in the next few years as hopefully there are fewer standardized cell sizes which would make something like this possible. Then again this is a product for the DIY market and many manufacturers would rather spend time designing a battery rather than helping you save money designing your own. Which is unsderstandable. I think I'm actually going to go with the SOK 24V batteries if they ever come out since I'd rather focus my energies (and money) elsewhere and I like their design. Thanks for keeping the converstation going in a thoughtful way. I look forward to more of your videos on battery design!
Why not just use wire as the bus bar?
Bit off topic but could it be feasible to build a power wall using cordless power tool battery as they are no tacking apart but clip them in on to a board with connectors just like putting them on the tool, it seems the simplest way to build off grid and plus the battery has a warranty. I don't fancy having to buy second hand and then doing capacity tests and soldering, I'm not lazy just not an electrician.
Like a commenter said below, charge to 80%, then limit further expansion with end plates of aluminum or wood with threaded rod. Tighten your bus bars and your done. As the cells discharge there may be some space between them, that's fine. No worries about cell terminals being damaged.
Much of the worst bloating occurs at higher levels of charge. So don't charge to 100%. Charge to 90%. The expansion pressure developed from 80 to 90% is negligible. Even from 80 to 100%, very little pressure is developed. I have the same bloated cells that Will has, and this has worked for me.
There must be a reason manufacturers say to compress or i.e., limit expansion of cells.
hi there!
I have the EVE specs for a 304 Ah lifepo4. I reread that manual a bunch of times because it does not give anything that will directly convert to psi. it is given in Newtons but not per sqr meter or other given area.
Except that they give the area of the cell. what I gleaned from from it was that the pressure they want on the cells is about equal to a cup of coffee per cell. .6 or about 5/8 lbs per cell. In other words just take up the air space between the flat part of the cells. However they want 3 6 mm bolts on either end of the cell. 6 bolts all together. They want a 7 mm aluminum splint to support it.
I am building 2 16s batteries.
I made 4 splints. The EVE busbars are laid out to fit side to side but they have a gap on the ends to accomodate the thru bolts. My lay out is 4 by 4. 2x8 would have been alot more aluminum. the more layers of battery the more expansion. I settled for 4 layers. I am using plastic separaters between each cell as recommended by the spec sheet..
I've said it before if one is dedicated to build a fixtyre air bags at the end will provide even force but that will def take more time than it's worth. Though batteries (cells) are a sizeable investment they're only getting cheaper and better technology is not far enough away to worry about a small amount of degradation. So I agree. Many many more reason to not bother with more than tape. Good luck and I'm sure you'll figure it out. Don't worry about expansion in my opinion.
I think that different manufactures don’t compress cells, bueacouse they don’t have incentive to do so. For them, after warranty, dosn’t matter how many total cycles you achive - thay waiting to sell another one ;) There are also potential failure mode when terminals are in stress from expansion. Lastly, there are mass production issues - is faster to glue cells together ect.
So if i would build battery, i would use torque wrench and, threaded rodes and plates to compress them and omega-shaped bus bars to connect them to achive 10+ years set… cheers! :)
Yeah, exactly
Can anyne please explain? What is the point of compressing, if in this case eventually it will most likely lead to blowout safety valve on the top of the cell (instead of inflating the cell)?
I'm building a 72v Tracker EV battery pack with twenty four Eve 280k Lifepo4 batteries. As suggested by 18650 Battery, I'm going to use 1" double sided tape at top and bottom of each battery and then use fiberglass tape to wrap outside. The 1mm foam tape will allow for the .5mm expansion/contraction in the middle of each battery. This will solve the bus bar issue as well as fix the batteries in place which solves the wear issue between batteries. I also plan on welding a 1/2" angle iron steal frame to go around perimeter so I can bolt the whole pack to the floor. Eve said their class A batteries do not need to be compressed. Maybe using the term "fixed" in the specs, they mean just that and not compressed at all.
After eight months of usage in my van, I will actually modify my two 280A batteries to add a fixture, not for the extended life, but mainly to prevent the cells from moving and better withstand the vibrations. I have had to go back and open them twice to tighten serrated nuts that had gotten loose. I already have insulating pieces of polycarbonate between the cells, but I intend to add spacers for cell expansion.
It really is important to check the "tightness" of the fasteners! Heat cycles will make them loose. Loose connection= bad.
Great video, appreciate your work. I personally think there are some key points missed. First point missed was compression in fixture. It clearly stated at 100% state of charge pressure should be 300kgf. Then rested for 60 minutes. Then discharged to 2.5v, At this time the capacity is changed to only charge to 80% I do understand how much these things cost. But to compare your 16 cell to 4 cell setup is not enough. I personally just purchased the Seplos Mason 135 16 cells from their DIY line. Epoxy sheets between the cells, all cells under compression within the fixture. Look at their video as to how a professional battery supplier tackled these issues.
For me in the future I will just have a fixture made add the Seplos BMS and add capacity.
Video???
Try building an enclosure from bosch rexroth profiles(or similar). Planning to build whole powerwall from it.
I suggested (2) 16S batteries in parallel a while back. Less hassle with the bus bars.
You do lithium battery. I have done this wiring diagram with AMG and AMG from many forms and combination. I can not afford the lithium batteries you use. This wiring diagram has had amazing results. Mega proud. I going to shair it with you in hopes just maybe you mint try it and see what results you can get. Google says. Any trolling motor. Last on an charge for 5 hours or less. I am guessing that's off an 100ah battery. My two battery hourglass configuration off of 2 20ah batteries. The lowest gade and over 10 years old but new trolling motor with brushes at 30% power. I stopped the none stop running test in an 5 gal bucket of water. At 12 hours. Wiring diagram. 2 batteries. 12v system. Positive to positive. Negative to negative. Off decharge. I battery positive. The other battery negative. I do have an 20 watt solar panel that is self contained on the system. The wiring is opposite of the two other wires. Yet in shade and clouds. Low watts. It did not count in my book. More then two batteries wiring diagram. 12v system. Positive to every positive. Negative to every negative. Decharge. All positives to an collection point. All negatives to an collection point. Then 12v and converter feed off of collection points. Same as solar. All positives to another collection point. All negatives to another collection point. Then the solar leads into from the collection point. One battery system has many different sizes of AMG batteries and a 200ah gel plus the other two batteries. I can stack as many batteries as I want in any size and rating. Amazing what one can do with 12v. Yet now my curiosity is can you do this with lithium batteries. What will be the effects. With mine. I took many different batteries and made them into one single battery. If you ever get some time. Could you please test this out. Thanks.
Thank you I have 280Ah batteries I’ve seen to use a cookie cutting sheet cut down between the batteries to ground the batteries
Seems like a woven copper strap would be the ideal busbar?
Use epoxy fiberglass FR4 sheets on the outside, and 1-2mm EVA foam tape between cells and a light clamping force, maybe the 12#. Use the pre-bent battery terminals. That combination will last for many many years.
Surprised no one makes fixtures/fittings/battery holders to use.
Seplos sells kits, which used to include one for their server rack battery with inter battery spacers and battery clamping.
They must get their battery clamping parts somewhere.
From this video I have concluded... that Will is spying on you and knows what kind of video topics you are coming out with next 🤣🤣
😂😂
Proof 👆
Next week on "As the World Turns" ... dun dun dun Bigger fish fellows; move along now; When these guys grow big breasts and a much higher estrogen count, then, and only then do they become interesting.
@@EdwardTilley you made my day. Thanks
While I assume you've said this in jest, I think it is GREAT that so many of those who've been working with batteries review each others' videos - and comment. We, the battery consumers, get the BEST information from most of the best around!
My *three years* involuntary experiment result on somewhat bigger RC pouch *lipo* packs, exploited daily. Over 1000 cycles from 3.3 to 4.2 but with conservative rates of charging and discharging.
Standard RC pack with soft tape and thin plastic insulation. Just a tad larger than average, 8Ah cells, charged with a maximum current of 3A and discharged with a 40A maximum load. Never exposed or allowed to reach over 40°C
The packs were held in a case that provided some level of compression but uneven, leaving one cell only held by it's edges. I only wanted the packs to stay in place and not rattle in the case when vibrating.
The outermost cells swelled noticeably and their capacity reduced
I went the custom bus bar route, 3/8 by 1 1/4 aluminum. If i did the math right, hope i did it is the same as 4/0 wire. then used noalox on all the joints. Time will tell if this was a good idea.
Why not flexible copper cables like traditional batteries?
use that home Depot 14 ga flat bar with 3/8 holes in it it's around 12.00 14.00 per 32 36 in long flat bar cut with grinder for length of bus bar needed
very cool...
Cap tape? Thumbs up and subscribed! I always wondered this? Data is excellent! Deductions excellent! >Gratitude! Nice Will reference! Bro, what 48v BMS u like?
660 lbs of force. Do it! Prevents delamination and provides cells with the proper mode of failure (the vent on top), would such an event need to occur...
I bought 3/8 x 1 aluminum stock and made all custom bus bars. Works great!
What’s the current carrying capacity that?
@@rorymakesstuff by my calculations 200 amps. I’ve pulled that and they stayed cold to the touch. I actually started finding other problem areas like my main bus bars and some other connections. We’ve built a few batteries this way, I’d recommend it.
What size cable should I use to make some flexible bus bars? for the 100Ah 8S 24Volt.
I typically follow the NEC recommendations for home residential wiring in my application.
www.usawire-cable.com/pdfs/nec%20ampacities.pdf
I use plastic cardboard between cells, wood at the ends and ratchet straps. Also use 35mm2 instead of busbars