Pack / Cell compression Optimized By Using Springs.
- Thread starter Bob B
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Has anyone considered gas tension springs? Basically the "pulling" version of the two gas springs that hold up your minivan trunk. Gas springs have nearly constant spring force compared to coil springs which have a linear spring force versus displacement. This might cost more than coil compression springs.
As I have iterated through multiple designs for compression trying to balance space constraints, it occurred to me that I wasn't fully considering the behavior of the plates. Specifically, if you put your constraints in the right location, and chose the plate thickness and material carefully, you can rely on the flex of the plate to provide some amount of "spring" compression while ensuring you don't exceed limits. This won't give you compression at low/zero SOC, but it's better than nothing and will result in a cheaper, simpler, potentially more space efficient containment at the expense of precision. I'm going to put a little more thought into it. Latest iterations (without this new approach captured, but it does have effective spring rates for different plate materials/thicknesses):
github.com/natecostello/van_two_point_oh/blob/master/electrical/battery/Battery_box_design.md
github.com/natecostello/van_two_point_oh/blob/master/electrical/battery/Battery_box_design.md
Bob B
Emperor Of Solar
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That's interesting the way you did the spacers .... can you post a picture from the top please.
Hello,
I got 16 cells from Luyuan and now I want to assemble them.
I have thought about compressing them to hold more charge and discharge cycles.
I have seen someone who directly uses a rod and squeezes the batteries on each side.
Does anyone know how many Nm I should put on the torque wrench to tighten ?
Thank you very much!
I got 16 cells from Luyuan and now I want to assemble them.
I have thought about compressing them to hold more charge and discharge cycles.
I have seen someone who directly uses a rod and squeezes the batteries on each side.
Does anyone know how many Nm I should put on the torque wrench to tighten ?
Thank you very much!
Hedges
I See Electromagnetic Fields!
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You need to start by calculating the force you want to apply to achieve desired pressure of area of the cell,
divide by the number of threaded rods to get force per rod
look up a torque to force calculation chart/web page and enter thread size, thread material (or coefficient of friction from another table), considering whether it will be dry or lubricated.
divide by the number of threaded rods to get force per rod
look up a torque to force calculation chart/web page and enter thread size, thread material (or coefficient of friction from another table), considering whether it will be dry or lubricated.
I'm convinced, and I'm building a very small building a very small battery here. I'm using 3/16 steel compression plates, rather than fire-prone wood, because my small battery size (120Ah, 12.8V as 4S) makes weight mostly irrelevant. (And the "bending force" resistance of those plates will be extremely high.) In contrast, storage space within the "battery compartment" of my RV Trailer is quite limited, I don't have a lot of room to mount longer springs (in order to make the compression force "more linear").
Now for my question: my spring size will be unique, as nearly everyone else is using larger cells (needing much more compression than my 270 pound total). But should I maybe "author" a Group Buy at The Spring Store, so that everyone can choose exactly the springs we want to get - and meet the $40 "minimum order amount as a group? I'd re-distribute the springs to their prepaid owners via Priority Mail "small boxes". PM to me if you would like to take part, and I'll create a "group buy" if I get at least 3 PMs in the next few days. Please advise, in the PM, of your COUNT of Springs, and the Spring Store URL for the specific spring you have chosen. (e.g.: The URL for the springs I would like to use, in quantity 4, is: https://www.thespringstore.com/pc102-590-7250-mw-1602-c-n-in.html -- even though they are NOT very linear at this extremely short length.)
Now for my question: my spring size will be unique, as nearly everyone else is using larger cells (needing much more compression than my 270 pound total). But should I maybe "author" a Group Buy at The Spring Store, so that everyone can choose exactly the springs we want to get - and meet the $40 "minimum order amount as a group? I'd re-distribute the springs to their prepaid owners via Priority Mail "small boxes". PM to me if you would like to take part, and I'll create a "group buy" if I get at least 3 PMs in the next few days. Please advise, in the PM, of your COUNT of Springs, and the Spring Store URL for the specific spring you have chosen. (e.g.: The URL for the springs I would like to use, in quantity 4, is: https://www.thespringstore.com/pc102-590-7250-mw-1602-c-n-in.html -- even though they are NOT very linear at this extremely short length.)
technerd1988
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foam is the way to go in a mobile application for sure. Thats what I did. I have such an uber low C rate though. I draw at most 200 amps while Im cooking and I have a huge bank, a little more than 1000ah with a bunch of 4s packs
zzyzx
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. . . and your recommendation for the type of foam to use is . . . ?
technerd1988
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I just used the kind you can get in a can at home depot or walmart....etc. I then sprayed the bottom of the box and used a piece of flat metal and evened it out. I then put each pack that I just taped together in the box and filled the voids with expanding foam. I just used cheap noco battery boxes from walmart since I dont have to ever look at em, and with the foam the things are solid and let you route direct leads through the sides because I don't like to do the double terminals thing, I prefer the negative to come from the bms and connect directly to my busbar along with the positive. I think its less failure points this way too. I've had no problems and I like to take back roads with lots of bumps. To be fair though, my converted box truck RV does have HUGE tires and its a dually ford from 1985 so nothing ever even moves where the batteries are. the suspension is super overkill for my use which is awesome
I've snipped your pictures of the 4S sub-assembly from this two-question "query" about it. Your new and bigger picture is way different, but I'd like to get your advice concering a small 4S battery which I am building (wth compression):
#1 You're using double layers of 1/2" plywood to handle the compression, and maybe to add some kind of additional "spreading" between adjacent cells. If my 4S cells will be free on the narrow and tall sides (not the bottom), do you think that I may simply put thin film or foam between the cells, avoiding all the extra insulation which the plywood added between cells?
#2, I've got some 3/16" steel plate lying around, of nearly perfect size for the bigger faces of my small (120Ah) cells. There is enough extra room to drill the corners for my spring-loaded compression rods. But it will be hard to attach to a nice base. The ONLY advantages of using steel plate are #1, it will have less bending and twisting when my rods apply compression forces; #2, the faces provide a good surface to conduct heat; and #3; they are fire-proof. But they are also conductive to electricity, and MUCH hard to attach to any outer case structure. Do you think I should use my plates, or switch to all-plywood?
Thanks in advance.
Im sorry, Im having a bit of a difficult time visualizing your description and I can imagine a few different ways you might be trying to describe. Would you mind posting a sketch?
Just like this earlier picture of yours https://diysolarforum.com/attachments/20210311_203444-jpg.40662/, but maybe using 8x8" metal plates on the ends (and probably skipping 2 layers of wood between adjacent cells). Wondering mostly whether wooden ends 1" plywood or 2x 1/2" plywood are "just as good" as 3/16 steel. Just your opinion, whether my prospective extra drilling trouble (for 4 compression rods) would have any value at all. Thanks in advance!
Just to clear up a little miss understanding. There isnt any double layers of plywood. In between the aluminum square tube from one side to the other, there is 3/4" plywood end plate, 3/4" plywood strip that is 3/4" wide, a cell, 3/4" plywood strip, 1/4" plywood, 3/4" plywood strip, a cell etc
Your cell arrangement is more like this?:
3/16" plate feels a bit thin to be used straight up if you want it to stay perfectly flat but would probably be perfectly fine. Id probably still stick an HSS from side to side. basically everyone skips the plywood intermediates that I used and some use nothing at all and so far they seem to be doing ok. An extra layer of foam or film wont hurt anything.
I ended up going with 3/16 steel, but not exactly for the rigidity. Based on some math, I can rely on the flex of the steel instead of using springs to provide compression in the general range we are looking for.
Current design is here: github.com/natecostello/van_two_point_oh/blob/master/electrical/battery/Battery_box_design_final.md
Concepts leading to it, including some comparative calcs on bending of various material thicknesses that may be of interest is here: github.com/natecostello/van_two_point_oh/blob/master/electrical/battery/Battery_box_design.md
WOW! I Love that you did all of the math for me. (And I would be incapable of applying the concepts and formulas with your skill, your effort didn't just 'save my time'.) The 3/16 plate will have a lot less flex than 1/2 plywood, probably being comparable to 3/4" plywood - but it also has the advantage of avoiding structural collapse, when the compression bars are mounted directly on the plate (with only a washer to spread the force of each spring onto a small area). With plywood, the outside face is likely to collapse underneath the washer. Your aluminum bars remove that problem, but at a slight cost in increased size. only at the cost of
Changed my mind (back to the steel plates), and getting out my box of diamond-tipped drill bits. Thank you, your work was wonderful!
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