Pack / Cell compression Optimized By Using Springs.

[Frick]

I’m just curious, I read through the whole thread and noticed everyone seems to be using the springs on the outside of the batteries over the rods and dealing with flex of the end plate, or building heavier to mitigate.

My my first question is why not make the fixture rigid with compression springs inside against the battery? For example, if it’s a 4 battery sandwich, it would be 5 batteries thick with a piece of plywood or metal against battery 4 and springs between that and the end of the fixture. Maybe something like a coil spring mattress if you will.

second question, I saw gas springs mentioned, but it seems creating a rigid fixture and using an air bag to a calculated pressure between battery and rigid fixture would also work. These air bags are commonly available for aftermarket suspension upgrades. while air is less permanant, it is also easy to adjust and work with. For that matter a small cluster of tennis balls between rigid fixture and batteries (using a rigid plate in between to protect batteries from localized forces) should yield the correct pressure.

In both examples forces are placed where they would directly resist battery expansion and not off to the edges relying on rigidity of fixture end pieces Which seems to me to stress out the edges of the batteries and not fully compress the centers where they would flex the most.

I’m enjoying the discussion you all have going and have seen some very nice prototypes of fixtures. Neat stuff.

 

[rickst29]

I’m just curious, I read through the whole thread and noticed everyone seems to be using the springs on the outside of the batteries over the rods and dealing with flex of the end plate, or building heavier to mitigate.

My my first question is why not make the fixture rigid with compression springs inside against the battery? For example, if it’s a 4 battery sandwich, it would be 5 batteries thick with a piece of plywood or metal against battery 4 and springs between that and the end of the fixture. Maybe something like a coil spring mattress if you will.

Frick, I'm not one of the main experts on this subject, but I think that I can answer this question: The walls of the battery cells are not very thick, and have much MORE flex than our compression plates. Your scenario is just like ours, except that you're ignoring the unmentioned face of the fixture against battery #1 (which needs to be just as good as the first piece of plywood, for OPPOSING the compression forces which remain present on the entire face of the first battery.)

Your 'end of the fixture' at battery #4 has one compression plate, and simply uses the end of the fixture in place of our nuts and washers. Nuts and washers alone are strong enough to handle the entire TENSILE force of the spring, trying to decompress. You are apparently proposing to use the fixture body, to handle this opposing force. Washers and nuts are both tunable AND smaller.

With a "coil spring mattress" scenario, you still need something like plate against the 4th battery (to spread around the force of individual coils). But most importantly, your scheme depends on the body of the "fixture" to handle all the opposing tensile force as well as the standard design, just 4 or 6 spring-loaded rods handle opposing tensile forces, coming from the compressed batteries and into the plates. In your one-sided scheme, AFAICT, the "fixture" is required to handle these forces through its face, corners, and sides - and its not tunable. I feel that the standard design is better, requiring significant strength within only the two end plates and the tension/compression rods.

 

[Bob B]

The idea of using springs at all is a little .... out of the box .... If we can find alternative methods for applying the force, that would be a good thing.

 

[Frick]

perhaps i didn’t properly describe what I am picturing, and I lack the ability currently to articulate it in pictures.

In the standard design I’m seeing used, the forces are applied on the outside of the battery, the corner of the battery being the strong resisting structural member, which Acts as a fulcrum deforming the end plate and releasing tension On the center of the span, which is the center of the battery. in any clamping situation I have seen, you can crank down on the bolts on the outside of the fulcrum and watch the plate bow away from the piece you are trying to clamp.

This can be overcome by the end plate being more rigid (thicker stronger better etc...) to a point, but flex it will. a dial indicator is a great tell tale on this.

My inclination is to put the forces where its required in a direct straight line. It would still be tunable as you would still have your fixture made of threaded rods. The springs would take the brunt of the inherent flex from the end of fixture and the piece used to protect the battery might likely see uniform force across its face.

that Said, this is a bit of building a better mousatrap, and there is likely infinite ways. You could be all ghetto about it and just slap some wood on the ends and wrap it with a couple rolls of tape at a lower charge state and have possibly nearly the same effect... or some wraps of heat shrink film and hit it with a heat gun.

 

[rickst29]

The idea of using springs at all is a little .... out of the box .... If we can find alternative methods for applying the force, that would be a good thing.

Well, you could always just crank up the nuts until you "feel" that you have decent compression. But, that would have less compression at lower SOC, more compression at higher SOC or higher current. Spring-loaded is far more adaptive, and does just the right adaptation - a bit more force when cells are "expanded", and a bit less force when they are "shrunk" down to their sizes during the original assembly (or even smaller).

 

[natec]

The idea of using springs at all is a little .... out of the box .... If we can find alternative methods for applying the force, that would be a good thing.

This was the whole basis for my final design. When you account for the flex of the compression plates, which will act as two springs in series with the compression attempted by the springs, you will find that the flex of fairly thick plates (3/16 steel) is substantial. If you choose the plate thickness and points of compression appropriately, you can eliminate the need for springs all together and rely on the flex of the plates to provide compression (at the expense of some accuracy).

 

[Topper]

If you're worried about deflection, Call up your local fab shop. Get them to press break a simple 7 7/8" wide (cell height) 3/32 plate with a 3/4" 90* lip at top and bottom. Cut to length (cell width plus bolts and wiggle room). Drill the plates for grade 5 - 1/4" all thread. Lay in a strip of foam (drawer liner) on the end plates. And done. I do like the springs idea but I like the K.I.S.S. rule..

 

[Gazoo]

The idea of using springs at all is a little .... out of the box .... If we can find alternative methods for applying the force, that would be a good thing.

I don't know if this has been mentioned. If it has I forgot. But....what about cutting a gap in the threaded rods and installing springs where the gap was? The springs would be pulling on the end plates instead of pushing on them as has been discussed. And tension could still be adjusted.

 

[Bob B]

I don't know if this has been mentioned. If it has I forgot. But....what about cutting a gap in the threaded rods and installing springs where the gap was? The springs would be pulling on the end plates instead of pushing on them as has been discussed. And tension could still be adjusted.

I think it was briefly at one point .... this is the way I initially thought of doing it .... with an expansion spring instead of a compression spring.... This would be a good way to do it if you didn't want the springs sticking out the ends ... but the compression springs on the end kinda took over.

 

[natec]

I don't know if this has been mentioned. If it has I forgot. But....what about cutting a gap in the threaded rods and installing springs where the gap was? The springs would be pulling on the end plates instead of pushing on them as has been discussed. And tension could still be adjusted.

That was concept C for me. It's not a bad idea. It's a little more difficult to find springs with the right characteristics, but they are out there.

 

[Frick]

Has anyone tried using fence post foam as a freeform elastomeric spring? I had the thought, build a rigid box an extra inch bigger length and width. Set batteries clustered in one corner wrapped in baking silicon paper, Inject can of foam into void on two sides. Should squeeze just about right force, be wonderfully even pressure, cost about $7 and take All of 15 minutes to do, except for the building of the box. I was thinking of using some pretty multicolored beech I have and some lacquer.

 

[Bob B]

Has anyone tried using fence post foam as a freeform elastomeric spring? I had the thought, build a rigid box an extra inch bigger length and width. Set batteries clustered in one corner wrapped in baking silicon paper, Inject can of foam into void on two sides. Should squeeze just about right force, be wonderfully even pressure, cost about $7 and take All of 15 minutes to do, except for the building of the box. I was thinking of using some pretty multicolored beech I have and some lacquer.

It would accomplish compression .... and secure the cells, but the amount of compression would be unknown. My only concern there would be if there is too much compression. Anything over 19 PSI is worse than no compression.
Do you know any details about how much pressure might be expected?

 

[Frick]

It would accomplish compression .... and secure the cells, but the amount of compression would be unknown. My only concern there would be if there is too much compression. Anything over 19 PSI is worse than no compression.
Do you know any details about how much pressure might be expected?

I dont have any numbers I could find. It is rated for pvc fence, like the flimsy stuff used around pools which flexes rather easily, so my guess is that compression would be minimal (1-2psi) and then would hold everything to the shape it formed in. Depending on the pressure the cells press outward with it should resist accordingly and provide stronger compression, flexing only a tiny bit.

 

[Zwy]

Start with this thread. https://diysolarforum.com/threads/another-cell-compression-thread-this-time-about-foam.16537/

Then here: https://diysolarforum.com/threads/compression-material-thoughts.20909/post-245041

I thought I didn't have a photo of the pack outside the camper where it resides now but I found one the other day. This was before adding the Schedule 40 pvc between the plates at the exact length needed for 25% compression of the foam.

 

[Frick]

Oooh. Thanks! It helps to see where things have already gone on the subject.

 

[Bob B]

A new one over here .... https://diysolarforum.com/threads/m...g-new-energy-technology-co.23066/#post-280867

 

[~Yuri~]

Итак, позвольте мне попытаться описать мою компрессионную сборку.
Я изучал опыт предыдущих товарищей на этом форуме. Я изучил таблицы данных и всю информацию, доступную здесь и на других ресурсах. И решил создать свой узел. Поскольку у меня батарея 16S, я считаю, что расположение с пятью шипами является наиболее приемлемым. 10 пружин взяты для более плавного регулирования силы сжатия. Пружины слева работают до 70 кг каждая, пружины справа - до 125 кг каждая. Когда пружины полностью сжаты, мы получаем силу 0,9 кг на квадратный сантиметр, что соответствует 12,8 фунтам на квадратный дюйм. (Мне проще метрические единицы).
Я купил пружины со склада в интернет-магазине, с заявленными характеристиками.
С тестовым сжатием даже 250 кг на площади 680 сантиметров я получил довольно хороший результат. Это около 0,4 кг на квадратный сантиметр или 5,7 фунтов на квадратный дюйм. Это нижний предел прочности на сжатие, который производители, такие как EVE, рекомендуют в своих таблицах данных.
Результатом полностью доволен, буду пользоваться.
Между ячейками находится тонкий (0,35 мм) стекловолокно. Шпильки, гайки, шайбы изготовлены из нержавеющей стали AISI 304. Хотя также можно использовать обычную оцинкованную сталь и метизы.
На шпильках есть прозрачный шланг из ПВХ. Для эстетики и изоляции.
Я использовал M8x1, этого диаметра, с моей точки зрения, вполне достаточно.
Я заказал пружины на заводе пружин с заданной силой максимального сжатия. Длина пружин в свободном состоянии 82 и 90 мм, толщина проволоки к пружинам соответственно 3 и 4 мм.
Боковые накладки - фанера толщиной 21 мм. Окрашено в цвет ячеек (только что покрасил).
Ниже прилагаю все фото, которые есть в наличии.


So let me try to describe my compression build.
I studied the experience of previous comrades on this forum. I went through the datasheets and all the information available here and elsewhere. And I decided to create my own node. Since I have a 16S battery, I find the five-spike arrangement to be the most acceptable. 10 springs are taken for smoother regulation of the compression force. The springs on the left work up to 70 kg each, the springs on the right up to 125 kg each. When the springs are fully compressed, we get a force of 0.9 kg per square centimeter, which equates to 12.8 pounds per square inch. (Metric units are easier for me).
I bought springs from a warehouse in an online store, with the stated characteristics.
With a test compression of even 250 kg on an area of 680 centimeters, I got a pretty good result. That's about 0.4 kg per square centimeter or 5.7 pounds per square inch. This is the lower compressive strength that manufacturers such as EVE recommend in their datasheets.
I am completely satisfied with the result, I will use it.
There is a thin (0.35 mm) fiberglass between the cells. Studs, nuts, washers are made of stainless steel AISI 304. Although you can also use ordinary galvanized steel and hardware.
The hairpins have a transparent PVC hose. For aesthetics and isolation.
I used M8x1, this diameter, from my point of view, is quite enough.
I have ordered springs from a spring factory with a specified maximum compression force. The length of the springs in the free state is 82 and 90 mm, the thickness of the wire to the springs is 3 and 4 mm, respectively.
Side skirts - 21 mm plywood. Painted in the color of the cells (just painted).
Below I attach all the photos that are available.

 

[Supervstech]

I added the English translation to the post.

 

[K8MEJ]

I added the English translation to the post.

Thank you!

 

[Zwy]

@~Yuri~, you should consider insulating the rods.