Battery box as Lifep04 compression plates

mystic pizza

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My LifePo4s will live in a steel welded battery box. There is little room in the box itself to add compression plates to the ends of the cells. The box, basically has a bottom, four sides, and a removable wooden lid.

What are people thoughts on using threaded rod between to sides of the battery box itself to act as compression plates on the cells?

The cells would need to be packed at the ends by about 10mm. That's all the available space left in the box

Thank you.
 

Short_Shot

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Most of us wouldn't put them in a steel box.

If you insist on doing so, slide some poron foam in with them and "shim" it to 25% compression to achieve its rated 11psi. I used a 1/4 inch piece on each end of my battery box.

I'm guessing if you truly have 10mm then a half inch thick piece of the stuff would compress to almost exactly what you need, and a couple sheets of thin plastic mat would make the difference up.

The box doesn't need to be compressed from the outside. It already would support the load if it's suitably strong at the edges.

Whether you can get poron foam at the right grade for 11 psi @ 25% compression is unknown, since we have no idea where you live.
 

Short_Shot

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Here's my wooden box using the stuff.

The dimension between the ends with the uncompressed foam is exactly 1/8" less than the overall length of my pack with the plastic mats in place. With half an inch of that foam that's 25% compression and 11 psi.

The poron is on the narrow ends of course.

20210928_102204.jpg
20210930_093445.jpg
 

mystic pizza

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Most of us wouldn't put them in a steel box.

If you insist on doing so, slide some poron foam in with them and "shim" it to 25% compression to achieve its rated 11psi. I used a 1/4 inch piece on each end of my battery box.

I'm guessing if you truly have 10mm then a half inch thick piece of the stuff would compress to almost exactly what you need, and a couple sheets of thin plastic mat would make the difference up.

The box doesn't need to be compressed from the outside. It already would support the load if it's suitably strong at the edges.

Whether you can get poron foam at the right grade for 11 psi @ 25% compression is unknown, since we have no idea where you live.
Thanks. I live in the UK. I am not sure what Poron foam is, but will look into it.
 

mystic pizza

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Here's my wooden box using the stuff.

The dimension between the ends with the uncompressed foam is exactly 1/8" less than the overall length of my pack with the plastic mats in place. With half an inch of that foam that's 25% compression and 11 psi.

The poron is on the narrow ends of course.

View attachment 68409
View attachment 68410
The steel box I have is already in place. I could substitute it for a wooden one, but the nature of how the box is fixed into a frame would mean even less room in the box than a steel one.
 

Short_Shot

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Yours can be steel and still use that foam method. Just a matter of measuring everything and ensuring the 25% compression by dimensions.
 

mystic pizza

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Yours can be steel and still use that foam method. Just a matter of measuring everything and ensuring the 25% compression by dimensions.
Its pretty thin steel, but the foam method sounds good if I can source some in the UK. Thanks for your input
 

Short_Shot

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And here's an article about using it for batteries.


 

Short_Shot

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Its pretty thin steel, but the foam method sounds good if I can source some in the UK. Thanks for your input
Please note that not just any kind of foam can be used as most types will undergo "compression set" which reduces the amount of force it'll rebound with over time.

This stuff does too but it's very very low.
 

mystic pizza

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Please note that not just any kind of foam can be used as most types will undergo "compression set" which reduces the amount of force it'll rebound with over time.

This stuff does too but it's very very low.
Poron sounds like a trade name. Is it just open cell foam?
 

mikefitz

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If you watch teardowns of ready built batteries on you tube that are using large prismatic cells, you will see that compression is not used, at the best the cells will be taped together.
I suggest installing with insulating material between cells and the metal and aiming for a 'snug' fit with the cells at around 50% SOC. High density closed cell foam or neoprene could be used.

Good discussion here,


I assemble similar to the picture below with high density foam sheet on each end plate ( metal or thick ply). The cells are held under slight restraining force whilst the side plates/ply are fixed in position.
Battery assembly.jpg

To overcome any issues with buss bars I use flexile braid.
buss bar1.jpg

I see you are in the UK,

high density foam sheet available on ebay , various thickness including adhesive backed .
Must be closed cell to avoid moisture issues.
high density foam
To isolate between cells use high temp baking sheet,


Mike
 
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Short_Shot

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I would add, the cells are installed on a boat of that makes any difference to compression.
If you keep them at reasonable temperatures it shouldn't be a major concern. A slight amount of compression from padding will probably be enough, with the padding being the more important of the two.

Just make sure your cells don't touch the steel case. Some are case positive, and while significant current won't flow you still don't want to deal with it.
 

mystic pizza

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If you keep them at reasonable temperatures it shouldn't be a major concern. A slight amount of compression from padding will probably be enough, with the padding being the more important of the two.

Just make sure your cells don't touch the steel case. Some are case positive, and while significant current won't flow you still don't want to deal with it.
Many thanks They are the small Thundersky 40 Ahr type cells, so I think the case is plastic. The foam will help with any vibration though, particularily if i put some beneath the cells too.
 

Short_Shot

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Oh.

You don't even need to compress those things as far as I understand it. I could be wrong but I'm under the impression the plastic case is stiff enough to handle its own internals.
 

mystic pizza

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Oh.

You don't even need to compress those things as far as I understand it. I could be wrong but I'm under the impression the plastic case is stiff enough to handle its own internals.
The Thundersky instructions do show compression plates, but that may be on the bigger cells, i'm not sure.
 
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