"Copper/nickel sandwich" buses for series connections

Thank you for posting those results. This kind of experimentation is vital to discovering the best possible materials and methods.

Since copper is four times as conductive as nickel, a 0.10mm thick copper strip (for series) is as conductive as 0.40mm thick nickel.

Due to the lower resistance of copper, a 0.10mm strip of copper converts less of the series watts into waste-heat.

It's high thermal conductivity also acts as a heat-sink, to rapidly pull heat away from the core of the cylindrical cell, compared to nickel...
 
Wow this thread is a potential game changer. I have some nickel coated copper strip but the Kweld doesn't seem to weld it. So the two strips need to start life as different pieces correct? People have had luck with .10 Copper and .15 nickel with Weld set to 50 joules?
 
By passing the welding current through two separate strips that are not touching, you force the current to pass through the cell-tip, instead of most of the current simply passing through the bus-strip from one probe-tip to the other...

SpotWeldCurrent1.png
 
I'm currently doing some soldering tests with the kWeld and 0.2mm copper sheet
I picked 0.2mm copper sheet for my battery project but on second thought, it's probably overkill. The battery will be a 16S8P 21700 cells, with a max pulse discharge current of 250A-300A. From what I have read, 20A per mm² of copper is good enough (correct me if I'm wrong)

Since I will solder large copper sheet like this : https://endless-sphere.com/forums/download/file.php?id=282314 and not just strips, the cross section area of the sheet is 190mm(width)*0.2mm(thickness) = 38mm². So, in theory the max current that is acceptable to flow in the copper sheet is : 38mm²*20A = 760A !

To get back to the point, I'm having hard time soldering 0.2mm copper sheet despites high energy setting on the kWeld (100J-125J)
Some welds are good (most on the negative tip), but the majority are too weak.
I'm currently using 0.15mm pure nickel on top of the copper, with full slot on both
The next step is to use steel nickel plated strips instead of pure nickel in order to see if there's any improvement. I have bought both 0.1mm and 0.15mm thickness strips today and I'm now waiting them

If it's still unsatisfactory, I will have to downgrade to 0.1mm copper sheet
 
The 0.15 copper sheet is what I am most interested in. It can handle the highest currents coming from 21700 cells. Even if 0.10mm copper works for the common 18650 cells, if there is a way to get the 0.15 copper to work on cylindrical cells, why not not use 0.15 on everything? It makes a great heat sink to pull heat out of the cell and dissipate it.
 
spinningmagnets said:
The 0.15 copper sheet is what I am most interested in. It can handle the highest currents coming from 21700 cells. Even if 0.10mm copper works for the common 18650 cells, if there is a way to get the 0.15 copper to work on cylindrical cells, why not not use 0.15 on everything? It makes a great heat sink to pull heat out of the cell and dissipate it.
Click to expand...

You can also double the layers or quadruple the layers :)
 
spinningmagnets said:
The 0.15 copper sheet is what I am most interested in. It can handle the highest currents coming from 21700 cells. Even if 0.10mm copper works for the common 18650 cells, if there is a way to get the 0.15 copper to work on cylindrical cells, why not not use 0.15 on everything? It makes a great heat sink to pull heat out of the cell and dissipate it.
Click to expand...

Seriously though I wonder if it is possible to heatsink the pack by putting a thermal pad between the copper sheet and the outer case.
 
I haven't really investigated materials with good thermal properties that were not conductive.

It's a worthwhile study, but the frequent suggestion is using high-amp cells so they run cool at normal loads, and also having a pack big enough to easily supply the normal amp work-loads.
 
Yes letting internal heat build up as a result of high C-rate discharges for longer periods

is inherently damaging to longevity.

Designing heroic cooling measures should not be required.

The ideal is raising Ah capacity until there is no detectable temp rise.

 
spinningmagnets said:
I haven't really investigated materials with good thermal properties that were not conductive.

It's a worthwhile study, but the frequent suggestion is using high-amp cells so they ruin cool at normal loads, and also having a pack big enough to easily supply the normal amp work-loads.
Click to expand...

I don't think there is many of them. People make a lot of insulating materials though.
 
john61ct said:
Yes letting internal heat build up as a result of high C-rate discharges for longer periods

is inherently damaging to longevity.

Designing heroic cooling measures should not be required.

The ideal is raising Ah capacity until there is no detectable temp rise.
Click to expand...

This is not really possible when weight, space and budget are all much more valuable factors though. This is why organised or clever design is necessary. Air cooling rather than water, copper rather than nickel, ferro fluid rather than air, are all leaning towards the qualities that you would expect to see in a light electric vehicle. Many of the things required to make this motor/bike/light ev tech better are already available in other areas of tech but it is just finding a way to apply or implement them.
 
Here is one company from Battery Digital Days that presented a few products that are both electrically insulating and heat conductive potting solutions:

https://www.epoxies.com/products/thermally-conductive/
 
If we do identify truly high thermally-conductive epoxy or polyurethane potting compound

ideally permanently flexible for help absorbing shock/vibration

That would also help a lot with **motors** where the windings have poor heatsinking to the outside case.

Embed a temp sensor first!
 
spinningmagnets said:
The 0.15 copper sheet is what I am most interested in. It can handle the highest currents coming from 21700 cells. Even if 0.10mm copper works for the common 18650 cells, if there is a way to get the 0.15 copper to work on cylindrical cells, why not not use 0.15 on everything? It makes a great heat sink to pull heat out of the cell and dissipate it.
Click to expand...

This is a clever idea...a little section of nickel on top of the copper to concentrate the weld heat. I tried spot welding copper directly and that totally failed with my Kweld.

Did you ever try .15 copper? How did that turn out?
 
Remember copper is **so** much more conductive, no need to go thick.

I really would love to find a way to make or get copper sheeting or strips that are thickly enough **plated** with nickel to give higher surface resistance.

Has anyone with Kweld tried welding a flattened segment of fine-stranded **tinned** boat cabling (UL 1426)?

That's pure copper on the interior of each strand
 
john61ct said:
Remember copper is **so** much more conductive, no need to go thick.

I really would love to find a way to make or get copper sheeting or strips that are thickly enough **plated** with nickel to give higher surface resistance.

Has anyone with Kweld tried welding a flattened segment of fine-stranded **tinned** boat cabling (UL 1426)?

That's pure copper on the interior of each strand
Click to expand...

Yes...exactly why I failed to spot weld copper directly. It was one of those "Hmmm...wonder if I can do...." moments and of course it didn't work. LOL!

If the copper was nickel plated, wouldn't that make the copper pointless? Using a small section of nickel on top of the copper focuses the heat to the spots on the copper so the copper welds to the cell. This isn't the same thing as nickel between the cell and the copper. Still...probably close enough that may not matter.

I messed with solder paste a while back to see if I could get the spot welds to have less resistance. It didn't really help much, but the solder did melt to the nickel and the cell. How about use solder paste with copper and the spot welder? I think there's enough heat to melt the solder and that ought to work just as well as trying to spot weld the copper to the cells. This would eliminate needing a small section of nickel or nickel plated copper.
 
Cowardlyduck said:
I got some copper sheet and going to try this soon.

My main concern is how to cut the copper in such a way as to avoid it overhanging too much and risking shorts from touching the wrong area/cell.
I guess the obvious answer is to cut it carefully...any other suggestions?

Cheers
Click to expand...

Shorts? What are those? Haha!
I reuse masking tape. All it needs to do is cover the adjacent cells long enough to let you do the weld. After that the nickel is no longer going to short on something it shouldn't. I cover the other cells around the weld with masking tape that I reuse over and over again while building a pack. No problems with shorts! Toss the tape when you are done. I haven't tried copper as presented here, but the tape ought to work for that too.
 
Cowardlyduck said:
I got some copper sheet and going to try this soon.

My main concern is how to cut the copper in such a way as to avoid it overhanging too much and risking shorts from touching the wrong area/cell.
I guess the obvious answer is to cut it carefully...any other suggestions?

Cheers
Click to expand...
Cover each section in mylar tape as you go. A small slip is all it takes for KFF. Make the pack in smaller sub sections to reduce the energy content, and do the final join at the end. Minimise the risk as much as practical.
10.JPG
 
spinningmagnets said:
When I want nickel ribbon in the USA, I usually get it from Keith at "18650 heat shrink and cell holders"

https://www.facebook.com/18650Shrin...TxB6BFW-dw0Wh5Y-Woh-5oGC3KHR2sivukcDIy6fEH6Ai

For copper, if you want to experiment before committing to a large build, you can get some in the jewelry section of a local large hobby supplies store, or ebay is reasonably fast. Here is 6x6-inches of 32-ga / 0.20mm for $7

https://www.ebay.com/itm/Copper-99-...683271?hash=item44407c97c7:g:KKgAAOSwkZldsQ5B

34-ga / 0.15mm, this link is twice as expensive at $14, but also twice as much area (bigger sheet), however...it might weld easier at a lower amp-setting due to being thinner.

https://www.ebay.com/itm/6-x-12-Pur...434298&hash=item3f504fe128:g:uu4AAOSwLs5XL46Y
Click to expand...

Noob question - are the caps on lithium cells the same material and thickness as standard alkaline cells?

I want to do some practice runs with one or more sandwich/slit techniques and a brick of AA's would be cheap to use as dummy cells as long as it's a valid test.
 
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