Spot Welding Copper Strips to 18650 Battery Cells

[spinningmagnets]

A peak of 23A per cell with a continuous 6A per cell can easily be handled by 0.10mm copper, which has the same conductivity as 0.40mm nickel without the high resistance.

I am curious about any results you could post about settings for 0.15mm copper, and if you do that, I would gladly paypal you the cost of buying the copper for the test.

 

[pbert]

I am curious about any results you could post about settings for 0.15mm copper

Sure i'd be happy to do a test with 0.15mm copper. Ive ordered my copper from ali express so will take a month before i can get it and post results though. No worries for payapl, thanks for offer but i'm happy to contribute.

 

[serious_sam]

I have some 0.1 copper sheet. I just tried an infinite slit with it and it welds much better than the 0.2mm copper. The problem is that the battery is putting out 60 amps continuous and so the 0.1mm copper is not enough as far as i know. I could order some 0.15mm and try with that but I would need to wait to receive it ...

0.1mm copper is plenty. You won't get full cell discharge continuous 30A from 40T. You will be heat limited in the cell.

Once again you're relying partly on compression contact with your fold over method. Not ideal or consistent. Especially at high current, you may end up with a noticeable imbalance.

I suggest having another try at the nickel/copper/cell sandwich, using the 0.1mm copper. Also, using nickel plated steel for the top layer will allow you to weld with slightly lower current (or get more penetration with equal current).

 

[pbert]

I suggest having another try at the nickel/copper/cell sandwich, using the 0.1mm copper. Also, using nickel plated steel for the top layer will allow you to weld with slightly lower current (or get more penetration with equal current).

I just tried the above, nickel plated steel with the 01.mm copper, and it works pretty well. You think that 0.1mm copper is enough for 30A ? I was looking at the ampacity chart i had found on a thread in this forum from Matador and it says that a 10mm by 0.1mm copper is good for 15 amps only. I just want to be sure i build a pack that wont overheat thats why i was using 0.2mm copper.

 

[serious_sam]

I just tried the above, nickel plated steel with the 01.mm copper, and it works pretty well. You think that 0.1mm copper is enough for 30A ? I was looking at the ampacity chart i had found on a thread in this forum from Matador and it says that a 10mm by 0.1mm copper is good for 15 amps only. I just want to be sure i build a pack that wont overheat thats why i was using 0.2mm copper.

It really depends.

I use 0.1mm copper at 30A per cell peak (VTC6A). I have 2 temp sensors: 1 on a cell, and 1 on a copper strip. There is no significant temperature difference in the copper relative to the cells. The whole lot heats up pretty evenly.

I would agree with Matador that 15A @ 10x0.1mm is good (i.e. that keeps the heat in the "optimal" range), but you could safely go higher with a little more heat, so something like 25A would still be "acceptable".

However, if you use the copper as a full sheet instead of individual 10mm wide strips, then it's at least 18mm wide per cell (width of cell plus spacing). Based on that, I calculate:
-> 18x18x0.1mm
-> 0.17mOhm
-> 0.15W @30A
-> 16.7A/mm2 @30A
-> All of these numbers are pretty good, IMO.

These numbers are obviously just guidelines. It really all depends on individual installations and what the cooling is (or lack thereof).

I'm getting deja vu.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680&p=1607061#p1606929

https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680&p=1607061#p1606870

 

[spinningmagnets]

Impressive

 

[pbert]

I'm getting deja vu.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680&p=1607061#p1606929

https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680&p=1607061#p1606870

Ok thanks for the details about using the 01.mm. I'm reading through the posts that you linked and it does look like devmonkey was doing the same thing i am doing deja vu indeed. I'll read through those posts and see what the outcome was.
Thanks for this.

 

[Hummina Shadeeba]

what would be a good conductor to use that could flex a lot? electric skateboards can have a lot of vibrations and flexing. I cant find a resource to compare the fatigue limits of pure copper vs pure nickel vs copper alloys vs other metals. Im seeing C15000 copper alloy with zirconium looking stronger than copper and with just a bit less conductance. maybe iron?

 

[serious_sam]

Better to have a rigid battery pack with compliant mounting to the board. A flexible battery pack is a terrible idea. IMO.

 

[john61ct]

Fine-stranded tinned wiring, ideally marine grade UL 1426 rated (BC-5W2 usually 105°C)

No movement at terminations obviously, just leave slack in the length portion.

 

[Hummina Shadeeba]

Fine-stranded tinned wiring, ideally marine grade UL 1426 rated (BC-5W2 usually 105°C)

No movement at terminations obviously, just leave slack in the length portion.

why would stranded copper be less likely to break than strip? they both in theory should have the same fatigue limit if the same thickness so wonder why not just use copper strip. wire comes with insulation but i can add that. i can get the same amount of copper across the series connections but thinner with strip taking less space, that would be the point, and makes it easier to build using strip welded to cells instead of also soldering multistrand

 

[markz]

Tab strips are super easy to bend and break, probably takes about 3 or 4 complete folds to break
Same goes for solid core wire
Stranded wire is good for flexibility

why would stranded copper be less likely to break than strip? they both in theory should have the same fatigue limit if the same thickness so wonder why not just use copper strip. wire comes with insulation but i can add that. i can get the same amount of copper across the series connections but thinner with strip taking less space, that would be the point, and makes it easier to build using strip welded to cells instead of also soldering multistrand

 

[Hummina Shadeeba]

Tab strips are super easy to bend and break, probably takes about 3 or 4 complete folds to break
Same goes for solid core wire
Stranded wire is good for flexibility

why would stranded copper be less likely to break than strip? they both in theory should have the same fatigue limit if the same thickness so wonder why not just use copper strip. wire comes with insulation but i can add that. i can get the same amount of copper across the series connections but thinner with strip taking less space, that would be the point, and makes it easier to build using strip welded to cells instead of also soldering multistrand

im pretty sure if you take a strand out of the multistrand wire and bend it to the same degree and amount of times as the sheet, if it's the same diameter, it will break at the same time.

10 awg wire is 6.7mm^2 of copper and if i have a sheet of copper 140mm long to span across my 4cells to other 4 connected in series, even if only .05mm thick it will have more copper at 7mm^2.

not nearly as capable of repeated flexing as soft multi-strand with its .006mm diameter strands but within the spectrum of harder multi-strand. https://www.maneywire.com/documents/Copper%20Stranding%20Chart.pdf

or maybe rectangular vs a round shape is a factor, or the finish.

Could do 0.025mm thick foil stacked maybe 4 layers thick. .025mm thick is thinnest over seen


Here’s .05mm strip I had on hand stacked 2x. These cells are hard to weld to on bottom. I see .025mm stuff out there and think possibly could make a nice flexible connection
https://imgur.com/a/zgOZjfx

 

[BobBob]

If you used bog standard wire such as 7/02, took a strand and bent it with the same bend radius as a 0.2mm strip it may fail at around the same time
Good high flex wire (such as Staubli/multicontact and probably Litz) tend to use 0.05 or even 0.04mm strands, sometimes in a coiled coil strand configuration and sometimes tinned as John61ct mentioned.
The thinner strands are much better at flexing than 0.1 or 0.2 strip, partly due to localised strain on strip and partly due to the thickness difference
The coiled coil configuration of the strands means that each strand goes from the inside to the outside of the bunch which reduces the strain on individual wires both tension and bend rad.
Tinning reduces corrosion and for higher heat this becomes more significant as the copper starts to react more
Insulation when part of the wire reduces bend radius and provides mechanical strength

 

[Hummina Shadeeba]

as you say thinner the better for being strong. i was doing .05mm strip i happened to have and see .025mm strip out there.

as you say with the strands in a wire i think it can take away any stress points. in my pics the sharp fold of the copper stip is no good in that regard and i did it in the interest of space but i think you could connect between cells with the strip with a minimal angle, like a loose ribbon, and could do a lot...in that regard.

in other regard.. you could add insulation. paint on some silicone even.
t
when you want a battery to flex on your skateboard or some other thing out there, adding lots of soldered on wire takes up some of the valuable little space. ideal would be flatter multistrand. flat multi stand.

but it does have i think a nice benefit over multistrand. in the pics with the strip going across all the cells they will stay more balanced.

 

[BobBob]

A bigger rad on the strip may take more space, distributing movement over a greater length reduces strain.
Movement an anything other than bend or stretch might mean worse strain (tranlational movement accross the batt axis for example)
The weld point may be a mechanical failure point but soldering thin strands also causes failures at the edge of the solder unless you fix the cable well (again as John noted)
High current accross the cells shouldn't be needed unless there's a lot of variation between cell resistance.
You've got me thinking copper sheet origami now - laser cuts in the strip - zig zags or lengthway cuts or something

 

[john61ct]

No solder terminations, use crimp connections.

Again, no flexing until you get well into the insulated wire body.

 

[Hummina Shadeeba]

How would you crimp spot-welded copper strip?

Why wouldn’t you want to flex wire other than well into the wire? you mean if soldered?

Maybe the bend radius need not be big depending on the amount of flex and more important it be consistent, but maybe as the metal is repeatedly bent it work hardens and the point of flex is thereby moved, or will the initially hardened point be first to break.

 

[john61ct]

I am not talking about strip, at all.

I am saying, where you are using the wire I spec'd to accommodate flexing,

abstain from touching that wire with any solder.

If you are connecting wire to strip, then do so via solid connectors with a wide contact surface area crimped to the wire,

perhaps quality marine tinned ring terminals, or flat-shaped ferrules.

The portion of the wire subject to flexion stress should start well away from that crimped terminator,

so there is no such stress on the junction area.

 

[goatman]

im waiting for 0.1mm copper right now to see if i can twist crimp with 18 gauge wire, lock the twist with a dab of solder and trim

this is 0.03 foil but its too thin

 

[BobBob]

How would you crimp spot-welded copper strip?

Why wouldn’t you want to flex wire other than well into the wire? you mean if soldered?

Imagine 1000 strands coming out of that block of solid solder, a gentle tug on the wire off axis is all concentrated on a few strands at the edge. The joint from wire to solder is rigid and any forces on the wire will be concentrated at that interface. Most of the strain occurs at the soldered joint.

Maybe the bend radius need not be big depending on the amount of flex and more important it be consistent, but maybe as the metal is repeatedly bent it work hardens and the point of flex is thereby moved, or will the initially hardened point be first to break.

Imagine soldering a single strand onto something large and heavy then shaking it about from a few cm away. It will allways snap next to the solder and pulling on the wire also puts all the strain right next to the joint. The wires next to it don't help as they do in a crimp

It's worth having good crimp tools and making sure the crimps fit both the conductor and the insulation.
The copper strands are almost solid metal in the most compressed section of the crimp but the reduction in compression is gradual and the lead-in to the crimp is a "bell mouth" shape that increases the bend radius. Further support is provided by the plastic cup around the insulation of the wire.
In the RAF they spend Thousands per crimp tool but wouldn't solder.

 

[Hummina Shadeeba]

i have some crimp tool and tons of crimps i never use really and end up always soldering. While crimps n ferrules may be better than solder for a wire that is repeatedly bent but i see no way to crimp .1mm copper strip to a ferrule and dont think it will be possible. it may be the best technique..im looking for second best maybe!

IF the strip goes straight from cells to cells as in my pic and and repeatedly bent where would it break first and how assuming cells bent maybe 10 degrees repeatedly along one axis and i paint on a "bell mouth" at either end where the strip comes from the cells, and assuming theres no smaller radius with the strip than would be with the multistrand..which is maybe an unrealistic expectation of the strip while not an issue for multi strand

i assume the metal eventually work hardens and breaks somewhere and that would be the same method of failure with the layered .025mm strip or if a crimped multi strand with strands that are .025mm and maybe take just as long

 

[john61ct]

Think of each section (sub pack?) involving strips, whether soldered or welded

as a rigid monolithic block, no flexing stresses allowed at all.

Then the flexible wiring with zero solder involved, connecting between those blocks, their connection ends well supported

so the crimped terminations are (on each end) part of the rigid block elements.


A spade-shaped ferrule end could be soldered to your strip, or more robust,

a ring terminal can be lock-bolted (Huck style) to it.

 

[Hummina Shadeeba]

Adding the multi strand with a soldered ferrule at the rigid side and then crimps to multistrand may be the proven standard for making a flexible connection and im sure would be good but i think there's alternatives that MAY be as reliable.


at this point im thinking if using stacked thin strip and there's a bell mouth made where connected to the rigid parts, maybe made simply by painting more silicone at the junctions, and the radius of the strips between the rigid parts stays consistent and even, like a bow, im wondering what could be a reason the strip would break sooner than multi strand if the same diameter strands and strip and why? hardening? and when a strip hardens is it more or less likely to break than many individual strands? maybe once a crack is formed in one of the strips its going to propagate easily.


wondering the physics of how the stacked strip would fail and how the multistrand will fail. I have no plans to make a battery with the bending thin strips and just curious.

 

[BobBob]

With a crimp the wire is supported at the ends and bending limited so the flex occurs in the loop.
1. With one or more strips, the mechanical fixing is via a tiny spot weld with perhaps 1% of the cross section of the strip so most of the strain may occur there.
2. The flex and vibration are likely to introduce strain other than rotating around an axis through the centre of your looped strip. The strip will not bend as well if the cells move side to side or if the strp vibrates weirdly.
3. The coiled coil high flex cable configuration means that the strand path is a tight helix following a more gently helix in the opposite direction following the path of the wire, supported by lots of other strands and all supported by insulation. imagine bending a slinky backward and forward until it snaps - you'd be there a while. Now imagine it was a straight piece of slinky wire - much more bend per wire section thus strain so if fails sooner.
The engineering complexity is way greater compared to that of the physics of bending a theoretical part and 3d.

Bit of wire from an old robot cable I designed 20 yrs ago.
The previous chap used high flex but not for mobile (easy to bend once) and the strands had migrated through the insulation in a rolling loop and shorted to each other intermittently dependent on robot position (worldwide and intermittent)