Spot welding overlapping nickel strips

Batteries, Chargers, and Battery Management Systems.
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Sepial   1 mW

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Spot welding overlapping nickel strips

Post by Sepial » Sep 12 2018 2:43pm

Hi,

I have DIY'd a spot welder (thanks to malectrics) that's working like a charm: I am spot welding 18650s with .15 pure nickel with a 3ms pulse and consistent holes in the strip when teared down. So everything is great except that when I try to spot weld the same nickel strips on top of each other I have to crank it up to 8ms. In my understanding I should be fine with the same pulse duration or even a lower one since the bottom layer is now more conductive. I am not worried too much as 8ms is still great, but the fact that I can't find anything about increasing pulse duration when spot welding nickel on nickel is aggravating my itch... Or maybe I am very bad at googling, there might be that...

Anyway if anyone had the same experience, or even an explanation, I would be glad to hear it.

Thanks

kdog   10 kW

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Re: Spot welding overlapping nickel strips

Post by kdog » Sep 12 2018 7:00pm

The decreased resistance leads to less heating which means no weld.... So you have to increase power, but it should weld well at increased amps, bc as you said the bottom layer is now more conductive.
As long as its welding and you're not getting excessive heat (evident by a blue discoloration between the welds) then it should be fine. The first layer of nickel protects the battery both thermally and from puncture from arcing, so it's pretty hard to damage the cell on the second layer.

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spinningmagnets   100 GW

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Re: Spot welding overlapping nickel strips

Post by spinningmagnets » Sep 12 2018 7:02pm

The more conductive a pathway is, the less heat is produced from a set amount of amps. It only the heat that makes the spot-weld.

In fact, this is the reason you are adding more conductor mass to the series connections. Your desired amps would create too much heat with only one layer. By adding a second layer, your bus material will run cooler.

The most desired path for the current is a straight line from one spot-welding probe tip to the other. However, due to the resistance of the material, some of the current takes a more curved path.

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spinningmagnets   100 GW

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Re: Spot welding overlapping nickel strips

Post by spinningmagnets » Sep 12 2018 7:04pm

HA! Posted at 6:00 and 6:02...we were typing at the same time.

I am a fan of overlaying copper on top of nickel, with the nickel only providing the easy spot weld.

Sepial   1 mW

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Re: Spot welding overlapping nickel strips

Post by Sepial » Sep 12 2018 7:56pm

Thanks guys, makes a lot of sense. Now my conscience is clear AND I understand what is going on.
I knew from here that welding copper is much harder but I thought it would be the case only as a top layer.
But now you got me interested spinningmagnets, what is the purpose of the nickel between the battery and copper? If it's not for an easier weld, is it that thermal and arcing protection? Is spot welding copper more prone to arcing because of the stronger weld? Can't it be mitigated with a precisely timed pulse? (just enough to do the weld, but no longer)

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spinningmagnets   100 GW

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Re: Spot welding overlapping nickel strips

Post by spinningmagnets » Sep 12 2018 11:02pm

It's almost impossible to spot-weld copper directly onto the 18650 cell ends. The cell tips are nickel-plated steel. However the path through the 18650 shell is thin. Nickel spot-welds easily, it's high resistance creates very hot spots precisely at the points where the welder probes are touching. Nickel melts at 2650F / 1350C

Copper of the same cross section can conduct the same amount of current while barely getting warm, but that high-conductivity / low-resistance also means it doesn't get hot when the spot-welder sends a jolt through the probes.

Here are 18650-cell bus-plates from ES member kdog
viewtopic.php?f=14&t=84680&start=125#p1319653

Image

When experimenting with copper sheet, I recommend to buy a sampler pack with these

thickness__common names
0.13mm___5-mil__36 ga (fragile, paper thin, not recommended)
0.20mm___8-mil__32 ga (minimum recommended thickness, 4 times more conductive than 0.20mm nickel)
0.25mm__10-mil__30 ga (recommended, cuts with common scissors)
0.33mm__13-mil__28 ga (acceptable for very high current)
0.40mm__16-mil__26 ga (very stiff, expensive, hard to cut with hand shears, not recommended)

xxxxxxxxxxxxxxxx

Here is my recommended nickel tab shape. The split down the middle makes the spot-welds more consistent and stronger while using a lower setting. If you attach the nickel tabs to the copper sheet before you add the cells, you can use as much heat as you like for that particular connection and nothing will be damaged. I would like to try a 3V / 500A MOT-based spot-welder. For the nickel-tab to cell connection I have a kWeld spot-welder.

SpotWeldTab4.png
D
SpotWeldTab4.png (22.68 KiB) Viewed 293 times
SpotWeldTab2.png
C
SpotWeldTab2.png (12.55 KiB) Viewed 294 times
You can easily cut common nickel strip with an abrasive disc on a Dremel
DremelDisc.png
B
DremelDisc.png (167.5 KiB) Viewed 302 times

Sepial   1 mW

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Re: Spot welding overlapping nickel strips

Post by Sepial » Sep 13 2018 5:07am

Oh my those bus plates are sexy... If only someone would manufacture them as 2p rolls like they do with slit nickel.
What I would love even more is manufactured 2p rolls with round nickel tab linked to larger copper sheet with wire fuses (tesla like), so I can spot weld them, no wire bonding required. If that makes sense.

Thanks for the heads up about nickel and copper, I would have got it all wrong... Seems it's hard to rely on theory and that practical experience is worth a lot more like it often is.

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spinningmagnets   100 GW

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Re: Spot welding overlapping nickel strips

Post by spinningmagnets » Sep 13 2018 9:11am

You can order nickel ribbon to be laser-cut to any shape. The four squiggly thin connectors to the centers on the right half are the "fuses". Plate shown is 2S / 6P if you don't add anything else.

Image

Sepial   1 mW

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Re: Spot welding overlapping nickel strips

Post by Sepial » Sep 15 2018 2:31am

Yes this design is great, I need to get a CNC laser cutter.

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