"Copper/nickel sandwich" buses for series connections

[spinningmagnets]

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...

[st35326]

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?

[spinningmagnets]

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...

[NexusG]

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 : 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

[spinningmagnets]

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.

[NexusG]

0.15mm copper sheet may be the sweet spot if it's easy to weld with the kWeld

[Darren2018]

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.

You can also double the layers or quadruple the layers

[Darren2018]

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.

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.

[spinningmagnets]

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.

[john61ct]

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.

[Darren2018]

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.

I don't think there is many of them. People make a lot of insulating materials though.

[Darren2018]

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.

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.

[hallkbrdz]

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/

[john61ct]

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!