High current connections for 18650 Pack?

I am trying to build a 6S2P pack with BAK N18650CNP cells (ordered from liionwholesale.com). These are rated for 30A continuous discharge, but I only plan on drawing 15A from each individual cell for a total of 30A across the 2P packs, and maybe a few short current spikes up to 50A from the entire pack.

I previously made such a 6S2P pack by soldering together the cells using 14AWG wire, and the pack worked fine for about a year. I’m now experiencing serious voltage sag issues with that old pack, so I now want to build a new pack by spot-welding together some new cells.

The problem is that I’m using a spot welder which can only handle up to 0.12mm nickel strip. I chose to use 0.1mm x 9mm nickel strip, which seems like it can only reasonably handle ~8.5A for extended periods of time, with bursts up to ~11A. I saw online that it’s not a good idea to stack nickel strips when spot welding, since nickel-nickel adhesion is poor. I also considered soldering wires onto the nickel strips after spot welding, but that defeats the purpose of spot welding since a lot of heat will be transferred to the cells upon soldering, and the whole purpose of spot welding is to avoid heating up the cells.

What have other people been doing to connect high-current capable cells when building 18650 packs?

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Not a spot welder myself as I havent tried it yet. But I’ve seen folks use a copper-nickel sandwich method to increase the current handling. They use a thin strip of nickel on top and a layer of copper sheet under it, that way the spot welder still has enough resistance to make a weld. Spot welding directly to copper just makes a short and most spot welders dont produce enough current to make sufficient heat and a weld.

What spot welder are you using if you dont mind me asking?

As for the wire, 14 gauge seems a bit thin. Even though it will probably handle the current levels you’re planning it might be worthwhile to use 12 or even 10 gauge. Hope this helps.

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Thanks for your answer, I’ll give the copper idea a try. I’m using this spot welder.

Another thought I had for improving the weld strength was to try doing 2 spot welds in the same spot – is this generally recommended?

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Not sure. I dont have enough experience. I think if the first welds arent sufficient then repeating that step isnt going to help.

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