I think this is still worth discussing. Its easy to do a bad solder job, but with some discussion, maybe we can assemble some tips to make soldered packs a better method than it currently is most often used.
By that I mean...I remember back when I was struggling to get-by financially...very rough times. At that time, I might have been able to solder together a pack, but there was no way I could afford a cheap spot-welder, much less a good spot-welder (I now have an arduino pocket-welder, and also the parts to make a 6-pence spot-welder, but haven't had the time to experiment with them yet I also want a MOT).
There is a guy on youtube who took thousands of laptop cells he acquired cheaply (low current), and assembled them into a DIY Tesla wall power-backup.
https://www.youtube.com/channel/UC0pBau ... 6sVdEOIUbA
I found him when I was looking for info and examples on fuse-wire. He also embraced the construction of having a thick copper series bar, and thin parallel connections, so using fuse-wire for parallel was intuitive.
The parallel connections carry the charging current, which can usually be around 3A-5A..the series connections collectively carry the full pack current divided by the number of series bars between P-sub-pack. Using doctor bass's method of one series bar in-between every two cells, a 6P string would have three series bars between every P-string. If you are using the 25R cell for high performance, the factory rates them at 20A, so 6P would be 120A for the pack (just throwing out round number to establish design principle). Three series bars for 120A is 40A each, so...the P-connections might see 5A during charging and even less when it gets to the balancing charge at the end, and the series connection is 40A each.
We have all seen the "ladder" style bus-plates made of nickel. There is one series connection per cell, and...the series and parallel connections are the same thickness and width. Nickel has a 20/100 conductivity rating compared to copper, which is poor. Why not copper? copper is soft so it adds no structural rigidity, plus it corrodes in humid and salty air (of course, if you use plastic cell-holders, pack rigidity is taken care of without needing stiff busses to help). Why not nickel-plated copper which has a much better 80/100 rating for conductivity over pure copper? It's more expensive, and pure nickel works OK for average performance.
Bottom line, in my humble opinion (IMHO)...nickel is used because it is corrosion-resistant, and it spot-welds very easily. I have seen videos of battery factories that have robots spot-welding dozens of batteries in the same amount of time it would take to one person to solder one battery pack, using the best methods available.
A fat soldering Iron tip means that it will not "cool off" during a 3 second soldering operation, and a high-watt soldering iron means that it will recover its highest possible tip temperature more rapidly. I have a 100W soldering iron with a fat tip. Once I used it to solder wires to connectors, I would never want to go back to a weaker soldering iron.
There is also a new idea that went un-noticed by most. An ES member mentioned that he put a skim-coat of solder on copper parallel strips, and it made the spot-welding onto the cell-ends easy. That is because copper is highly conductive (making spot-welding difficult), but...solder has some resistance.
That is my only issue with solder. It is actually not a good conductor, and it adds resistance to any connection. It DOES add a solid connection that seals out corrosion from reactive air (oxygen, humidity, and saltiness near oceans). Like nickel-plated copper, a thin skim-coat of solder might not be too bad. Pure copper with a skim-coat of solder "might" be better than pure nickel. Or at least not worse, with the added benefit of a serious cost-savings over buying a spot-welder. My 100W "fat tip" soldering iron is over ten years old, can most spot-welder owners say that?
When I researched flat copper ribbon (as opposed to common round cross-section wire), I found solar-PV panel connection "flat wire" that was made of copper, and coated in solder. The skim-coat of solder made it easy for field-techs to make connections without needing three hands, and the solder (tin / lead mix) is fairly corrosion-resistant when left outdoors in the weather (unlike the copper ribbon core).
I suspect that if I was in a situation where soldering onto the cell-ends was my only option, I would want to know what is the best method...even if I preferred spot-welding, if the situation allowed it.
Rough-up and clean the wire and cell-ends.
Use flux (I don't know what it is, or how it works, but people smarter than me insist it is a requirement for best results)
Pre-tin the wire (or flat ribbon) with solder
Pre-tin the cell-end with a thin skim-coat, and do this rapidly so the heat does not penetrate into the cell.
let everything cool off, then...
Press the wire/bus against the cell-end, and apply "just enough" heat for the solder on both parts to bond.
side note: If you also embrace the doctor bass method of having one series connection in-between each pair of P-group cells...connect the series bars to the parallel ribbon (whether nickel or copper), BEFORE you make the final attachment to the cell-ends.
HBPowerwall...individual cell fusing
HBPowerwall...soldering vs spot-welding (soldering can be OK)
Ebikeschool.com...Can you solder 18650 batteries? (he thinks, not a good idea)