Confused about the benefits of welding copper to a battery...

[TRS300]

Please bear with me... I'm about to begin building two 60v 25ah 16s 5p batteries with a 150 amp bms. Due to the higher current of this build I'd like to build the series strings to handle around 35-40 amps comfortably.
I've been reading up on options that use copper and obviously the copper sandwich comes up often. I've considered other options also that connect 8 gauge copper wire to 0.3mm nickel tabs, then weld the tabs to the battery...
I know that doing this is not welding the copper to the battery, and this is where the confusion enters...
The battery cell caseing and tab are not copper... So even if you weld copper to the case / tab power is still flowing through "non-copper" components to get to the copper weld spot. So how is this any different to soldering copper wire to a thick nickel strip and welding the nickel strip to the battery? All you are doing is adding a slight extension to the non-copper battery surfaces. And the current only needs to move through a few mm of nickel to get into the thick copper wire soldered to the nickel.
The point of the my confusion is the the battery case is not copper anyway, so what is driving the desire to weld 0.1mm thin copper to the case?

 

[fechter]

I think guys do it because the nickel alone can't handle the current in some builds. Soldering copper wire to nickel and then welding has worked well for me. You really only need to do that on the ends where the main wires attach. Between cell groups, 0.2mm nickel sheet will have plenty of capacity.

Here's a 12p pack I built. The nickel on the end cells was folded over after welding.

 

[TRS300]

Here's a 12p pack I built. The nickel on the end cells was folded over after welding.

I did something similar on my 48v 60amp battery. I like the way your battery is held together. Are you binding the cell groups using some sort of tape?

 

[fechter]

Yes, it's fiberglass tape. The cells already had that when I got them. Won't melt and makes enough space between cells to prevent shorts. There's also silicone glue between the bottom and the Nomex paper they're sitting on.