I am not an expert, but I thought those welds look like friction stir welds, which made me think they would have exposed the underlying copper if those tabs were made of plated material. But that was just a guess.
No, the friction welds used in battery connections typically just join the materials where they touch each other, like a traditional electric spot weld; they're not friction stir welds that mix the materials to a substantial depth. Look at the tiny wires that attach each end of each 18650 or 2170 cell in Tesla module to the aluminum bus plate - it looks like the wire has been smashed onto the bus plate and stuck there. Anyway, this module is similar to any other LG Chem module of pouch cells, so the people who have worked with them at the cell connection level can confirm how the welds work.
Looking at this picture again, and I feel like we are missing something obvious. I also assumed we were looking at the back of the module, but now that I think about it, I do not see that the cells are actually wired in series... This would explain why there was no voltage - you might be measuring cells that are not connected anymore! I am starting to suspect that there must be interconnections on the reverse side of the orange cover plates? Can you post a picture of the front and back of the exposed modules, plus the inside of the cover plates?
Yes, they're wired in series. Each of those visible connections is presumably the positive tab of one cell (one one side of the copper bar) connected to the negative tab of the next cell, supported and located (and connected to the BMS) by the copper bar. There are 12 cells in series, so there are module positive, module negative, and 11 inter-cell connections... there appear to be six here on the back and so the the other seven (module positive, module negative, and 5 inter-cell) must be on the front.
The feature which makes this module look unusual - which threw me at first - is that the cells have one tab (terminal) on each end. Pouch cells more commonly have both tabs on the same end, each less than half the cell width, so connections in those modules alternate side to side as the current flow path works its way from one end of the stack to the other.
I can only guess that the soldered points have no voltage relative to anywhere because they are some sort of isolated intermediate point in the monitoring circuit for each cell.
But yes, photos of the front and back, both with covers removed, would make everything more clear.
As for the BMS taps, Brian, wouldnt you need 7 connections for a 6 cell module? It is true that you will need one tap from the far side, but it has the terminal stud that can serve as the connection point, right?
These are 12S (not 6S) modules, so you do need 13 BMS connections, 7 of which (including the two terminal studs) are on the other (front) side.
Here's a text drawing attempt to represent what appears to be the module configuration:
< Front Back >
(+)--[+ cell 12 -]__
/----[- cell 11 +]-/
\----[+ cell 10 -]__
/----[- cell 9 +]-/
\----[+ cell 8 -]__
/----[- cell 7 +]-/
\----[+ cell 6 -]__
/----[- cell 5 +]-/
\----[+ cell 4 -]__
/----[- cell 3 +]-/
\----[+ cell 2 -]__
(-)--[- cell 1 +]-/