Hello Harlequin,
I attached 3 images, please check them.
I was thinking about the wires between the cell modules.
On the first image I have rearranged your CellTop module schematic, and for the better visibility, I have removed everything that I don't need for the "experiment"
On the second image I put two of this circuits behind each other, and I added a shape representing the copper connector between the two battery cells, and I have added simple 2 lines for the twisted pair wire also.
Nothing special, just to show that the VCC of the first cell, and the GND of the next cell is connected.
And because of this, I can reposition the resistor R2, as shown on the third image.
This way we don't need the twisted pair, only 1 wire (just as they did:
Battery Management System (BMS)).
However, this way the optocoupler becomes unnecessary also, am I right?
The BMS on the above link really doesn't use optos, and they can handle cells up to 255. They use twisted pairs only at the first and at the last cell, to connect them to the master unit. (and in one more case: if the battery pack is divided into multiple banks, and the banks are quite far from each other. Only the last module of Bank0 and the first module of Bank1 are connected this way.)
What is your opinion, can this make the design even simpler? (by the cost of the opto. Maybe a transistor needed to invert the signal, or it even can be handled inside the PIC?)