The Bolt EV technical papers presented at SAE last year give the nominal pack voltage as 350v.

That is correct 3.65 x 96 =350.4 V nominally

This discussion doesn't make a lot of sense to me as the individual cell form factor, power, and capacity specifications ultimately dictates pack construction and the necessary series-parallel connections (although there are a few choices to be made in order to optimize power vs storage)

The parallel connected cells designate group/triplet/module (and hence pack) capacity in Ah

Then you simply string those in series to achieve your desired pack voltage (to be compatible with he various power electronics, motor controllers, inverters etc) and peak power (V x I)

3 parallel cells were used in the Gen1 as they were (in 2011) a large format LG 15Ah prismatic cell which sets the triplet and ultimately pack capacity to 45Ah @ 3.75V nominally (usually ~65% SOC but it depends on the discharge curve )

96 triplets strung together in series made it ~360V nominally (total of 288 cells)

360V x 45Ah = 16.2kWh total energy capacity

By comparison he Gen2 cell has a capacity of 25.5Ah so the 2P96S results in a cell group/duo capacity of 51Ah (pack capacity of 51Ah as well)

52Ah x 360V = 18.360kWh (an 8% increase in pack total energy capacity using 1/3 fewer cells only 192 in total)

Tesla uses the diminutive Panasonic 18650 cell which has a capacity of 3.3Ah that is 13% of the Gen2 LG cell capacity (identical nominal voltage of 3.75V)

In the P90D a pack module construction of 6S74P creates a modular assembly with ~22.5V(nominal) and ~257Ah (444 cells)

By connecting 78 of those 3.3Ah cells in parallel raises the capacity of the module (and hence pack capacity) to ~257AH and then putting 6 of those groups in series raises the module voltage to 22.5VDC

Tesla then connects 15 of those modules in series which does not change the cellular capacity (still 257Ah) but raises the voltage to ~375VDC with a total of 6,660 cells

375V x 265Ah = 96.375kWh

So it's really the CELL characteristics and the design goals in terms of peak power and storage (and to some extant the physical limitations) that will dictate pack construction in terms of the required series - parallel connection.

WopOnTour