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Discussion Starter #1 (Edited by Moderator)
I thought I'd bring this up because I know that the Bolt EV shares the same pack configuration as the Volt (96s3p), but I'm wondering if that is still the best pack configuration. It makes sense to me for a small battery pack like we find in the Volt. However, does it make still make sense for larger battery packs?

One of the reasons I ask is charging rates. Most of the DCFC stations available charge at 500 V, and the pack voltage on the Bolt EV is relatively low (charging at the lower end of SOC starts at about 330 V). Technically, the i3 charges at a faster rate because its pack voltage appears to be about 5% higher.

What would be the advantages and disadvantages of going with a 100s3p or even 120s3p pack configuration?
 

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AFAIK, the Volt cells peak at 4.20v (403vdc max) so I'm thinking the Bolt will do the same.

96 is a magic number. The next one is 128s. These numbers are divisible by 32/16/8 which makes them computer friendly and balancing friendly.

128 probably made more sense though as it is divisible by 64 as well, but that's 538v which... uh... might make it DCFC and 3ph AC fast chargers more expensive?
 

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Just as a note the Gen II Volt is actually a 96S2P configuration. Gen I is a 96S3P.

Also the nominal cell voltage of the Bolt EV is lower than that used in the Volt. The Volt's nominal pack voltage ends up around 360VDC while the Bolt EV's nominal pack voltage is 344VDC.

I do think higher pack voltages will come in the future. However today under 400VDC is common because I think there is a large set of electronics built around handling those types of voltages. Once you start getting into higher voltages you start getting into more customised and much more expensive electronics.
 

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Discussion Starter #4
Interesting. The 128 cells makes sense.

As for the pack voltages, my Torque Pro registers the Bolt EV's pack voltage at 352 V. That formula might be off, though.
 

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Interesting. The 128 cells makes sense.

As for the pack voltages, my Torque Pro registers the Bolt EV's pack voltage at 352 V. That formula might be off, though.
Is that a full charge voltage or a nominal voltage? The 344VDC number I got is a nominal number from literature used for GM dealer service technicians.
 

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Interesting. The 128 cells makes sense.

As for the pack voltages, my Torque Pro registers the Bolt EV's pack voltage at 352 V. That formula might be off, though.
At what state of charge? Voltage climbs as SOC rises. It would be interesting to note the voltage vs SOC curve.

DOH!! Beat me!
 

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Cell voltage is mostly proportional to SOC. Especially in the middle 65-70% range that Volt uses. There is a max charge voltage, that Volt only sees briefly at the end of charge when the OBCM goes into constant voltage charge mode (using the max charge voltage). During discharge, the cell voltage drops, depending on load (KW). The computer must make a determination of SOC based on voltage, KW draw and total KWHr use, as total capacity is reduced at higher load levels.

96S is based on a max pack voltage design around 400V (and max charge voltage). Going higher will increase the cost of the electronics, but can decrease the cost of wire (at the same power level, higher voltage needs less current).
 

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AFAIK, the Volt cells peak at 4.20v (403vdc max) so I'm thinking the Bolt will do the same.

96 is a magic number. The next one is 128s. These numbers are divisible by 32/16/8 which makes them computer friendly and balancing friendly.

128 probably made more sense though as it is divisible by 64 as well, but that's 538v which... uh... might make it DCFC and 3ph AC fast chargers more expensive?
Gen1 Volt's cells peak at just over 4.000 volts (<4.009); pack voltage ~384 volts at full charge.
 

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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
 
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