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Regarding theoretical pack capacity not corresponding with cell specs.

billysb

New Member
Feb 17, 2020
2
0
Norway
I have some questions I can’t seem to find specific answers to, and hopefully you can help me understand.

Tesla uses heavily customized Panasonic cells for the battery packs based on the 18650 format. More specifically the NCR18650A/B-cells with different rated capacities. I’ve found spec sheets for the commercial NCR18650A-cells with typical capacities of 2900 mAh and 3070 mAh.

Some maths to summarize what I do (and don’t) understand:

From what i know the rated capacity will (ideally) increase with cells connected in parallel and the nominal voltage will ideally increase linearly with cells connected in series.

I’ve read from multiple sources that the 85 and 90 kWh packs are the same, and contains the same amount of cells (7104).

Theoretical energy:
[Wh] = n [no. of cells] * Rated capacity [Ah] * Nominal Voltage [V]

Example: for the 85 kWh pack where there are 7104 cells with nominal voltages of 3.6V and rated capacities of (ish) 3.1 Ah. The cells are distributed as 16 modules with each module containing 6 «groups» in series where each group has 74 cells connected in parallel.

[Wh] = (74 cells/parallel * 3.1 Ah) * (16 modules/series * 6 groups/series * 3.6 V) = 79280.64 Wh
Equally (but simplified) = 7104 cells * 3.6 V * 3.1 Ah = 79280.64 Wh (much lower than said 85 000 Wh).

I understand that there are internal resistance and that the rated capacity is specified for upper and lower voltage being 4.2 and 2.5 V respectively, but can you help me understand the significant deviation?

I am also wondering if any of you have any spec. sheets for the 3.4 Ah cells that supposedly are used in the 100 kWh pack.


All help and discussion would be greatly appreciated.
 

billysb

New Member
Feb 17, 2020
2
0
Norway

r1200gs4ok

Active Member
Jul 17, 2019
1,370
467
Irvine
I had read somewhere that the 2020 Raven 18650 cells had a different chemistry than the previous model s....I am still trying to find that article
 

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