Large Prismatic cells and small cylindrical cells in BEVs

[Jeff N]

Erik posted in that Facebook link above (comments) that he hasn't seen degradation. Whether it's true, who knows?

Which Facebook link? I don't see one.

 

[stopcrazypp]

As mentioned, it is not possible for there to be no degradation. Most likely whoever did the measuring did not have the equipment or data available to measure capacity consistently.

Keep in mind that the chemistry choices should be kept separate from the cell format choices. The 18650 based pack Tesla chose can safely be used for high density LCO and NCA chemistries, as well as all chemistries used for large prismatics, but the other way around is not true.

On paper, the LMO spinel / NMC chemistries should yield significantly higher cycle life (as in multiple times more) than Tesla's current NCA chemistry. I'm not sure if this is necessarily borne out in the real world (Tesla's packs have been doing very well in terms of degradation also).

 

[techmaven]

As mentioned, it is not possible for there to be no degradation. Most likely whoever did the measuring did not have the equipment or data available to measure capacity consistently.

Keep in mind that the chemistry choices should be kept separate from the cell format choices. The 18650 based pack Tesla chose can safely be used for high density LCO and NCA chemistries, as well as all chemistries used for large prismatics, but the other way around is not true.

On paper, the LMO spinel / NMC chemistries should yield significantly higher cycle life (as in multiple times more) than Tesla's current NCA chemistry. I'm not sure if this is necessarily borne out in the real world (Tesla's packs have been doing very well in terms of degradation also).

This.

It is not possible to build the Model S with a 85 kWh pack weighing ~1200 pounds using the chemistries used in the gen 1 Volt or Leaf cells. It might be possible to build the 85 kWh pack with the NMC chemistry used in the upcoming Bolt, but with a modified (lighter) cooling system. Also, Tesla couldn't achieve their target price without going with 18650s. Basically, without going with 18650 NCA's, Tesla couldn't ship the Model S at it was in 2012. Now, in order to safely use NCA's, they had to have a number of mitigation strategies for thermal management and safety that might not be necessary in later chemistries. Using a cylindrical form factor is part of the safety issue with using NCA cells. It is understandable that other OEMs didn't want to go down the route of using NCA chemistry in their designs, especially with the knowledge that other chemistries would be coming down the pike. Now, high voltage NMC didn't work out, but they were hoping that would allow them to leap frog NCA.

Higher cycle life for really large automotive battery pack is often unnecessary. As it stands, the original 85 kWh battery pack is likely getting 1,500 to 3,000 charge cycles cycling at about 60% DoD of the pack to about ~15% degradation. Assuming ~200 miles a charge cycle, that's 300,000 to 600,000 miles. That's well into the likely service life of overall vehicle. The physical integrity is likely to be just as much an issue as cycle life at ~15+ years necessary for most vehicles to hit well into that mileage.

 

[thrix]

Interesting discussion, but I think there is one important point missing.
When it comes to manufacturing in large numbers NOTHING beats a cylinder.