Pushmi-Pullyu
Well-Known Member
When Tesla started building the 2008 Roadster, 18650 cells (made for laptop computers) were the cheapest way to buy kWh of li-ion batteries. Later, the 18650 batteries which Panasonic made for the Model S (and later the Model X), made to Tesla's specifications, were the highest energy density batteries used in any EV. So far as I know, they still are, with the exception of Panasonic's 2170 cells. There has been a lot of argument over whether or not those actually have a higher energy density than the 18650 cells currently made by Panasonic for Tesla, so I won't offer an opinion on the subject.
A higher energy density let Tesla put a high-capacity battery pack into its cars without making them that much larger, which meant Tesla's BEVs had a longer range than anybody else's. To a large extent, this is still true. But to claim that a higher energy density is the only advantage of using small cylindrical cells... well, that's simply not true.
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Let me try to create an objective overview, comparing cell types in what I think are the two best types of battery packs used in BEVs:
Tesla's battery cells: Small cylindrical cells (18650 & 2170) from Panasonic
Pro:
Higher production means higher volume, and lower cost from the advantage of scale
Smaller size enables higher energy density, allowing smaller pack size, significantly reducing the cost of the car
body
Metal case means simple, strong battery pack architectureRound shape means space between the cells, helping dissipate heat safely
Relatively small size means a high surface area to volume ratio, again helping dissipate heat safely
Large numbers means the failure of a single cell has little impact on the pack's capacity
Con:
Small size means large numbers are required, which also requires a large number of connections
Round shape makes coolant loop architecture more complex, as the loops must be curved around the cells
Higher energy density means greater risk of overheating and fire, which has required Tesla to build various fire
prevention systems into its packs
Despite fire prevention systems, there have been a small number of battery fires in Tesla carsGM's battery cells: Moderately large pouch cells from LG Chem
Pro:
Pouch cells are cheaper than other types of cells, all else being equal*
Larger pouch cells means fewer cells are needed per kWh, and thus fewer connections needed
Pouch cells have flat sides, making cooling loop architecture simple
Pouch cells can be packed tightly, side-by-side, with no gaps between the cells, allowing smaller pack size,
significantly reducing the cost of the car body
Lower energy density chemistry means lower risk of fireCon:
Pouch cells are soft-sided, requiring supporting frames, meaning complex pack architecture
Packing cells tightly side-by-side yields little area for radiating heat away (most heat must be carried away via
conduction)
Larger cells limits energy density because more heat builds up in each cellLarger cells means the failure of one cell has a greater impact on pack capacity
Lower energy density means larger pack size for a given capacity, requiring a larger and more expensive car body
*Of course it's not equal in this case, because Panasonic makes a far greater number of kWh of cells for Tesla than LG Chem makes for GM, so that gives Tesla a significant cost advantage via volume discount.
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Overall, I'm not sure which approach has the greater advantage of pack-level energy density. (Perhaps someone else can supply figures for pack-level energy density.) Tesla's use of round cells means there must be some space between the cells, but those cells have higher energy density. GM's cells are flat and are packed tightly side-by-side, but the cells themselves have lower energy density. Perhaps it's best to say that both approaches result in good (or perhaps even superior) pack-level energy density, but use different approaches to get there.
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