Bosch Battery Specialist Disputes Tesla's Advantages with 4680 Cells

Tesla keeps saying that its 4680 cells are a massive competitive advantage. Despite that, it recently asked Panasonic to start making them as soon as possible. The Japanese cell maker said that will not happen before 2023. It seemed that the only issue was manufacturing them. However, Bosch now gives us another perspective. Being larger, it may also present temperature concerns.
Tesla's 4680 cell 6 photos
Photo: Tesla
Tesla's 4680 Battery CellTesla's 4680 Battery CellPanasonic's  Financial 2021 Fiscal Results presentationTesla Battery DayTesla's 4680 Battery Cell
That idea was shared in a post on Bosch Research Blog by Jake Christensen. He is the company’s Chief Battery Modeling Engineer and works at the Research and Technology Center in the U.S., where he directs the Energy Systems and Materials Modeling department. Summing up, he creates the mathematical models that allow Bosch to save time developing its cells.

Christensen analyzed the concepts Tesla said made the 4680 cells unique, such as the tabless design. The engineer corrects that: according to what he could see in the pictures the EV maker released, it is actually a multi-tab design – or a shingled spiral of tabs that have been laser patterned. That creates a continuous tab architecture instead of a single-tab design, with this tab connected to the ends of the cathode current collector and the anode current collector.

Considering each cylindrical cell has rolled-up electrodes measuring around 1 meter (39.4 inches), the Tesla design shortens the total electrical path length from 3.5 m (137.8 in) to about 75 mm (2.95 in). That would be why the EV maker believes it would help increase energy density and reduce resistance and, consequently, heat generation inside the cells. The problem is that resistance is not the only source of higher temperatures in the battery.

Christensen stresses that most of the heat generated by cells comes from the electrochemistry itself. The Bosch engineer also mentioned that “a larger cell will generally be slower in rejecting that heat.” In other words, unless Tesla has a trick in the chemistry itself, 4680 cells tend to present higher temperatures than those in smaller cylindrical battery formats such as 1865 and 2170.

As EV owners already know pretty well, it is not a good deal to have hot batteries in a car. Apart from the risk of thermal runaway, they also degrade faster if they get too hot. Thankfully, Christensen said that the multi-tab architecture could help avoid lithium plating, which occurs in fast charging. The issue is that Tesla said the 4680 cell and its “tabless” solution “removes the thermal problem from the equation,” which needs to be verified.

With the info Christensen has, that’s not correct. The higher radial thermal resistance due to the larger cell diameter would be a problem in the long term. It could make these cells degrade sooner, thanks to faster parasitic reactions.

The entire blog post deserves a read. As Christensen points out, Tesla may have some elements that it did not disclose and prove thermal concerns are genuinely out of the equation with this cell. However, that can also be the reason for these cells to have taken so long to reach any vehicle – if they even did in the first place.

Until an independent party disassembles a battery pack with these cells and tests them, we’re just concerned for the customers with batteries in their cars that may not be mature enough for production. If they were, Panasonic would be manufacturing them – and that will not be the case until 2023 in the best-case scenario.
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About the author: Gustavo Henrique Ruffo
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Motoring writer since 1998, Gustavo wants to write relevant stories about cars and their shift to a sustainable future.
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