Elon Musk weighs in on electric vehicle battery form factors
What form factor is best for electric vehicle batteries? Most would agree: safety first. That said, are there battery form factors that are safer than others? While you can ask a variety of "experts" their opinion on the matter, Tesla CEO Elon Musk is probably a good candidate to start with — after all, Tesla sold almost 80% of all electric cars sold in the US last year.
Above: Elon Musk discussing the new 4680 battery form factor at last year's 'Tesla Battery Day' presentation (Source: Tesla)
It turns out that Elon recently opened up a bit more regarding his views on EV batteries. Musk began by answering a question on Twitter about different EV battery form factors and the use of pouch cells. He said that heat propagation (technical term: TRP which stands for thermal runaway propagation) seen in large pouch cells can be dangerously high.
“Probability of thermal runaway is dangerously high with large pouch cells. Tesla strongly recommends against their use,” Elon Musk tweeted.
Pouch cells have come under some scrutiny of late due to GM's Chevy Bolt recall related to a series of battery fires, as reported by CleanTechnica. According to Steve Hanley, "The Bolt’s battery packs are made up of pouch cells, which are essentially layers of cathodes, anodes, and separators that are flooded with liquid electrolyte and encased in a flexible polymer pouch."
In turn, is there a better approach to EV batteries? Twitter weighed in once Musk provided his initial input. “So smaller, reinforced, pressure protected prismatic cells for iron based cells (LFP) are good & safe, and steel cylindrical for nickel (and iron) are the overall design sweet spot?” — a tweet inquired from Twitter handle Tesla Facts. It appears Elon agrees.
Above: Tesla tab-less battery cells (Source: Tesla)
Musk further explained that cooling a cell with a larger form factor can be a challenge because the cooling loop to the center is a longer distance. This high cooling loop makes it harder to prevent hotspots (or heat spots). “Then, pressure & heat released from large cell in weak bag make it impossible to stop whole pack from burning,” Musk tweeted.
Tesla unveiled its 46mm diameter and 60mm length (4680) form factor battery cell at the company's "Battery Day" in 2020. Since this is a larger form factor cell as well, I thought I should ask Elon if Tesla also had to deal with heat propagation issues with the 4680.
However, he did not reply to our official account directly but indirectly addressed the question, “Our new cell is 46mm diameter with steel shell & even that was huge challenge for propagation resistance,” Musk said.
Taking a closer look at the Battery Day presentation, vehicle teardown expert Sandy Munro noted that Tesla has changed its battery cooling mechanism. Previously, the batteries were cooled down by placing the battery coolant tubes between the cell walls.
The newer battery packs with 4680 cells will be cooled down by placing the coolant tubes above and below the cells, an effective technique to dissipate battery heat. Coupled with the tab-less design that reduces the cooling loop, Tesla appears to have discovered an optimal approach for battery thermal management.