In his latest tweetstorm, the vibrant and knowledgeable CEO of the Silicon Valley-based automaker Tesla (TSLA) has opened up a bit more about EV battery packs and the 4680 cells thermal management.
Musk started tweeting by answering a question about different battery pack form factors and the use of pouch cells. He said that heat propagation or more technically called thermal runaway propagation (TRP) in large pouch cells is dangerously high. This is the reason Tesla is against the use of large pouch cells in its electric vehicles.
“Probability of thermal runaway is dangerously high with large pouch cells. Tesla strongly recommends against their use,” Elon Musk tweeted.
“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?,” Musk agreed to the theory presented in this tweet from a Twitter account Tesla Facts.
He further explained that cooling a cell with larger form factor is 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 2020 Battery Day. Since this is a larger form factor cell as well, I thought I should ask Elon Musk if Tesla also had to deal with heat propagation issues with it.
However, he did not reply to our official account directly but he answered 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 explored 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, the right way to dissipate battery heat. Coupled with the tab-less design that reduces the cooling loop, Tesla has found the secret sauce for optimal battery thermal management.