Tesla's 4680 battery supply chain has been severely impacted as partner L&F Co. has written down the value of their supply deal by 99%, from $2.9 billion to $7,386. This suggests a significant drop in demand for Tesla's in-house battery cells, primarily used in the Cybertruck. The Cybertruck has been struggling with demand issues, and Tesla has been offering discounts and incentives to move inventory. The reduced demand for 4680 cells may indicate that Tesla is not ramping up production as expected.