Robin Zeng, the founder of the world’s largest EV battery company, says Tesla CEO Elon Musk’s big bet on 4680 cylindrical cell technology “is going to fail and never be successful.” Zeng, the chairman of China’s Contemporary Amperex Technology (CATL), tells Reuters that when Musk visited China in April, “We had a very big debate, and I showed him. He was silent. He doesn’t know how to make a battery.”

Tesla’s “tabless” 4680 cells, which are used in some of its cars, including the Cybertruck, are supposed to have “five times” more energy capacity, and the company announced in September it had produced 100 million of them. A recent report by The Information said Musk had given the team working on the batteries an end of the year deadline to deal with costs and other problems.

CATL batteries, meanwhile, go inside everything from Tesla vehicles in China to Ford EVs in North America like the Mustang Mach-E and F-150 Lightning. The company specializes in lithium iron phosphate batteries (LFP), which generally don’t get as much range as various cylindrical cell units used in many Tesla vehicles.

While Zeng wasn’t impressed with Musk’s battery knowledge, he thought he was good with chips, software, hardware, and “mechanical things.”

Zeng also commented that Musk’s problem is “overpromising” timelines, which he often does habitually, especially regarding Full Self-Driving technology. “Maybe something needs five years. But he says two years. I definitely asked him why. He told me he wanted to push people.”