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

Tesla’s 4680 “table” cells, which are used in some of its cars, including the Cybertruck, are claimed to have “five times” more energy capacity, and the company announced in September she had produced 100 million. A recent report by The information said Musk gave the team working on the batteries an end-of-year deadline to iron out costs and other issues.

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

Although Zeng was not impressed with Musk’s knowledge of batteries, he thought he was good with chips, software, hardware, and “mechanical things.”

Zeng also commented that Musk’s problem lies in “overpromising” timelines, something he usually does often, especially when it comes to fully autonomous driving technology. “Maybe something requires five years. But he says two years. I definitely asked him why. He told me he wanted to push people.