When Tesla unveiled its ambitious 4680 battery program in 2020, Elon Musk promised nothing short of a “revolution.” The larger cylindrical cells — 46 millimeters in width and 80 millimeters in length — were supposed to slash costs, boost driving range and help Tesla reduce reliance on outside battery suppliers.

But recent developments suggest that achieving the company’s long-promised battery independence may be proving far more difficult than Musk envisioned — underscoring the technological edge Korean battery-makers have built over years of large-scale production experience.

Tesla has quietly begun testing its in-house 4680 batteries in some European Model Y Long Range vehicles, replacing battery packs supplied by partners such as LG Energy Solution and Panasonic. The move was intended to demonstrate Tesla’s growing in-house battery capabilities, but it instead triggered fresh scrutiny over the real-world performance of the cells.

According to US media outlet Electrek, Tesla recently replaced partner-supplied battery packs in European Model Y rear-wheel-drive vehicles with its in-house 4680-based 8L battery pack without notifying consumers. Some customers viewed the change as a downgrade, citing reduced driving range and charging performance.

The previous 5M battery pack — based on smaller 21700 cylindrical cells supplied by LG Energy Solution — had a capacity of 82-84 kilowatt-hours, compared with around 79 kilowatt-hours for Tesla’s pack. The shift reduced the vehicle’s independently rated driving range by 52 kilometers, from 661 kilometers to 609 kilometers.

Tesla’s cells also appeared to lag behind in energy density, at 244 watt-hours per kilogram versus LG’s 265 watt-hours per kilogram, according to the report. Charging performance was reportedly weaker as well, with Tesla’s pack taking roughly 40-45 minutes to charge from 10 percent to 80 percent, compared with an estimated 27-30 minutes for LG’s battery.

According to a senior researcher at a major Korean battery-maker, the controversy highlights a broader reality in the EV battery sector: Making batteries larger does not automatically translate into better performance.

“Simply scaling up battery size does not necessarily lead to higher usable capacity,” the researcher said on condition of anonymity.

The challenge lies in how electricity and heat move inside the cell. While smaller batteries such as 21700 cells are relatively easier to optimize, larger 4680 batteries require electrons and heat to travel much farther distances, increasing internal resistance and making uniform reactions across the cell far harder to achieve.

As a result, some portions of the active material may become underutilized despite the battery’s larger physical size, limiting real-world performance gains.

The researcher compared the process to cooking ramyeon.

“Cooking one pack is easy, but making 10 packs evenly at the same time requires much tighter heat control,” he said. “Otherwise, some noodles end up undercooked while others become overcooked. Large-format batteries face a similar challenge.”

Safety and structural requirements add further complications. Larger batteries require thicker casing and stronger support structures, reducing the amount of space available for actual energy-storing materials.

Tesla’s latest setback underscores the manufacturing advantage Korean battery makers have built through years of large-scale production experience.

“Korean battery makers have accumulated years of manufacturing optimization know-how involving thermal management, charging speed, quality control and yield management,” the researcher said. “Large-format batteries require enormous amounts of testing and fine-tuning before they can achieve stable mass production.”

The researcher added that Tesla may also have adopted relatively loose standards for long-term battery consistency compared with Korean suppliers.

“If small compromises continue accumulating at the cell level, the performance gap becomes much more visible at the pack level over time,” he said.

Tesla has long sought to reduce reliance on outside battery makers. Since partnering with Panasonic for 21700 cylindrical battery production at Gigafactory Nevada in 2017, the company has steadily expanded efforts to internalize battery technology as part of its broader strategy to lower EV costs and secure supply chains.

“Controlling batteries effectively means controlling roughly 30 to 40 percent of an EV’s cost,” said an industry source who wished to be unnamed. “Producing batteries internally can reduce supplier dependence and help capture margins that would otherwise go to battery manufacturers.”

Still, the latest developments suggest that battery manufacturing may be far more difficult to internalize than Tesla originally expected.

Korean battery makers, particularly LG Energy Solution and Samsung SDI, have expanded investments in next-generation 46-series cylindrical batteries and are widely viewed as holding a technological lead in commercialization and manufacturing stability.

For the industry, Tesla’s latest battery controversy is becoming less about a temporary consumer backlash and more about the hidden complexity of scaling advanced EV battery technology for mass production.

hyejin2@heraldcorp.com