Panasonic on Monday unveiled a prototype large-format 4680 cylindrical battery cell for Tesla, The Wall Street Journal (subscription required) reported.

The electronics giant and primary Tesla battery supplier is aiming to start test production as early as March. It marks an important step for Tesla's future battery plans, and an about face of sorts for Panasonic.

The 4680 cell format was at the center of Tesla's Battery Day presentation last year—and one of the keys to cutting battery costs. The name refers to the cell's larger size—46 millimeters wide by 80 millimeters long. For reference, the Panasonic-supplied cells currently used in the United States-market Model 3 and Model Y are 2170-format—21 millimeters wide by 70 millimeters long.

Cylindrical cells - Panasonic hinting at 4680 progress

Cylindrical cells - Panasonic hinting at 4680 progress

Even earlier this year Panasonic was skeptical of the format change, suspecting that the larger cells would have thermal issues. In a rare interview, Panasonic EV battery boss Yasuaki Takamoto said large-format cells could lower EV manufacturing costs, but noted that they were more susceptible to particle contamination, which can lead to battery fires.

Tesla plans to use the 4680-format cells as part of a structural battery pack in the upcoming Texas-built Model Y, and then in the Cybertruck and Semi, among others. Other automakers and battery suppliers have expressed interest in the format as well.

Lucid CEO Peter Rawlinson recently also sounded bullish on the idea of larger-format cylindrical batteries. The Lucid Air is using 2170-format cells supplied by Samsung SDI in the initial Dream Edition, then "next-generation" cells from LG for the Grand Touring model down through the rest of the lineup. Both Samsung SDI and LG are also reportedly pushing ahead with plans to make 4680 cells or at least explore large scale production.