Which battery brand is fully owned by Berkshire Hathaway?

The battery brand fully owned by Berkshire Hathaway is Duracell.

Duracell traces its roots to the 1920s work of inventor Samuel Ruben and businessman Philip Rogers Mallory. The company became especially known for compact alkaline batteries and the advertising image of the Duracell Bunny, which symbolizes long-lasting power.

Procter & Gamble acquired Gillette in 2005, bringing Duracell into P&G's portfolio. In 2014, P&G agreed to transfer Duracell to Berkshire Hathaway in exchange for Berkshire's shares in P&G. The transaction closed in 2016, making Duracell a Berkshire-owned operating business rather than merely a stock-market investment.

Rayovac, Panasonic, and Energizer are separate battery brands, which is why they are plausible distractors but not correct here. Berkshire's ownership also covers Duracell's operating companies and manufacturing activities, not just a trademark.