Shares of Fisker Inc. FSR, -4.13% sank 4.2% in premarket trading Monday, after the electric vehicle maker announced plans to sell $340 million in convertible debt, with the potential for the amount offered to double to $680 million. The notes, which will be due 2025, will be convertible at any time at the investor’s option into common stock at an initial conversion price of $7.80, which represents a 30% premium to Friday’s stock closing price of $6.00. Fisker said it plans to use the proceeds of the offering for general corporate purposes, including an additional battery-pack line. The stock has tumbled 17.5% year to date through Friday, while the Global X Autonomous and Electric Vehicles exchange-traded fund DRIV, +0.95% has run up 31.4% and the S&P 500 SPX, +0.67% has gained 14.6%.