Tesla Inc’s chief executive Elon Musk has suggested the U.S. electric carmaker may be able to mass produce longer-life batteries with 50% more energy density in three to four years. Researchers have said the energy density of Panasonic’s “2170” batteries used in Tesla’s Model 3 is around 260Wh/kg, meaning a 50% jump from the current energy density which is key to achieving a longer driving range. Tesla said its Battery Day will take place on the same day as its 2020 annual meeting of shareholders on Sept. 22.