BMW is preparing to launch an all-electric version of its popular X3 offroader by 2020. BMW and Mercedes are betting they can mass produce new electric cars based on conventional vehicles, defying skeptics who say they will need more radical designs to head off the threat from Tesla and other start-up carmakers. There are two ways to make battery-driven vehicles: use a clean-sheet design like Tesla, or a traditional vehicle platform that can use all types of motor: combustion, electric or a hybrid of the two. Electric motors are smaller than petrol or diesel engines, so electric vehicles designed from scratch can benefit from better interior packaging which allows a bigger passenger space. The problem: their unique design…