Some of China’s top auto makers, facing among the toughest national emissions rules in the world, are lining up behind an emerging battery technology that provides a way to boost fuel economy to near hybrid performance. The Chinese car makers—including Geely Automobile Holdings Ltd. GELYY 2.81 % and FAW Group Corp.—in the next two years will begin selling gasoline-powered cars with 48-volt battery systems that provide additional energy for steering and other high-energy devices. These cars offer a lower-voltage building block to better fuel economy than the about 200-volt systems in the best-selling gasoline-electric hybrids. Although some U.S. and European auto makers have signaled plans to follow suit, it is unclear when they will join the fray. Analysts say 48-volt lithium-ion batteries are one of the most effective ways to create “mild hybrids,” vehicles that use smaller batteries and an electric motor to boost fuel economy without adding thousands […]