Investigators concluded last year's battery fire that led to the grounding of Boeing Co.'s 787 Dreamliner jets was due to insufficient design and testing. The Federal Aviation Administration approved both design and testing after Boeing certified that overheating in one cell of the lithium-ion battery couldn't spread to others. The National Transportation Safety Board blamed both for not expecting how the power packs might fail, and cited battery maker GS Yuasa Corp. for poor manufacturing. The fire that occurred on Jan. 7, 2013 was the longest grounding of a large commercial aircraft by U.S. regulators since jets were introduced in the 1950s. Boeing has already redesigned the battery to include more protection around the cells to control overheating. The company also created a steel case to prevent any fire from spreading and a tube that vents fumes outside the fuselage.