An investigation into the overheating of a lithium-ion battery aboard a Boeing 787 Dreamliner in January 2013 has been wrapped up by the Japan Transport Safety Board (JTSB) without saying what caused it. The aircraft operated by ANA Holdings Inc was forced to make an emergency landing in Japan due to the battery glitch, which followed another battery-related incident less than two weeks earlier in Boston and prompted authorities to ground the global fleet of Dreamliners for three months. Japan's transport authority said in its final report on the ANA incident that a lack of appropriate testing may have contributed to the overheating of the battery. The JTSB, however, said that during the device's development, engineers failed to identify the possibility that a short circuit in one cell could spread to other cells because they did not ground the device. This may have caused the battery to overheat, reported Reuters. "The test conducted during the development phase did not appropriately simulate the on-board configuration, and the effects of the internal short circuit were underestimated," the JTSB said in its 115-page report. Boeing said its tests on the battery were in line with regulations. "At the time, the industry standard was to conduct such tests on an electrically isolated battery. Those standards are unchanged today," said a Boeing spokesman in Tokyo. The report recommended that the US Federal Aviation Authority should "provide instructions to aircraft manufacturers and equipment makers to perform equipment tests simulating actual flight operations". Boeing won approval for its Dreamliner jets to resume flying after it redesigned the battery compartment to isolate any future thermal events and vent hot gases directly outside the aircraft, according to Shipping Gazette.