Boeing Co.'s chief engineer Michael Sinnett speaks during a press conference on Boeing 787 jets in Tokyo, Saturday, April 27, 2013. Sinnett said Saturday that changes to the lithium-ion battery system are fully sufficient to ensure the aircraft's safety, although the company has been unable to find the cause of the original battery failures earlier this year that led to groundings of the plane worldwide since mid-January. AP Photo/Itsuo Inouye TOKYO (AP) - Boeing Co.'s chief engineer for the 787 Dreamliner said Saturday that changes to the lithium-ion battery system are fully sufficient to ensure the aircraft's safety, although the company has been unable to find the cause of the original battery failures earlier this year that led to groundings of the plane worldwide since mid-January. Michael Sinnett gave a briefing on the revamped battery to reporters in Tokyo after Japanese and U.S. regulators gave carriers permission to resume 787 flights once battery modifications are made. The new battery system is designed to prevent a fire, and to contain one should it occur with an "enclosure," a casing around the battery to prevent heat from being released in the aircraft, Sinnett told a news conference in Tokyo. "Even if we never know root cause, the enclosure keeps the airplane safe, it eliminates the possibility of fire, it keeps heat out of the airplane, it keeps smoke out of the airplane, and it ensures that no matter what happens to the battery, regardless of root cause, the airplane is safe," he said, adding "in some ways it almost doesn't matter what the root cause was." He said Boeing has identified over 80 potential causal factors and addressed all of them in the new design. The 50 Dreamliner jets in service worldwide were grounded in mid-January after incidents with smoldering batteries occurred aboard two different planes, leading to hundreds of cancelled flights and revenue losses. Japan's two biggest carriers have the most 787s - All Nippon Airways owns 17 of the jets, while Japan Airlines has seven. They have begun installing the new batteries over the last week, and airline officials said commercial flights would resume around June as the safety improvements are expected to take several weeks to finish. It takes five days to completely retrofit one airplane, Sinnett said, and repairs to nine jets are almost complete. New batteries are being shipped from Japanese battery maker GS Yuasa to the airlines, he said. ANA is planning to conduct a test flight using a modified Dreamliner in Japan on Sunday. The only U.S. airline using the 787 is United Airlines, which owns six. Japan is mandating additional safety measures including one test flight after the new system is installed. Operators will need to monitor the new battery system during flight and authorities will require airlines to conduct a detailed sampling inspection of the batteries after a certain period of use. Special training for all on board personnel including the pilot on 787s is mandated, and airlines are to disclose information on safety measures taken on the 787 to the public. Boeing has 840 purchase orders of the plane so far. Sinnett declined to comment on cost for repairs worldwide. He plans to meet with executives from ANA and JAL during his Japan trip. Associated Press writer Emily Wang contributed to this report.