Japan's ANA replaced faulty batteries on Dreamliner last year An ANA Dreamliner sits on the tarmac after an emergency landing at Takamatsu Airport in western Japan on January 16. ANA: 787's main battery failed to start normally on three occasions last year Batteries have become a focus of the Boeing Dreamliner 787 safety investigation An inspection of the Japan firm that makes the batteries found no serious problems Authorities worldwide continue to ground the aircraft amid safety concerns (CNN) -- A Japanese carrier has revealed that it replaced faulty batteries on its Boeing 787 Dreamliner fleet last year, as aviation authorities worldwide continue to ground the aircraft amid safety concerns over the devices. In an interview with the New York Times, All Nippon Airways (ANA) said the aircraft's main battery failed to start normally on three occasions and had to be replaced along with the charger. In other cases, one battery showed an error reading and another, used to start the auxiliary power unit, failed. All the events occurred from May to December of last year, the report said. Megumi Tezuka, spokeswoman for ANA, told CNN Wednesday that 10 lithium ion batteries on its 787s were replaced in 2012, with a low charge being reported on some of the devices on board the aircraft. READ: 'No big problems' with battery maker However, she said the problems were discovered during maintenance and therefore categorized as minor troubles. She added the airline was not required to report the faults to regulators. The news comes as an inspection of the Japanese company that makes the batteries found no serious problem with the devices. While investigators found several issues in quality control at the company, they did not uncover any serious problems that can be linked to recent incidents involving Dreamliners, Yasuo Ishii, director of air worthiness standards at the Japanese transport ministry, said Tuesday. Two recent incidents -- a fire aboard the Japan Airlines aircraft in Boston's Logan International Airport on January 7, and a smoke alarm aboard a plane flying over Japan on January 16 -- prompted the Federal Aviation Administration (FAA) to ground all Dreamliners in the United States, and other nations across the globe quickly followed suit. Boeing, meanwhile, has said its technical experts are working "around the clock" and are focused on "resolving the issue" and returning the 787 to service. There are currently 50 wide body Dreamliners in service globally, with several hundred on order. ANA was the first carrier to use the 787 and currently has 17 in service -- more than any other airline.