The safety investigation started after one of the 787s operated by All Nippon Airways made an emergency landing in Japan when its main battery overheated.
Earlier, a battery in a Japan Airlines 787 caught fire while parked at Boston's Logan International Airport.
Two weeks ago the US Federal Aviation Administration said both batteries had leaked electrolyte fluid, and there had been smoke damage to parts of the aircraft.
The FAA said airlines must demonstrate battery safety before flights could resume, a statement that effectively meant airlines had to ground their 787s.
Boeing, which competes against Europe's Airbus, has halted 787 deliveries. Boeing has orders for more than 800 Dreamliners.
However, Japan's transport ministry has said that no faults have been found with the battery used on Boeing's 787 Dreamliner.
It has raised fears that there will be no quick fix to a problem that meant all 50 787s in service were grounded.Attention has now shifted to the electrical system that monitors battery voltage, charging and temperature.