They also faulted the plane's maker and the Federal Aviation Administration for designing and approving a battery that didn't protect against such a failure.
An inspection of the GS Yuasa manufacturing plant in Japan where the battery was made found that flaws and debris in lithium-ion aircraft batteries were going undetected, according to the National Transportation Safety Board report. Investigators were able to rule out other possible causes of the short circuit such as overcharging, external heat, or improper installation, the report said.
Boeing failed to anticipate, when testing the battery's design, that a short circuit in one of its eight cells might lead to uncontrolled overheating known as thermal runaway, which would spread to the other cells and cause them to vent smoke-like vapours and catch fire, the report said. The Federal Aviation Administration was faulted for not catching the design deficiency when it approved the batteries for inclusion in the plane.
The January 2013 fire in one of two batteries installed aboard a Japan Airlines 787 parked at Boston's Logan International Airport was followed nine days later by a smoking battery that forced an emergency landing in Japan by an All Nippon Airways 787. The incidents led the FAA and aviation authorities around the globe to ground the entire 787 fleet for over three months while Boeing redesigned the airliner's batteries, which run the plane's auxiliary power systems and also the cockpit electrical systems.
The short circuit occurred in the battery that runs the plane's auxiliary power system.
The events were a severe blow to Boeing's prestige. The 787, a long-range, wide-bodied airliner introduced in 2011, was the company's newest and most technologically advanced plane.
After the incidents, Boeing made extensive changes to the battery and its charger, including more heat insulation, holes to vent any flame or smoke outside of the plane, and lower charging levels. The case that contains the cells was also strengthened. Boeing has said the redesign, approved by the FAA, should contain any overheating inside the battery and vent any smoke outside the plane.
There was a third incident in January of this year when vapours were discovered coming out of a battery in a Japan Airlines 787 parked at Tokyo's main international airport. Boeing officials said the problem appeared to have been contained by the battery's redesign.
The NTSB report falls short of confirming a connection between the manufacturing flaws found by investigators and the short circuit that sparked the Boston battery fire, Kenneth Quinn, an attorney representing GS Yuasa said in a statement.
"The root cause of this internal short circuit remains elusive," he said.
Boeing spokesman Doug Alder expressed confidence in the redesigned batteries and the 787 are safe.
The 787 is the world's first airliner to make extensive use of lithium-ion batteries, which are lighter, recharge faster and can hold more energy than other types of batteries. The batteries help make the plane about 20 per cent more fuel efficient than comparable airliners, a popular feature with airlines whose biggest expense is the cost of jet fuel. There are more than 200 of the planes in service and another 1,000-plus on order.
Lithium-ion batteries, however, are generally more prone than conventional batteries to overheat and ignite if they have a manufacturing defect or if they are damaged, overcharged, or exposed to high temperatures. Airbus, Boeing's chief competitor in the global airliner market, recently said it will use lightweight lithium-ion batteries in its new A350 beginning in 2016.
Boeing contracted with Thales Avionics Electrical Systems, a European manufacturer, to design the 787's electrical power conversion system, which includes the two batteries. Thales in turn contracted with GS Yuasa to make the batteries.
Thales inspected the GS Yuasa plant in Japan that made the batteries, but failed to realize that plant's x-ray scans of finished batteries weren't sensitive enough to detect minute flaws and debris, the report said. Boeing and FAA officials didn't visit the plant until after the battery fire.
Boeing's battery testing concluded that short circuiting wouldn't lead to a fire and that the chance of a smoke event was one in 10 million flight hours. Instead, there were two battery failures when the entire fleet had clocked less than 52,000 hours.
Since the FAA didn't have safety regulations for lithium-ion batteries as installed equipment in planes when the 787 was designed, the agency and Boeing jointly developed special safety conditions the plane's battery system should meet.
The board said earlier this year that the government failed to properly test the batteries and relied too much on Boeing for technical expertise. It also said the battery testing plan proposed by Boeing and approved by the FAA didn't include testing of what might happen in the event of a short circuit.
The board previously recommended that new designs of lithium-ion batteries be subject to tests that induce short-circuiting under conditions similar to the conditions the batteries experience when they are installed in planes. The board also recommended that the FAA consult with outside experts such as scientists at the Department of Energy when evaluating new batteries rather than relying exclusively on the aircraft manufacturer's employees and contractors.
The NTSB's findings directly conflict with the FAA's own internal study released in March, which said the agency had "effective processes in place to identify and correct issues that emerged before and after certification." In May, the FAA cleared the 787 to fly far over the ocean, up to 5 1/2 hours away from the nearest emergency landing site.
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