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Federal investigations said Sunday they had ruled out excessive voltage as the cause of a battery fire on a Boeing 787 Dreamliner in Boston this month, widening the mystery into what led to the grounding of the world’s most technologically advanced jet, after a second battery-related problem last week.

With investigators focused on the plane’s lithium-ion batteries, the National Transportation Safety Board (NTSB) said an examination of the data from the plane’s flight recorder indicated the battery did not exceed the designed voltage of 32 volts.

The fire aboard a Japan Airlines plane Jan. 7 at Logan International Airport in Boston occurred after the passengers had gotten off.

Last Wednesday, a battery problem on another 787 forced an All Nippon Airways jetliner to make an emergency landing in Japan. That episode prompted aviation authorities around the world to ground the plane.

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The Federal Aviation Administration said last week it would not lift the ban until Boeing could show the batteries were safe.

But with investigators on a global quest to find out what went wrong, the safety board’s statement suggested there might not be a rapid resumption of 787 flights.

On Friday, Japanese safety officials, who are investigating the second battery problem, suggested that overcharging a battery might have caused it to overheat. Pilots decided to make an emergency landing 20 minutes after takeoff after receiving several alarms about the battery and smelling smoke in the cockpit.

American investigators are helping with that inquiry, which Japan’s transportation-safety board is leading.

Speaking after the NTSB’s statement Sunday, a Japanese investigator said their inquiry was not as far along.

The Yuasa Corp. of Japan, one of the world’s leading lithium-ion battery manufacturers, produces the 787 batteries. France’s Thales makes the control systems for the battery.

The battery is part of a complex electrical system that powers the 787.

Like many other components and structures, Boeing outsourced much of the manufacturing to partners around the world.

The NTSB said the lithium-ion battery that powered the auxiliary power unit, a small engine used on the ground, had been examined in the safety board’s materials laboratory in Washington.

Investigators have also examined several other components from the airplane, including wire bundles and battery-management circuit boards as well as the battery-management unit, the controller for the auxiliary power unit, the battery charger and the power start unit.

Even though it appears the voltage limit wasn’t exceeded in the case of the Japan Airlines 787 battery that caught fire in Boston, it’s possible the battery failures in that plane and in an All Nippon Airways plane that made an emergency landing in Japan may be due to a charging problem, according to John Goglia, a former NTSB board member and aviation-safety expert.

Too much current flowing too fast into a battery can overwhelm the battery, causing it to short-circuit and overheat even if the battery’s voltage remains within its design limit, he said.

The battery is like a big sponge, Goglia said. You can feed it with an eye dropper or you can feed it with a garden hose. If allowed, it will soak up everything it can from the garden hose until it destroys itself.

There are so many redundancies and safeguards in aviation that when an accident or mishap occurs, it almost always is the result of a chain of events rather than a single failure, Goglia said.

Information from The Associated Press was included in this report.

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