The National Transportation Safety Board believes an internal short circuit within a single cell inside a lithium-ion battery led to a fire aboard a Boeing 787, shedding new light on the battery problem that has grounded every one of the 50 Dreamliners in service worldwide.

The agency said Wednesday that it has completed disassembling the 32-volt battery that caught fire on a Japan Airlines 787 after passengers had disembarked in Boston on January 7. Investigators found evidence that the fire – called "thermal runaway" – started with a short circuit in cell no 6. There are eight cells in the 63-pound lithium-ion battery, and the NTSB said it found evidence that cell no. 6 sustained multiple short circuits. Investigators have ruled out mechanical damage as a cause of the short, as well as the possibility that the short circuit occurred between the cell and the battery case. Rather, the damage to the case containing the battery was caused by the fire that resulted from the short.

"The short circuit came first, the thermal runaway followed in cell no. 6 and it propagated to the other cells," NTSB chairman Deborah Hersman told reporters in a press conference this morning. Hersman said they have yet to find the cause of the short circuit but are looking at several possibilities.

"We are looking at the state of charge of the battery cells, we are looking at manufacturing processes and we are looking at the design of the battery," she said.

The new information came the same day that Boeing flew a 787 from its paint facility in Texas back to its factory north of Seattle. The flight was a one-time-only ferry flight approved by the Federal Aviation Administration, and only the pilots were allowed aboard. They closely monitored the battery during the flight to Paine Field and experienced no problems during the three-and-a-half-hour trip. It was the first flight of a 787 since the entire fleet of Dreamliners was grounded pending an investigation by the FAA.

Late today the FAA announced they will allow limited test flights by Boeing to collect data about the battery and electrical systems during flight. The test flights will be flown over unpopulated areas with just the Boeing crew on board the aircraft. Like today's flight, pilots will carefully monitor the batteries and will be required to land immediately in the case of a battery malfunction. One of Boeing's 787 flight-test work horses, ZA005, will be used for the flights.

Boeing, along with investigators in the United States and Japan, have focused on the lithium-ion battery from the start. And today's announcement that the problem appears to have started with a short circuit within a cell is exactly what battery expert Dr. K.M. Abraham suggested was the problem when we spoke with him last month. The lithium-ion cells within the 787 batteries use a graphite-coated copper anode and a lithium cobalt oxide-coated aluminum cathode. The anode and cathode are separated by a very thin polyethylene film known as the separator.

The separator is roughly the same thickness as cellophane and behaves in a similar way. There doesn't need to be a tear or a hole to create a short circuit that can cause thermal runaway. The material is very thin - typically around 25 microns, according to Abraham - and small irregularities in the thickness can be enough to lead to problems. A section of the separator that is just 20 microns thick might be enough.

"It could be a stretch, it doesn't necessarily have to be a big hole, just a weak point where you have low resistance," Abraham said. "It can be a problem when you have such a very large surface area electrode where there is a lot of inhomogeneity in the current distribution."

The variable thickness of the separator material could be a result of manufacturing, but also could occur during charging and discharging of the battery. A very small short might lead to the growth of a lithium crystal within the battery cell.

"Sometimes what happens is you start with a very small dendrite growth due to an internal short," Abraham says of the small fibers of lithium metal that can grow in the cell, "but it gradually heats up because you can pass current through it and heat up that location."

And just like cellophane, the separator can shrink when it is heated, Abraham says, "once it starts heating up slowly it will shrink and then a small short will become a massive short."

Abraham, agreeing with comments made by Tesla Motors CEO Elon Musk, said the relatively large cells in the 787 battery pose a problem. The large surface area of each cell increases the chance that an irregularity could lead to a short, Abraham said. The problem of the separator changing thickness due to heating is something addressed in the batteries used in the Chevrolet Volt. The separator is less likely to change thickness due to heating, according to Abraham.

"That was overcome in the Chevrolet Volt separator where they reinforced the separator with ceramic particles to mitigate the shrinking problem," he said.

According to the NTSB, the separators used in the 787 batteries are not reinforced with ceramic particles.

The NTSB chairwoman told reporters the agency also is looking into the certification of the Dreamliner. Because the 787 is replete with new technology, the FAA had to create nine special conditions for use of the lithium-ion batteries. Much of the testing was performed by Boeing, which performed several rigorous tests including one so grueling as to be called "abusive" to the battery, Hersman said.

Boeing saw no evidence during testing of a problem that would lead to the cell failure propagation the NTSB saw in the battery that caught fire in Boston. Hersman also said Boeing's tests indicated it was extremely unlikely that a battery problem would generate smoke, much less fire. The difference between the predicted likelihood and the recent events did not sit well with Hersman.

"The design and certification assessment and the assumptions that were made were not born out by what we saw," she said, speaking about the battery fires in Boston and Japan. "We had two events in two weeks on two separate aircraft. The fleet has less than 100,000 hours and [Boeing] did not expect in their assessment to see a smoke event in but less than 1 in every 10,000,000 hours."

In a statement released after the NTSB press conference, Boeing said it welcomes the progress being made in the investigation. The company also said it worked within FAA guidelines during certification.

"The 787 was certified following a rigorous Boeing test program and an extensive certification program conducted by the FAA," the statement read. "We provided testing and analysis in support of the requirements of the FAA special conditions associated with the use of lithium ion batteries."

Hersman said the NTSB will release an interim report within 30 days. That means the fleet most likely will remain grounded at least another month. Boeing is working on several possible fixes to the issue, including designs that will better protect the electrical bays and aircraft from any potential fire damage.

Meanwhile, several airlines, including All Nippon Airways and United, continue cancelling 787 flights many weeks into the future.

Correction: The story has been updated with the following corrected quote from Dr. K.M. Abraham, "but it gradually heats up because you can pass current through it and heat up that location."