US safety investigators are examining battery of a Japan Airlines' 787 that caught fire in Boston last month

The US's top transportation safety investigator has questioned the "assumptions" regulators used before clearing Boeing's use of controversial lithium-ion batteries on its grounded 787 Dreamliner.

The national transportation safety board (NTSB) has been investigating the battery of a Japan Airlines 787 that caught fire at Boston's Logan airport in January.

NTSB chairwoman Deborah Hersman told reporters on Thursday that tests showed a failure in a single cell of the battery spread to the rest of the battery in a way unanticipated by Boeing.

Before the Dreamliner's approval for flight the company had discounted such an event, Hersman said.

The Japan Airlines battery fire and a battery alarm that led to an emergency landing for an All Nippon Airlines 787 days later led to the grounding of Boeing's hi-tech model. Boeing had calculated that the lithium-ion batteries would produce smoke less than once in 10m flight hours, Hersman said.

"The 787 fleet has accumulated less than 100,000 flight hours yet there have been two battery events resulting in smoke less than two weeks apart and on two different aircraft," she said. "Assumptions used to certify the battery must be reconsidered." She said those assumptions "were not met".

The fire in the Japan Airlines battery started with multiple short-circuits in one of the battery's eight cells, said Hersman. The short-circuits led to an uncontrolled chemical reaction, known as "thermal runaway", that spread to the rest of the cells. The fire started when the sixth of the eight cells failed.

Boeing was required to perform special tests to certify the lithium-ion batteries for the 787. These types of batteries have caught fire in many other situations including in cell phones, cars and light aircraft.

The FAA has strict guidelines for passengers carrying lithium-ion batteries as cargo because of fire concerns. The 787 is the first large passenger plane to use them so extensively.

Boeing's certification tests had shown no evidence of internal battery failures spreading from cell to cell, Hersman said. The root cause of the fire had yet to be identified.

The NTSB is checking whether contamination or internal battery defects led to the fire. A report of the safety watchdog's findings is expected in 30 days.

Having identified one issue, Hersman said, the NTSB would now examine the battery's design, how the unit's eight cells are linked, how it was charged and how it was manufactured.

Ultimately, she said, the FAA and not the NTSB would determine when the Dreamliner were allowed to fly again. But any hope of a swift resolution appears to have gone.

Boeing has orders for about 800 787s and is developing a new version. All 50 planes that have been delivered have been grounded and those on order have been put on hold until the battery issue is fixed.

The grounding is likely to lead to a vigorous examination of lithium-ion batteries in passenger planes. The Airbus A350, expected to be ready next year, uses them as well.