NEW YORK — Federal investigations said Sunday that they had ruled out excessive voltage as the cause of a battery fire on a Boeing 787 in Boston this month, widening the mystery into what led to the grounding of the 787, 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 said data from the flight recorder indicated the battery ‘‘did not exceed the designed voltage of 32 volts.’’ The fire aboard a Japan Airlines plane on Jan. 7 at Logan International Airport occurred after passengers had left.