WASHINGTON (AP) - A Boeing plan to redesign the 787 Dreamliner's fire-plagued, lithium-ion batteries won approval on Tuesday from the Federal Aviation Administration (FAA), although officials gave no estimate for when the planes would be allowed to fly passengers again.

The plan includes changes to the internal battery components to minimise the possibility of short-circuiting, which can lead to overheating and cause a fire. Among the changes are better insulation of the battery's eight cells and the addition of a new containment and venting system, the FAA said in a statement.

A series of tests, including flight tests, must be passed before the 787 can return to service. So far, flight tests of two 787s with prototypes of the new battery design have been approved, the agency said.

The plan is an outline for a recertification of the plane's batteries, the FAA said. The 787 has two identical lithium-ion batteries, one of which is located toward the front of the plane and powers cockpit electrical systems, the other toward the rear and used to start an auxiliary power unit while the plane is on the ground, among other functions.

Every item that is part of an airplane, down to its nuts and bolts, must be certified as safe before the FAA approves that type of plane as safe for flight.

The 787 fleet worldwide has been grounded by the FAA and civil aviation authorities in other countries since Jan 16, following a battery fire on a Dreamliner parked in Boston and a smoking battery that led to the emergency landing of other 787 in Japan.

"This comprehensive series of tests will show us whether the proposed battery improvements will work as designed," Transportation Secretary Ray LaHood said in a statement. "We won't allow the plane to return to service unless we're satisfied that the new design ensures the safety of the aircraft and its passengers."

The cutting-edge airliner's troubles have raised concerns that the FAA has ceded too much responsibility for evaluating the safety of new aircraft to manufacturers. To save manpower, the FAA designates employees at aircraft makers and their subcontractors to oversee the safety testing of new planes. 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 every 10 million flight hours. Instead, there were two battery failures when the entire fleet had clocked less than 52,000 flight hours.

The FAA's approval of Boeing's battery plan "is a critical and welcome milestone toward getting the fleet flying again and continuing to deliver on the promise of the 787", Mr Jim McNerney, the aircraft maker's CEO, said in a statement.