However, that fire was due to problems with the test rather than the batteries themselves, he said.
"There are multiple backups to ensure the system is safe," Birtel said. "These include protections against over-charging and over-discharging."
But John Goglia, an aviation safety expert and former National Transportation Safety board member, said, "It certainly sounds like based on what has been released so far that we have an issue of the battery charger or some other source providing too much energy to the battery."
He said too-rapid charging might cause the electrolyte fluid in the batteries to overheat, leak and catch fire.
If the incidents are due to overcharging batteries, that might be good news for Boeing, Goglia said. A flaw in the aircraft's electronics that permits overcharging would likely be much easier to fix than having to replace the aircraft's lithium batteries with another kind of battery, he said.
Another possibility is a manufacturing defect in the batteries themselves, Whitacre said. More than other types of batteries, lithium ion batteries rely on very thin sheets of material internally to separate the negative and positive poles. The slightest flaw can cause a short circuit, overheating the flammable electrolytes.
"It's a delicate ecosystem that you are creating inside it and you have to manufacture it with perfect integrity," Whitacre said. "Then you have to keep it in the right voltage range and be very safe with its environmental conditions."
Jim McNerney, Boeing's chairman, president and CEO, sent the company's employees a letter Friday expressing confidence in the 787 and vowing to return the plane to service. "I remain tremendously proud of employees across the company for the decade of effort that has gone into designing, developing, building and delivering the most innovative commercial airplane ever imagined," he said.
The attraction of lithium batteries is that they are significantly lighter than other types of batteries. That saves fuel, which is airlines' leading expense. They also charge faster and contain more energy. And they can be molded to fit into odd space on airplanes, which most other batteries cannot.
The only other airliner using lithium batteries is the Airbus A380, which makes only limited use of the batteries for emergency lighting. However, Airbus is working on another airliner, the A350, expected to debut in 2014, that will make more extensive use of lithium batteries.
Boeing's headaches with the 787's lithium batteries are likely to cause European safety officials and other regulators around the world to take a harder look at the new Airbus plane's batteries, safety experts said.
"I think they're going to have a learning experience here that probably is going to result in future modifications for anybody who wants to design an aircraft and use this type of battery technology," said Robert Fiegl, chairman of aeronautical science at Embry-Riddle Aeronautical University in Prescott, Ariz.
The FAA, like aviation regulators in other countries, relies on the aircraft manufacturers to test their planes to make sure they are safe. FAA's certification engineers validate that testing and ensure that the level of safety meets FAA regulations. Boeing developed the safeguards for the 787s lithium batteries, but they had to win FAA's approval first.
The safety certification for the design, manufacturer and assembly of the 787 — a process that requires FAA approval each step of the way — was different in some respects from other aircraft because the Dreamliner employs so many cutting-edge technologies, safety experts said.
Besides its use of lithium batteries, the 787 is the first airliner whose structure is mostly made from composite materials rather than aluminum. The aircraft also relies to a greater extent than previous airliners on electronics to operate, rather than hydraulic or mechanical systems.