Regulators need to rethink how they approved the batteries on the Boeing 787, a top U.S. safety official said Thursday, adding a new and potentially time-consuming wrinkle to the plane’s grounding.

National Transportation Safety Board Chairman Deborah Hersman said regulators must review the “special conditions” used in approving lithium-ion battery technology on the 787 Dreamliners after two battery-related safety incidents in a matter of days.

The 50 Dreamliners in service were grounded worldwide Jan. 16, after a fire on a parked 787 in Boston and an in-flight problem on another plane in Japan. The groundings have cost airlines tens of millions of dollars, with no end in sight.

“There have now been two battery events resulting in smoke, less than two weeks apart, on two different aircraft,” Hersman said. “The assumptions used to certify the battery must be reconsidered.”

Boeing investors took the news in stride, pushing shares 2 percent higher Thursday to close at $77.43. Analysts said the market was focusing on the wrong issue: the short-term question of fixing the battery versus the longer-term prospect that the whole battery system might need to be approved again.

If the battery needs to be re-certified, “you’re talking about changes to the 50 they’ve delivered, significant amount of engineering commitment on the 787-9,” said Ken Herbert, an analyst at Imperial Capital in San Francisco. “I see this as still having a significant amount of question marks.”

Boeing’s stock has risen 3 percent since the 787s were grounded, despite the headaches that has caused the planemaker and the demands for compensation.

Because finished 787s are piling up undelivered and Boeing customers are already agitating for compensation, that could complicate Boeing’s assumption that the grounding would not significantly affect its finances this year.

“The market is focusing on the battery short circuit, which implies a simple fix,” said Carter Leake, analyst at BB&T Capital Markets. “But they’re missing the much bigger issue, which is the questioning of the certification process. Hersman is basically saying we’re questioning the original certification altogether.”

Hersman mentioned nine so-called special conditions the FAA set in 2007 in approving Boeing’s use of the battery, and the plan to allow the battery to burn itself out if it caught fire, because the risk was considered extremely remote.

Boeing’s certification tests put the chances of smoke from a 787 battery at one in every 10 million flight hours.

“The 787 fleet has accumulated less than 100,000 flight hours, yet there have now been two battery events resulting in smoke less than two weeks apart on two different aircraft,” Hersman said.

The special conditions and the design assumptions are part of a broad review that the FAA launched last month, before the second battery incident. Hersman said the NTSB was not yet making any further recommendations.

Hersman also said the NTSB has isolated the source of a Jan. 7 battery fire in Boston to a single one of the battery’s eight cells but still has not found the root cause of the fire.

The NTSB plans to issue an interim factual report in 30 days, though the decision on returning the plane to flight rests with the Federal Aviation Administration.

In a joint statement, Transportation Secretary Ray LaHood and FAA Administrator Michael Huerta reiterated that the FAA’s comprehensive review was ongoing.

“We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward. The leading experts in this field are working to understand what happened and how we can safely get these aircraft back into service,” they said.

— Reuters