The two-month investigation found nothing unusual in any of the electrical components linked to the battery, which were all tested. These include the battery charger, its monitoring unit and a starter for the auxiliary power unit.

The report detailed a sequence of events lasting about 36 seconds during which the battery performed erratically and was drained of its power and the auxiliary power unit stopped. But it did not provide an explanation.

The battery first showed a one-volt drop in its designated voltage of 32 volts. Three seconds later, the battery current sharply increased for four seconds, indicating that current was flowing into the battery, instead of out of it. Current then began to flow out of the battery, but the voltage kept dropping. The voltage fell to zero and jumped back to 28 volts three times.

Boeing’s plan to redesign the battery would add insulation among the eight cells in the battery to minimize the risk of a short circuit cascading through most or all of them. The company also proposed adding systems to monitor the temperature and activity in each cell. It would enclose the batteries in sturdier steel boxes to contain any fire, and it would create tubes to vent hazardous gases outside the plane.

The 787 is the first commercial airplane to use large lithium-ion batteries for major flight functions.

Boeing has delivered 50 787s to eight airlines, and officials said it could replace the batteries in them quickly once a new design was approved. The company has much at stake with the plane, which is the first commercial jet to be built mostly of lightweight composite materials. Boeing has orders for 800 more of the planes.

Aviation analysts said the plan would probably protect against the main problem that the safety board had identified, a short circuit in one of the cells that can set off a chemical reaction, leading the battery to overheat.

But investigators in Japan have suggested that something else may have caused the battery on an All Nippon Airways 787 to emit smoke on a flight on Jan. 16. They said the battery may have been hit by a surge of electrical current from another part of the plane.

via New Details on a 787 Fire, but Little Headway in Inquiry – NYTimes.com.