The Boeing 787 battery fire in Boston last month was caused by a battery short circuit that led to a thermal runaway condition, the National Transportation Safety Board (NTSB) has concluded.
NTSB investigators who did forensic analysis of the Boeing battery said the charred components indicated the temperature inside the 63-pound pack had exceeded 500 degrees Fahrenheit during the January 7 incident at Logan Airport. Evidence showed that one of the pack's eight cells short circuited, causing the temperature to soar.
"Our review of the flight data recorder data shows that the voltage of the battery dropped from a full charge of approximately 32 volts to approximately 28 volts," said NTSB chairman Deborah Hersman in a press conference on Thursday. "This drop is consistent with the charge voltage of a single cell."
NTSB chairman Deborah Hersman said last week that investigators have concluded that a short circuit occurred in cell number six of the pack's eight cells. (Source: NTSB)
NTSB investigators ruled out mechanical impact damage and external short circuits as the reason for problems inside the battery, Hersman said. They did not know, however, whether the short circuit was caused by battery charging or by defects in the design and construction of the unit, or some other problem.
Experts contacted by Design News said a temperature of 500 degrees can create serious problems for a lithium-ion battery pack. "It's high enough to cause the cells to bloat and possibly to burst into flame," Donald Sadoway, John F. Elliott professor of materials chemistry at MIT, told us in an email. "Remember, the electrolyte is volatile and flammable."
Boeing said it still has not announced any specific ways to fix the 787 batteries, despite reports in various news media that it has already settled on a fix. The Wall Street Journal reported on Friday that the company "is looking at increasing the separation between cells in the lithium-ion batteries to reduce the potential hazards from heat or fire spreading within the batteries and adding enhanced heat sensors."
"There has been no public decision by the company on what these answers might look like or, specifically, what our teams have been working on," Boeing spokesman Marc Birtel told Design News. "We're not going to talk about a fix until we have something to share that's at the point of approval by the FAA."
Improvements like the ones mentioned by The Wall Street Journal have been the subject of much speculation over the past month. Two weeks ago, Tesla Motors CEO Elon Musk publicly stated that the 787 battery pack design was unsafe, and experts such as Sadoway and Elton Cairns of the University of California have suggested that an active cooling system would provide an additional layer of safety for a cost that would be miniscule compared to other 787 sub-system costs.
Sadoway again said last week that the Boeing batteries would be safer with active cooling. "That's what GM does in the Chevy Volt," he told us.
Another story came out yesterday, saying that Boeing intends to provide more space between cells. It's worth mentioning, however, that Boeing's spokespeople are not commenting, so we don't know for sure.
"The gaps between the cells will be increased." as immediate solution will allow a breathing space for further improvements. SAFT mentions in their battery manual:
"Un espace de 30 à 50mm doit être laissé libre sous les modules pour permettre la désoperculation des éléments en cas de problème."
Google translation - I love this tool
A space of 30 to 50 mm must be left in the modules to allow uncapping elements in case of problems.
The problem still exist but this short solution will reduce the cascade effect in case of a cell failure and will improve the local cooling - trough enhanced sufrace contact with the air environment.
I saw a graphic that Boeing was going to stick a vented containment around the existing battery box. I doubt that will satisfy the FAA and NTSB. Aside of the whole qualification time lag, why wouldn't they swap the Lion battery units with NiMH-based assemblies? Since they seem to have the space to put a box around the existing box, they could definitely go to a lower energy density battery technology.
this link shows some of the aviation incidents involving smoke, fire, extreme heat or explosion - and we continue to fly. 787 battery issue is a marketing issue, I hope the Dreamliner wil not change the name to Incubusliner.
@g_ost: It's hard to tell whether Airbus's decision to switch to NiCd for the A350 is more about public relations or safety. They've been flying A350s with lithium-ion batteries successfully so far. But being able to say that they don't have the "bad" battery chemistry that Boeing has may give Airbus an advantage with customers and regulators.
Due to 787 battery problems Airbus dispensed for A350 to the installation of lithium batteries. The first aircraft will be delivered with cadmium batteries, not lithium batteries. But the first test flights will attend Airbus with lithium batteries.
For large capacity multi cell series/parallel batteries each series stack typically will have its own controller. If thermal runaway begins in one cell, the consensus seems to be removing the charging current will NOT stop the runaway. See the nfpa link:
Other links regarding marine applications of lithium-ion batteries indicate that monitoring the individual cells (series stacks) is absolutely critical to prevent runaway. If this degree of protection is reasonable for a boat, I can't imagine it isn't practiced on an aircraft.
Yes, isolating each cell physically and electrically is doable, at a cost - but, if due to manufacturing defect or degradation over time, a cell develops an internal short, is it sufficient to just stop the charging current? Or will the stored energy through the short circuit lead to thermal runaway anyway? Does anybody here know?
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