The Boeing 787's high-profile battery fire may have been the result of an engineering double-whammy: an energetic battery chemistry combined with a possibly inadequate cooling system.
Battery experts who spoke to Design News this week said that the 787's lithium-ion batteries employed a cobalt oxide cathode, which is known to be more prone to overheating than other lithium battery chemistries. If that chemistry was used without extra measures to draw heat away from the pack, it could be a problem, experts said.
"It's a no-brainer," Elton Cairns, a professor of chemical engineering at the University of California and a nationally known battery expert, told us. "If they used a cobalt oxide chemistry, then the battery should use a cooling system."
An NTSB engineer examines the casing from the battery involved in the JAL Boeing 787 flight in Boston. (Source: NTSB)
Although Boeing has not said whether the 787's lithium-ion battery packs use any kind of active cooling system, experts who saw photos of the packs said it looked unlikely. "The images I saw indicated that there was no active cooling system and this battery pack has many cells stacked close together," Donald Sadoway, the John F. Elliot Professor of Materials Chemistry at the Massachusetts Institute of Technology, wrote in an email to Design News. "So you need active thermal management."
Boeing representatives told Design News that their lithium-ion battery pack used specific measures to prevent overcharging. "There are multiple back-ups to ensure the battery system is safe," a Boeing spokesman told us. "That includes protection against over-charging and over-discharging."
Boeing representatives did not know whether the battery packs included cooling, however. And cooling was not mentioned in a five-page transcription of a Boeing media call explaining the incidents.
The 787's use of lithium-ion batteries for the auxiliary power unit is said to be a first, which is one of the reasons why the batteries are being scrutinized so heavily. The National Transportation Safety Board (NTSB) X-rayed batteries from a January 7 fire aboard a Japan Airlines Boeing 787 at Logan International Airport in Boston. The NTSB team also did CT scans, disassembled the battery, and examined flight data recorders to determine if it exceeded its design voltage of 32V.
On January 20, investigators said that the battery did not exceed its prescribed voltage. Since then, the agency has continued to look for the root cause of the problems, which have occurred on two Japan Airlines flights and one United flight.
Ervin is quite right! Besides that, any design that required active cooling for reliable operation would be a very poor choice in a situation where reliability and safety are top priority. Just look at the Japanese nuclear plant failure as a verification of that concept!
In addition it is certain that the folks at Boeing had verified that the design had an adequate margin of safety. So that would put the cause of failure in the supplier quality area, which, as has been reported quite a few times in Design News over the years, has been one of the ongoing problems with the project. Unfortunately, just because we have provided detailed and adequate specifications for items to be provided by suppliers does not assure that those specifications will always be met.
Cabin altitude (internal cabin pressure at altitude) for the 787 is 6,000 feet maximum. Plane cabins are not usually pressurized to sea level at altitude because of the stress on the airframe, although there is a trend to increase cabin pressures. 8,000 feet cabin altitude is generally the minimum pressure.
I like how everyone is assuming that active cooling is needed (and they seem confident about this.) However nowhere in the article is there a statement of current input or current output. We do not know the amount of power going in or out and cannot make an educated guess if active cooling is needed. I am sure if they were drawing a few W's an hour out of that battery the sheer size alone will handle cooling. There is a sentence that states that current input and output is controlled which leads me to believe that the battery had sufficient cooling.
Any equipment that goes into a plane is tested for thermal cycles. Not saying this one was only reviews of the reports can prove that. But I will be surprised if it was an engineering flaw and it had anything to do with cooling. If I took a guess it would have to be the rest of the electrical system has a fault. it could be one item or multiple items.
However on a project of this scale and expense and complexity it would be wise to not guess and wait for the investigators to do their job and inform the rest of the world what the problem is.
Paul I am far more optimistic on this one! I simply cannot believe that Boeing who has lived by the DO 254 credo for the better part of its existence could ever opt for standard COTS without rigourous climatic testing to verify the specs were met or exceeded. This would constitute a watershed in unacceptably poor risk management and I am not prepared to take away the benefit of the doubt, at least not just yet. If you are correct then we can both share the same kleenex in a corner, but I simply cannot believe that scenario.
I've always assumed that large commercial aircraft capable of cruising at high altitudes had some way of boosting cabin pressure, and if so why not go to full atmospheric? But in my experience, pressure does noticeably increase during a descent (ear problems), so you may be right, it's allowed to drop well below atmospheric. Even so, vibration is still the logical culprit.
Yes, the analysis needs to examine undamaged batteries from the same manufacturer and date code. Preferably samples that have already seen some use. In my own experience with medical devices - yes they need to be highly reliable as well - battery manufacturers want to offer their standard processes even for custom batteries. The problem here is that they want you to take RoHS compliance and No-Clean solder systems. This leads to unreliable circuitry when exposed only to thermal and humidity cycling. Add stress associated with a high vibrational environment and the reliability slips even lower.
I'm certain that Boeing did a lot of testing and much of it done in parallel in an attempt to minimize schedule slip. Unfortunately, some problems only reveal themselves when certain of the stressors are tested in series - just as they are experienced in the real world and under electrical load.
The other aspect of testing that is critical here is the pass-fail criteria. Simply meeting operating specs isn't good enough at this point. The investigators need to gently disassemble the units and along with everything else examine the circuitry under 20x to 40X magnification looking for the beginnings of electrochemical issues such as surface salts, dendrites and the more difficult to detect, tin whiskers. You will be hard pressed to find a component these days that doesn't have pure tin on the leads.
A problem like this rarely has a single cause. To or more corner case conditions must come together in just the right way. This is why serial testing of multiple assemblies under operational load is necessary.
From My earlier comment on these Lithium batteries, I want to hear from Boeing that these batteries are not COTS purchased batteries from the CLINTON era for purchase COTS to save money... Off the shelf devices DO NOT WORK ANYWHERE except on earth in a typical building. Now I want to see evidence that these batteries went through some BOEING testing and was accepted as properly built components worth of aircraft flight for 20 years in service. I bet they dont have that information available and this gets settled out of court with Uncle Sam!!
I share your scary thought, Chuck. I will always give Scientists and Engineers the benefit of the doubt, but as we are remembering the 27th anniversary of the loss of the Challenger's Crew today, sometimes margins of risk get bundled together into an "acceptable flight risk". Not suspecting anything nefarious, but with such a complex system, sometimes it is only possible to rank relative risk in hindsight.
nelso7926, I agree completely. No injuries and certainly no fatalities. I'm sure Boeing has tested and retested this system so I would certainly hope the issue is inadequate cooling and not the lack of cooling. With that being the case, are all of the failures on the ground? Are any experienced in flight? Also, can anyone tell me if there are redundant systems for this device? I don't think so but do not know.
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