Whether or not the battery exceeded its design voltage, however, experts believe a cooling system was critical. Lithium-ion battery chemistries in general are "energetic," they said, and the cobalt oxide varieties of lithium-ion are particularly so.
"Not all lithium-ion batteries are created equal," Cosmin Laslau, a research analyst for Lux Research, told us. "None of them should fail. They are all essentially safe. But in the event of a failure, lithium cobalt oxide would fail earlier than the other types. Chemical bonds in lithium cobalt oxide will release oxygen earlier." Experts say the release of that oxygen can, in rare cases, lead to fire.
Many engineering teams around the world choose cobalt oxide chemistries, however, because it offers energy densities that can be up to 25 percent higher than other types of lithium-ion, such as manganese spinel (used in the Chevy Volt) and phosphate-based systems.
To counteract the higher energies, big, lithium-ion batteries in general are often used in conjunction with cooling systems, no matter whether they are cobalt-, manganese-, or phosphate-based. The Chevy Volt, for example, employs liquid coolant that circulates through 1-mm thick channels machined into 144 metal plates sitting between its lithium-ion manganese spinel cells. Similarly, the Prius PHV plug-in hybrid uses specialized fans, intake ducts, and 42 temperature sensors to actively monitor and cool its lithium-ion battery.
To be sure, the 787's 63-lb battery pack is smaller than those of today's typical electric cars, which can often exceed 400 lb. But experts said that lithium-ion batteries of all types need ways for heat to get out. "Size does make a difference," Cairns told us. "But the size of that (Boeing) battery is still substantial. If the cell casings are touching one another or have inadequate space to allow for natural convection cooling by air, then you're in for trouble."
Cairns said that he hadn’t personally seen the Boeing battery pack, however, and didn't know if Boeing engineers had provided any means for the heat to escape.
Battery experts who spoke to Design News repeatedly stressed the fact that all types of lithium-ion batteries can be safe and successful, if engineered properly. The question still being answered is whether Boeing engineers did that. “They should have stress-tested the battery with charging system as it it is installed in the 787,” Sadoway said. “I myself wouldn't fly in a 787 at this point."
The litium battery must be observed in its used enviroment. We are talking low atmospheric pressure and the batteries could be outgassing causing a rupture between layers. Include with that plane vibration from jet engines could play in deterioating loose layers within the battery. The colder temperature at that altitude could have cause contractions the batteries should not have seen. As I remember the batteries should have thermal monitoring device and balance circuits to prevent heavy discharge and charge. So Rapid short within the batteries due to enviromental such as atmospheric condition not designed into the battery compartment would give a clue why the thermal sensing did not have time to respond and shut down.
You cannot analyze this by the damaged batteries alone. And new set for enviromental testing needs to be done to characterize the destruction.
It looks like control electronics reside inside the battery pack, so yes they would be subject to rising temperatures AND if electrolyte sprays on the control board all bets are off. This does not appear to be an intrinsically safe design.
This was a massive integration effort with literally thousands of verification tests. Notwithstanding it's relative criticality, if this is the biggest issue Boeing can pat themselves on the back, but (as previously mentioned) they must move expeditiously to arrest the problem, accept full culpability, and implement a lasting fix before their market share begins to suffer. The Defense industry has thought me that these post-production woes are intrinsic in a project of this magnitude; thank God the problem was discovered without catastrophic consequences.
Battar, your principle is sound but the article suggests that the cooling system may have been inadequate. Seems to me they might not have asked the question properly...and in a critical application safety margins for proper operation would be pretty important.
Many Lithium ion batteries are not actively cooled but do manage not to burst into flames. Most laptop batteries, camcorder batteries, etc., manage to function without active cooling.
Looking at the photos of the Boeing 787 battery pack I can only make out a single pair of high current output pins that ties the battery to the aircraft's power bus. If so, then all charging and discharging is going through that single pair of contacts, which means the cell monitoring and charging circuitry must live inside the battery. So, if the battery got too hot, possible runaway condition, it could have fried the controller thus disabling any chance of shut down or external alarm.
It would be foolish to not have additional wiring to enable external monitoring and control of the pack. Virtually every Lithium ion camcorder battery communicates with its host as do many other consumer and professional battery packs.
Not having active cooling and external monitoring of system temperature for an aircraft battery system would seem foolhardy.
There is someting fishy,for decades I have used embedded temperature sensors in all batteries, it is a part of my designed cell balancing and battery management systems, surely sudden temperature surges would be recorded -of course it will not prevent explosion due to O2 accumulation. I have seen cost saving measures causing accidents -but in aerospace!!
It would seem that not enough is known about the characteristics of this chemistry when used in aircraft systems where high altitudes are encountered. If a cooling system is deemed necessary, it will require redundancy for safety purposes. This of course will require futher testing and approval by the FAA for airworthyness. Perhaps a step back to a known and proven battery technology could be used temporarily at the cost of reduced capacity, but at least it will get the aircraft flying once again until a new, improved battery design can be readied.
I was a Boeing Engineer for a while and I worked on the 767 and 777 projects. I know something about Boeing. Yes, weight savings is very important. BUT SAFETY is the highest priority. So high, in fact that no technology is used that has not been thoroughly tested and confirmed to be safe. I really don't understand why this is happening, (and I will not guess, as I am an ME) we will find out eventually. Oh and yes, "The original Engineer isn't even in the picture" is somewhat true. Once an engineering design is completed by "The original Engineer" The design is circulated through all the engineering departments for their blessing, and modification (if any) before it is allowed to be installed in an airplane. The design may or may not look like the original after everyone has gotten their hands on it.
The division I worked for several years ago had a similar Lithium-ion battery 'thermal event' on a much smaller scale in a consumer market portable radio... We did not experience any fire, but there certainly was sufficient energy released to create a lot of smoke and reduce the radio to a bubbling mass of melted plastic. The cause seemed to be a short resulting from insulating materials being capable of sliding with vibration... The entire experience was perplexing...the experts were telling us it couldn't happen, yet it was!
Since we are all in the guessing mode, I'll guess too.
The weight savings was a big temptation so they calculated the risks and made a decision to go with this battery.
The surprise came in real life when the battery ended up working much harder than anticipated. Accessories?, add-ons?, custom build? (Sales says yes, yes, yes, and the original engineer isn't even in the picture.)
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