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."
I guarantee you that was not expert advice. Protection circuits are placed to insure that the amount of power drawn from a battery allows for safe operation. True we would like to have active cooling but:
Can any one guess the cost of heat sinks?
The added weight because of the heat transfer unit?
The cost of the delivery of Air, water, or fluid flow through the device (added weight as well)?
The cost of developing and testing the equipment so that it will not impact safety of the system during operation in the event of a failure?
The cost of maintaining the equipment?
No I don't think there is any logic in the claim that active cooling is required simply because other systems tend to use it. Best and cheapest way is to hire an engineer specializing in thermal characteristics of that device and insure that heat rise of the internal components does not pose a threat to the system and limit power draw that way. You supply more deep cycle power for less weight (weight is a major factor in aviation and duration of emergency operation) and insure that your device has passive cooling. This is a guess and it seems that the end design goal to this battery was this. I have dealt with devices that require active cooling in the airframe (I specialize in the hottest part of the airplane the engine). Testing components that require active cooling is a long stretched process. The analysis of the heat transfer unit alone could easily become a 1000 page report. Not to mention the cost added to the unit.
I checked the CT scans made by NTSB and I'm surprized to see the deformation of the individual cells. The protective cell case looks weakly designed when the requirments for aeronautical applications include strong external pressure variations. I would say also that the large terminals could initiate case cracks during battery assemby or life time.
I would not tip on battery manufacturing process issues much more on battery cell integration solutions. It looks like a poor engineering solution.
You hit on the long term problem. The composites have not had enough in-use time to verify long term durability. Accelerated testing, which has been done on the types of composites used in aircraft, is useful, but not definitive. The best accelerated testing is achieved when the results can be checked against a known record accumulated in the actual operating enviroment. That has not yet been possible at the depth and scale of the current expansion of composites usage.
It's true that we don't know yet what current or voltage was going in or out of the battery. But electronic battery management -- i.e., protection against overvoltage and overcurrent -- by itself is not considered sufficient for cooling of a big lithium-ion battery, the experts told us. "Just having protective circuits is fine, but it's absolutely insufficient," Elton Cairns of the University of California said when we asked him. "There's no avoiding the generation of a certain amount of heat. Ant time you operate a battery, heat is generated." It's worth noting that all automakers who use lithium-ion batteries also also use battery management ICs to monitor voltage, current and charging rate. But even while they use battery management ICs, they also all use cooling systems -- either liquid-based or air-based.
@Ann: On the other hand, a pessimist would say that the battery problems are just keeping the planes from accumulating enough flight time for the problems with the structural composites to become apparent yet. Fix the battery problems and get the planes flying again -- for long enough to get some fatigue cracks going -- and pretty soon you'll have structural components failing.
I'm not saying that I necessarily endorse that view, but that's the way you need to think if you want your designs to work.
I am with you on this, but obviously something went wrong. Even in the most careful design, sometimes there are unforseen problems. Then again, it's pretty well known that lithium ion batteries can overheat and possibly explode...so I guess it will take awhile before we know the real story.
FYI, Elizabeth, in case you missed it, there was a lot of criticism of composites in aircraft early in the game last year due to some minor problems with the 787's composites, as well as composite issues with some Airbus planes.
William K, you bring up a good point about specs and suppliers. Unless you do a lot of quality control on those suppliers, you open yourself up to potential problems.
Years ago I was at a spacecraft manufacturer. We had a ball bearing lab. Yes, ball bearings. These are used in gyroscopes, reaction wheels and other mechanisms. We did start to contract that out, but the quality was so bad that we brought it back in house. It was that important. Not long ago I talked to a local manufacturer of components for industrial machinery. His supplier convinced him to take some foreign made ball bearings. Well, they failed in customer installed equipment in a matter of months. He could test for most things, but did not have the facility to test for hardness. Guess what the problem was?
I could go on and on about companies that I have consulted with that contracted out manufacturing and were burned by it. Generally the smaller companies go out of business. Boeing's experience with the 787 is a cautionary tale. An aircraft manufacturer is not just a brand and design house. Some of the people posting here have said they would not fly in a 787. These are informed people. Boeing has some work to do.
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