The classic question. Was it a manufacturing problem in the battery pack that caused the cell to short or did the pack overheat and then a cell failed, as so many people looking from the outside have suggested?
the cell presented is not the one from 787 but the technology should be similar.
The 787 battery pack has 8 cells serial hard connected. The cells can not be individual switched off in case of failure. The damaged cell still getting power from the good cells and aggravate the issue.
I understand that, but did the cell fail from being overheated, or did it become hot because it had failed? The folks at Tesla and others have said that this type of pack requires active cooling. If that's true will an overheated cell fail by shorting, or was the cell poorly constructed, shorted and then caused the thermal runaway?
Like I've said before, if the battery is prone to runaway thermal failure, it really needs to be designed to be jettisoned like a fuel tank when it gets into trouble, or insulated and isolated in such away that it can burn itself out without smoking up the aircraft or risking further damage. Additional thermal sensors alone are not going to fix this problem! I'm also concerned that charging is not done on a cell by cell basis as is the case with all of the consumer Lithium Ion battery packs I'm familiar with. This is the only reliable way to catch faulty cells before they fail catastrophically or to insure proper charging.
Charging the cells in series configuration requires perfect matching of each cell. And if they don't track with age, serious overcharging of some cells could result! One of the benefits of individual cell charging is being able to measure cell temperature and control the charge cutoff accordingly. In a series strung pack, the weakest cell, the one that attains end of charge temperature first, would have to force charge termination for the entire pack. There is no way to use the voltage curve to safely terminate the pack's charge if one cell starts to go bad. In fact, if a cell's voltage gets depressed due to a partial internal short it might result in a prolonged charging cycle which would further damage the bad cell.
With Boeing indicating they want to add more temperature sensors to the battery, I wonder if they presently only monitor the entire packs temperature and not individual cells.
Well written! Boeing's engineers no doubt picked up on potential problems with the battery system, and Liaison Engineers who are involved with nonconformance would have written up a disposition to look into the situation. The problem is that the disposition may have been transferred by A.N. Other to the next higher assembly for resolution until it reached flight test. By that time the original concern which raised the nonconformance would be a faded memory. Flight testing would address the perceived problem, find nothing untoward during flight testing and sign off the paperwork without necessarily contacting the original Liaison Engineer who wrote up the disposition...they may not be with the company anymore.
This is a common scenario. Only Liaison Engineers with a roving commission can actually check up on a travelling nonconformance to ensure that it is actioned within a sensible time period. With the large number of nonconformances that emerged during the DreamLiner episode paperwork could get buried quite easily.
With regard to the comment about bloated cells...that's likely to be on the broad face of the prismatic casing...#6 is sandwiched between #5 and #7 so it has nowhere to go....the vent would normally open to relieve the bulging pressure...did this happen?
It's all rather perplexing given that the technology is pretty well understood. It makes me wonder if something else is going on. If you start with the thought of what might be different about an airplane, pressure cycling comes to mind.
I suppose that that there might be other issues outside the normal concerns that were not considered.
Thanks g_ost for the link to a presentation made by Yuasa that shows their products used in satellites...
It is the typical business presentation, proudly showing their modern producton installations and many achievements... but, wait. Did I see the word "FIRE" in page number 46 (slide 41 of 49) where in paragraph where they report the test named "Heat to vent" , they state the following phrase:
Cell vented when cell temperature reached to approximately 145 degrees C.
A small amount of gas was emitted from the cell. Approximately 10 minutes after the venting, fire emission was monitored."
At least they know that under overheating, the cell did catch FIRE.
Let´s now tell Boeing they need to add a note to the 787 FOM (Flight Operations Manual), requiring pilots to land within 8 minutes after battery problem, assuming 2 minutes for proper passenger evacuation, that is.
Since the electrolyte of Lithium Ion batteries is flammable, fire emission is common during an extreme venting failure. That is why the battery is in a metal box that is supposed to contain any fire. The box is not designed to contain smoke. This is vented to the outside while in flight.
The FAA accepted this solution assuming that it would only happen once in 10 million hours according to the model. Since this happened twice in about 100,000 hours there is an obvious issue with the earlier assumption.
There is also some controversy as to whether the fire was actually contained in the box as it was supposed to be.
Yes, it does raise more questions. Because they've isolated the short circuit to cell number six, they should now be in a better position to track down the reason for the short. Hopefully, we'll be getting more news soon.
Experts Sadoway and Elton Cairns "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."
My first thought was weight. The Chevy Volt is a grounded vehicle that must generate enough electrical power to propel itself over pavement at a reasonable rate of speed...relatively slow compared to the 500+mph (ground speed) of a large commercial airline.
Like most other mechanical contraptions we've invented, turning electricity into power and power into motion is a big issue. The heavier the machine, the more power it takes to make it move. And thus more electricity. More batteries. More weight.
Wouldn't adding an active cooling system add considerable weight? And take up precious space?
Great point, plasticmaster. Adding an active cooling system would indeed add weight. Depending on the type of cooling system used, the weight of the battery pack could jump from 63 pounds to a little over 100 pounds. As you point out, an additional 40 pounds tends to make aircraft engineers unhappy.
Yes, did it short due to poor construction, being deep cycled too often or being baked too long in that sealed box with no active cooling?
It's almost like having a murder case where the victim was poisoned, shot and stabbed by three different people and the coroner's report states that the victim died due to cardiac arrest.
If it's a manufacturing flaw, then Boeing is off the hook and they look good due to the limited damage caused. If it's an external problem, where the batteries are being utilized more than intended, then it indicates insufficient testing and/or new model growing pains and doesn't really indicate a significant problem at Boeing. If it's an overheat problem due to ignorance or post-testing/pre-production design changes, then it tends to reflect poorly on Boeing.
I guess we will only find out if Boeing volunteers the information.
When I wore a military uniform (never mind which color) I remember we had some communicaion equipment which had lithium batteries installed. The equipment was labeled "Do not airlift" because no one trusted the batteries to behave themselves in the air. Nevertheless, I don't remember ever seeing one of these boxes damaged by battery failure. Maybe the correct fix is to switch to a safer battery technology (LiFePO, fof example). Less energy density, but the airlines can make up for it by charging obese passengers a surcharge (they are a prime example of low energy density). Sorry for not being politically correct.
As any model aircraft amateur who has been into electric powered models can attest, Lithum batteries are a peculiar animal. They need dedicated chargers, a lot of Tender Loving Care and some luck too. They can go crazy from time to time. A heavy walled ceramic pot is not out of place when charging them. (May I suggest you to take a look at some photos of completely burned down cars when someone left a small battery pack charging inside).
I'm surprised that some overly 'creative' engineers at Boeing went on using this technology on a plane, when everyone that has some real-life experience with Lithium cells has developed a lot of respect and a healthy dose of distrust too. Subcontracting/Outsourcing this set of components is another reason for failure, as responsibility is thrown around and diluted all too easily. It will not be easy to precisely know what happened, as bad reputation is going to be "managed" in order to perform a crisis handling that will pretend that the original battery chemistry selection was either very "advanced", or "bold" or a "breakthrough"...
The best thing is this - We will create a battery technolgical leap with this issue. If the analysis is correct the solution has already solved itself -fix the initial or isolate the components of the battery so a small section at 500 degress will not ignite more of the battery or surrounding componets and the issue cannot migrate to the rest of the battery. I commend Boeing for using this technology. If every issue they ran into was solved by adding a cooling system (weight) and revering to the tried and true the plane would be a 707 with lipstick on it - just like a typical marketing department requested. Engineering is not for the faint of heart. In ten years we'll look back at the 787 as a relic that pushed us forward. It will create battery powered cars that actually make sense and cents.
We all know that lithium batteries are not the state of the art.Funny is that I have an old Samsung 225 with ORIGINAL battery for 10 years. Lithium batteries are bit strange.They should not be discharged completely like SOME others can.I found out that NiMH are also praclically trash if you discharge them fully.So, some kind of trickle charge is a good idea.I am onot a chemist, but I suspect that once the chemistry of a battery goes to extreme ,be it under , or over limits , that battery is unuseable.As far as temperature goes lithium batteries have been used on ships for many years with a great success.Cooling(or in case of airplane ,sometimes heating) and stable pressure is a key here.Unfortunately planes have no natural constant cooling and experience huge changes in an air pressure.
I'm confused. Cooling can be achieved using liquid, gas, or air. VW did it with the Beetle (using air) to cool the engine. A transatlantic flight will see outside temperatures of -65°F. Couldn't this air be harnessed in some way to act as a cooling agent on these batteries? (providing that's the real problem)
Wouldn't be possible to put some intelligence into these batteries?
I can see having a simple monitor for voltage and temperature of each cell. If one cell got hot or reduced it's voltage a redundant cell could be switched in. Of course the monitor would also report status upstream to the aircraft electronics.
Surely this has been considered in the past. Does anyone know why it is not practical?
As someone has said above, the 787 battery that failed was made by several cells connected in series in order to provide the required useful voltage. Those cells are hardwired. If you want to be able to electrically separate any of the cells, you would need to take the terminal wires to a selector switch, togehter with the wires of any extra cell. Those extra connections would reduce the reliability and the output current capacity, demand a comparatively large additional space and would have a sizable weight increase, largely negating any weight/space advantages of the Lithium chemistry. In regards to your statement about "smart" batteries, they are already "smart", as all cells are monitored individually with thinner wires in order to be able to be "balanced" by a dedicated circuitry during recharge. Cooling will only solve a high temperature problem if it is inside the thermal runaway limit, but won't help at all if the cell goes bad by itself. Trying to keep the resulting flame contained or conducted to a safe place is feasible in theory, but will again reduce the weight-size savings of the selected Lithium chemistry... so, was it a sensible selection to start with?
Actually there are a number of ways to keep the internal series connection intact and still monitor and charge each cell separately! You can do so with floating chargers/voltage sensors that measure and send charging current across each cell individually.
This is obviously a more expensive approach, but doable. And if you want fewer connections outside the battery box you put the charging and monitoring electronics inside but design it in a fail safe mode that will cut off charging if the circuitry gets too warm. Look at some of the smart camcorder batteries. They have no more than 4 terminals to the outside world, charging current, common, output current and data.
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?
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.
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.
@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.
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.
First, It could very well be that the cells were OK, but were badly installed, or badly monitored, or badly handled or used outside their true limits... all by others and not by the original maker. Or it could have a cell internal failure, which could be blamed directly on their manufacturer. At this moment, we DON'T know.
But what we do know is that the Lithium chemistry in general (with its different and different behaviour varieties), is certainly not intrinsically safe, as is has much more energy density and requires considerable care in its use and implementation. There have been numerous fires caused by them in many places, some aboard planes.
On the other side, the airplane manufacturer has the absolute responsibility of releasing a throughly tested and safe design. I respectfully disagree with those believing that a given "advance" or "huge leap" is the responsibility of a commercial plane manufacturer. At least not on airplanes meant to cross oceans between continents with hundreds of human beings aboard. Would the 787 fire happened on a long ocean crossing and causing a downed plane, would the urge of "progress" justify that?
I like how active cooling is mentioned again? obviously it was not the cooling that caused the thermal runaway. It was an electrical failure. Would active cooling help? or would it put other systems in danger? say systems more critical than the battery? I don't believe that active cooling is the solution... the miniscule cost that is so readily referenced is not so miniscule. If Boeing paid the miniscule increases in cost to every component that needed to be boosted to unrealistic standards they would have to lift up a tank into the air not an airplane and development costs might double or triple.
Let me explain:
The cost is not miniscule its actually quite large. Development of such a system alone will run in the hundreds of thousands and then the fire tests that it has to go through will be in the same range.
The weight added will be significant. Remember this weight costs you every time that plane lifts up flies and lands that's a lot of money...
and last but not least "It's not needed"
However I do agree with the general consensus that it would be nice if batteries did not short during operation that's bad mojo.
Of course, that's why the root cause is of critical importance. It does no good to cool a battery that is being damaged by overcharging. Since the pack that's failing is used for back-up avionic power, it sounds like the pack is being damaged by overcharging and cooling wasn't included in the design because the discharge level is so low. Mind you, I said "it sounds like".
The power control unit that this battery is tied to has current control for charging and discharging or so the initial post in DesignNews said. So that controls the charging. If you limit the power in (or out) then you can have static cooling. Basically because the current draw is limited just thermal conduction of the solid material is sufficient to cool the battery. It seems to be the case here. If i was to take a gues i would say it was quality control, the design should have been tested and was most likely tested during certification of the plane and the component.
The article implied that the NTSB is done with their investigation. I certainly hope this was not the final report.
Although they were able to rule out a few potential causes, they DID NOT rule out an inherant design flaw (either inside the battery or outside in the charger or the load). Will Boing be required to follow up with a report of its own along with mitigation? Or will Boing put some extra sensors on the battery unit and continue on as if that solves the problem (that remains undefined?)
Published information has indicated that there are 2 sets of Lithium ion batteries on a 787. One to start the APU and the one that caught fire that is designed to operate the aircraft flight systems in case of an engine or system electrical failure. If accurate, this suggests that the battery under discussion is constantly charged and only load tested to verify capacity prior to each flight. Other published data indicates that Li-ion batteries do not tolerate overcharging so each cell is monitored and the charge current varied based upon temperature and voltage. A shorted component within a cell in a series stack with no immediate temperature rise would be trying to drive the normal charging current through a short circuit. Heat rise would be extremely rapid, if the localized heat exceeded the maximum before it was detected, the fire would possibly have started before the charging current was removed by the monitoring circuit. The fact that Boeing has been replacing batteries is not in of itself an indication of a malfunction. Removal and analysis of in-service batteries provides a timely opportunity to correlate performance with expectations. The NTSB coming-out with this report should serve to quell public fears about the mystery of the fires. Now we need to know why did a short circuit happen and how do we prevent it in the future.
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.
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.
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