Imho, they did not bother to mitigate cell failure spreading to other cells as this would be weight and size prohibitive (likely worse then NiCd and not near certain to work either).

Rather, the approach seems to be to contain a _rare_ failure inside a solid box and mitigate over pressure by an elaborate venting system out the pressurized vessel.

It worked, twice, resulting in a fart out of the tail pipe and some leakage and one bent box likely due to internal arcing ignited oxygen fed combustion after breakdown of the electrolyte.

All that seems acceptable for a rare failure. It is by no means the equivalent of a hub failure.

What is unexpected and not reasonable is that 2 out of 100 or so batteries in service fatally fail within about year. Two points really do not add up to a curve, but the fact still is of great concern

@YRP

You are right, my bad.
NTSB, not Boeing visited securaplane.

BTW, just found this:
U.S. NTSB reviewing whistleblower claims in 787 case - chicagotribune.com

So, the chargers are not identical.

For those who never heard about Lithium batteries, and if the RC guys are taking it seriously..........

Complete Guide to Lithium Polymer Batteries and LiPo Failure Reports - RC Groups

Li Batteries & Vibration
 

The Yuasa battery being used in the 787 and what appears to be its prismatic cells are unfamiliar to me but our experience with all Li batteries is that there is a threshold vibration level that will cause rapid cell damage and lead to shorting behavior. In our aerospace application we've tested pretty much every cell type and none of them are particularly tough. Our application has a considerably higher vibration level than most transport aircraft however. I am sure that the battery in question was qualification vibration tested before it ever flew but I wonder if the actual in-service vibration level of that battery installation is what was expected. Our experience is that if the vibration threshold is exceeded the failure is prompt. The operating temperature of these cells is a issue as well. Just how hot does that compartment get when the aircraft is on the ground?

@Hetfield,

Very interesting, would have thought both are identical. Guess these are customizations of the same basic design.

From page 15 of www.oalj.dol.gov/Decisions/ALJ/AIR/2008/LEON_MICHAEL_v_SECURAPLANE_TECHNOLO_2008AIR00012_%28JUL_15_2 011%29_172333_CADEC_SD.PDF
Improper to work with the battery "open loop".

Good to look again into the charger history. Also the history of the batterie's installation, was the main battery ever used as an APU battery? Can a bus "charge/reverse flow" the battery without the chargers involvement?

The charger is only part of the story. The installation should have prevented the mess by protecting the battery from any charge.

Other than electrical management failure, thermally induced thermal runaway of many cells without electrical involvement after one cell failed (for any cause) seems less likely but is not impossible.

What makes cells fail? Design/manufacture, storage, overload or deep discharge or overcharge or the bus reverse flowing. The investigation will determine which.

Overcharge of individual cells seems a more likely scenario to me than the cells itself. But it does not have to be the charger, it could also be the bus.

saptzae,

Thanks for your interesting post. Could you possibly, however, help me translate this clause into intelligible English for us laymen?

" ...resulting in a fart out of the tail pipe and some leakage and one bent box likely due to internal arcing ignited oxygen fed combustion after breakdown of the electrolyte."

:{

Do you mean something like:
"...resulting in a fart out of the tail pipe; and some leakage and one bent box, due to internal arcing-ignited, oxygen-fed combustion, after breakdown of the electrolyte."?

What a difference punctuation can make. Or, if that's difficult (as it often can be); shorter, simpler sentences. There are no hard and fast rules, of course, but a degree of logic can make the difference between understanding and misunderstanding.

As you may know, a lady called Lynne Truss once wrote a book called "Eats, shoots and leaves", which might describe the actions of a gangster in a restaurant. The gunman, however, is a panda. Asked by the waiter to explain the carnage he has created, he produces a badly-punctuated wildlife manual and says: "I'm a panda, look it up." Finding the relevant entry in the manual, the waiter reads:
"Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves."

Sorry to pick on yours, because PPRuNe is jam-full of ambiguous and/or unintelligible posts: many written by intelligent, experienced pilots and engineers whose first language appears to be some kind of English!

Regards,
Chris
[With apologies to John Tullamarine for the thread-drift.]

Ironic
 

Shipping details of B787 Main,Apu & FCS batteries states:

@A33Zab


There were at least two fires on UPS freighters with Li-Ion loaded.

One (747F) was a hull loss, both pilots died.

Chris Scott
 

You've got it right on the APU starting- the l/h ASG can get power from either ship power or external only. The r/h can get it from 3 sources.

The APU battery provides power for external lights when towing, and can also provide power for manual RAT deploy, and, of course, APU start. Its charger is powered by the F/O instrument bus. The main battery provides brakes, captains audio control panel and a dome light when towing.

I have no idea of the current draw when starting the APU.

Different Threads.
 

For my dough, this thread carries the weight. Here are pilots, engineers, and experience. This thread is Platinum, R+N, Copper.

The topic deserves both, but if it's FAA, politics, or scuttlebutt, R+N seems like the proper spot.

Here be the real deal.

The real deal
 

Salute!

Thank you, Lyman.

In my quest to find out more about AF447 I found this site a few years back.

We have many experienced folks here that have survived due to training, innate skill, and learning from those who went before. .Not just pilots, but engineers and wannabes

We have had many spirited exchanges here, but I have yet to see a lack of respect for the posters' thoughts and theories and explanations of how things work in the real world.

The JAL incident seemed a lot more potent than 'a fart out of the tailpipe'.
An equipment fire that takes the Logan Airport fire dept 40 minutes to put out is a safety of flight issue, imho.

As yet there has been no confirmation or denial that the LI cargo was responsible for the fire. Any recommendations have been precautionary only.

etudiant ....... The JAL incident seemed a lot more potent than 'a fart out of the tailpipe'.
An equipment fire that takes the Logan Airport fire dept 40 minutes to put out is a safety of flight issue, imho.


The NTSB report from the "Grounded" thread states the fire department fought the fire from 10:49 to 12:15.

Eighty Six Minutes.

etudiant? mois aussi.... :ok:

True!

Why?

Revenue?
Money?

Who got the balls to exlude Li-Ion on airplanes?

Briefing contents
 

Hi,

In a first analysis we may see they discarded failures (like a major short circuit) in the DC busses as causes of the problems.

This means:

The problem was not due catastrophic load to the batteries (excessive current draw)

My comment is because considering certainly there is no discharge control (perhaps will be required for Li Ion in aviation) the analysis is concentrating in:

1) Cell issues (defect, etc.)
2) "Circuitry" like protections, charger including design, algorithms, etc.

There are other points to comment. I´m further analysing.

E.g. the point on cell overcharge is intriguing. Are the cells charged individually in this design? IMO should be for airliners batteries.

With over "US$ 10 billion grounded" and other costly consequences the briefing shows we are distant from a solution (using Li Ion).

An immediate decision to revert to Ni Cd´s appears urgent and inevitable.

Can be implemented? Surely yes. Implications? We would need data not publicized, to estimate. Certainly less costly than the current situation.

Very serious issue. Painful teething! And a "virtual stalemate" (If Li Ion usage insisted).

I'm pretty sure there is, it would be inconceivable to connect an uncontrolled load (ie DC starter motor, etc) directly to any Li battery. All of the "big" loads on the 787 are AC or driven from the +/- 270V DC bus and have no direct connection to the battery buses. The APU starter is a starter/generator which has its own (load-limiting) controller when using the APU battery for start power.

The simplified graphic in this post shows high-power distribution.

We're getting closer to the reason for at least one of the failures. Flightglobal writes that a short was found in cell # 5 of the JAL battery.

Story with pix here.

They don't know if the short was the chicken or the egg(cause or effect).

They have the results. They don't know what the triggering event was or the sequence of events.

rottenray:

If a cell grossly overheated, it wouldn't be surprising to find that the
separator material broke down eventually, whatever it's made of.

What they are also saying is that they are not sure if an overcharge
condition occured in a single cell, which would have caused thermal
runaway affecting the rest of the battery.

Still early days, but my money is still on the charger or temperature
sensing within the battery. Yuasa have been shipping these cells for
years and they are even used for space applications, so find it difficult
to believe there's a fundamental design fault in that area...

Regards,

Chris

Some time in the past, a poster (here or somewhere else) mentioned that an inspection showed wiring errors.

I wonder what they were and where they were at.

If a harness was mis-wired so that charge sensing was swapped between two cells - cell A being read as cell B and vice versa - it would eventually lead to an overcharge situation.

If the swap happened on cell temp, then a cell going into thermal runaway would be allowed to do so as the corrective action would be applied to the wrong cell.

Wow.

Current control of dangerous batteries (787 design lacks it?)
 

rottenray

On inconceivable look to the battery picture. Certainly the design doesn´t control the CHARGING current independently in each cell. :{

This may explain BOTH INCIDENTS!

And on discharge control:

1) How you would design it? What kind of part to perform it?

2) We will learn soon to confirm. I suspect there is not this feature in 787 main battery (main one is my bigger concern)

Mac

This picture just shows the high current cell conections (strips) and the sensors wiring. There is no provision for independent cell charging! :{

http://www.flightglobal.com/Assets/G...x?ItemID=49342

Rottenray:

I didn't see that post, but it seems unlikely, since the manufacturing
process for such an item would have have visual inspection and test rigs
to ensure that the wiring was correct. and that it functioned as expected.

If voltage sensing were swapped between two cells, i'm not sure it would
make that much difference if all the cells are charged in series, since
you can only vary the overall charge rate for all cells, not individually.
Would make a lot of difference if there were a charging circuit for each
cell, though from the pics of the internals, doesn't look like that is
the case.

There's another point that may be relevant as well: It's not clear from the
pics if there is a temperature sensor per cell, or a single sensor for
the enclosure. I don't have figures, but the cells could have significantly
less thermal mass than say lead acid or nicad. ie: they might heat up much
faster than the latter. They also have a fairly low max operating temp of
65 C. If you have a single sensor at one side of the enclosure and a cell
starts to overheat at the opposite side, would that single sensor detect the
overheat condition in time to shutdown the battery ?.

May be irrelevant, if there's a temp sensor per cell, but the failure also
raises the question of whether they tested the overall system for all
fault conditions, including that to destruction...

Regards,

Chris

I wasn't aware you were referring to independent current control. And I'm not aware of any multi-cell Li battery packs which do limit current on a cell-by-cell basis.


I read somewhere that temp sensing is overall, rather than cell by cell.

Given the relatively "light" weight of these cells, and the corresponding lower rate of thermal inertia, I'd guess that a cell could overheat in one area before a sensor located on the opposite side could sense the climb. Perhaps thermal curves could be modeled to predict this, but if I were designing the bugger I'd certainly have at least 2 sensors per cell.

Cheers!

Safer design when using critical batteries in airliners
 

rottenray:

Before editing i made implicit first point was on CHARGING. I consider a safer design to charge the cells INDEPENDENTLY. Why not?

Another point on safer design. There are certainly better locations for this DANGEROUS battery.

http://regmedia.co.uk/2013/01/25/boeing_787_fire.jpg

syseng68k

From published data, it certainly appears that testing to destruction was not done. That is inconceivable; Boeing certainly would have done that, if only to know what solutions were available to conform to the regs. If there was fire...

That was Boeing's position, initially, that the battery performed as designed.

If they hadn't done the testing to arrive at a default solution to failure that did conform, they would not have pleaded to FAA: "give us instructions on a fix..."

Which suggests they don't have one, ergo, they did not test the system to failure that did not comply.

Scary

Parallel charging, overload, mis-wiring
 

I apologize for my earlier analogy, it is a lot worse than a fart out of the tail pipe.

Parallel charging
There will be differences between cells, a few %. The cell of the lowest capacity determines the total charge-able and discharge-able battery capacity. Cells age, available capacity diminishes over time and varies more and more amongst cells.

Parallel charging enables to micro-manage the charge per cell in particular when cells are individually loaded (battery tapped). Very rare scenario in this day and age.

Here, batteries are discharged in series. Discharging must be stopped when the cell of the lowest charge is empty and charging must be stopped when it is full. Parallel charging would have little if any practical benefit beyond micro-management.

Parallel charging would require one isolated inverter per cell, capable of the maximum fast charge current + associated heavy wiring. 4V 30-60A x 8. Please consider size, weight, connection, cooling and reliability.

Actually, switching and rectifying losses depend mostly on the current and in practice would be almost same for a single 4V inverter and a 32V series charger.

Electrically, a parallel charge arrangement here would have 1/8th of the efficiency of the current 32V charge arrangement.

Then, these eight chargers must be managed and monitored!

No thanks, KISS :)

Overload
Generally, Li based cell chemistries will tolerate transient overload less than others. Au, Ni or Pb based cells can handle lots of abuse as these do not chemically deteriorate, at least not very quickly.

Important to all batteries is that the cells match, by effective inner resistance, throughout the event. Gross mismatch could even reverse a weak cell. The result would be rapid cell deterioration or outright destruction.

A partial cell failure, resulting in capacity loss or effective inner resistance increase, could trigger this scenario.

Mis-wiring
Swap of monitoring connections would identify the wrong cell to the management system, but would likely be detected by a production test.
This is a serial charge and discharge arrangement. Operationally, the weakest cell counts. Mis-wiring by swapping amongst cells would have no effect on charge/discharge management.

Battery and cell monitoring
 

Temperature would be used to limit fast charge current. Management system will lower max charge current at higher temperature.

Cell voltage change precedes temperature change. Once the temperature rises substantially, it will be too late to manage.

When a cell arcs internally or shorts, the voltage will fluctuate a lot and reduce to near zero.

When a cell electrically opens, voltage across it will revert and the battery shows a high inner resistance.

Important is to keep individual cell voltages in the allowed range at _all_ times, or disconnect the battery.

Thus, per cell temperature monitoring would not be a big benefit to battery management.

Important is that charge current be reduced quickly when any cell's voltage approaches it's limit, and that it never increases the design limit.

When _any_ cell over/under/reverse voltage is detected, the entire battery must be taken off-line (disconnected) within a very short time to prevent secondary damage to it.

Failure scenario
 

There three main management issues.

a) Cell production quality
b) Cell operational management
c) Cell failure handling.

It was reported that a cell was found shorted. We do not know whether this was the initial failure, or whether due to a) or b). It does not really matter because c) should have worked better, and may be what did the real damage.

If short was the initial failure, from an electrical perspective:

1. A cell fails and shorts - (reason either a) or b) above
2. Cell voltage drops, removing (most) of its 4V share of the 32V
3. Bus voltage remains essentially the same at 32V
4. Other cells take up 4V / 7, thus are subjected to severest over-charge in terms of voltage and current
5. More cells fail in a cascading manner
6. Combined per-cell energy and the bus feed battery destruction

Shorting

It is not going to be a solid short from one moment to the next. The energy is just too great. Cell could not ever dissipate it's 200 - 300 Watt hours quietly. It would short, arc, burn out the short and do more damage along the way. There would be over pressure, relieve valves opening, electrolyte blown out. More shorting as separators fail by the heat. The whole cycle continues until energy is dissipated and some spot shortened can not be burned out.

Throughout all this, terminal voltage will fluctuate and other cells be subjected to voltage and current spikes, until the next one fails and the cycle repeats itself even faster.

There is no need for parallel charging if you have a battery management system (e.g. balancer etc.).

Lithium Battery Failures

Management
 

@Hetfield

Yes, serial management is sufficient. All it must do is to keep cell voltages in range. It's important for any battery chemistry. It's critical for Li based cells, which are sensitive also to short-term over-discharge.

Importantly, when cell management fails, failure mitigation must kick in. It seems that it did not.

rottenray:
A temp sensor per cell becomes more important where fast charging is used.
There will be significant temperature rise and that coupled with a low
max temp of 65 C means that a single temp sensor to cover such a large
area is not enough. From what i've read, the fast charge timescale is about an hour
30mins.

2 would provide redundancy, but if the temp and voltage sensing were
combined into a single chip device, it could be embedded into the cell
casing at very little cost and have enough diagnostics to detect either
the voltage or temp sensor's failure.

There's another point as well: It looks like there are printed circuit
boards inside the battery casing. Hmm: pcb's + electrolyte = trouble,
even if the boards have a conformal coating. You only need a single cell
case to split, for whatever reason and you have electrolyte and vapour
which could play havoc with electronics. Inside a battery casing is
the last place I would put a pcb, especially if that is handling the
sensors

Regards,

Chris

Battery shutdown?
 

syseng68k,

Quote from your post#143:

If you have a single sensor at one side of the enclosure and a cell
starts to overheat at the opposite side, would that single sensor detect the
overheat condition in time to shutdown the battery?

How do you shutdown a battery that is experiencing a thermal runaway? I thought that a runaway in a lithium-ion battery cell could not be stopped.

By the way, thanks for all your very informative posts!

Saptzae,

Thanks for your latest posts. For a layman like me, they seem to summarise the issues facing Boeing, Securaplane, Thales, and (perhaps) Yuasu very clearly; particularly your description of the cascade effect following the shorting of one cell during charging.

Could you just clarify the last sentence of this part of your penultimate post, please (my bold)?

“Shorting
It is not going to be a solid short from one moment to the next. The energy is just too great. Cell could not ever dissipate it's 200 - 300 Watt hours quietly. It would short, arc, burn out the short and do more damage along the way. There would be over pressure, relieve valves opening, electrolyte blown out. More shorting as separators fail by the heat. The whole cycle continues until energy is dissipated and some spot shortened can not be burned out.”

Also, do you think that a manganese (spinel) type of Li-ion battery would have been a safer choice than the cobalt type, and would its performance be adequate for the task? If so, could its retrofit be one of the options currently under consideration?

grebllaw:

Once there's a runaway condition, it's probably too late. The whole
idea is to have accurate and timely info from the cells, to allow the
electronics to stop the charge or dump the load before runaway happens.
If the battery is managed correctly, it should never happen, other
than from gross component failure.

LI is far more fussy about operating temperature range and min / max
cell voltages than either lead acid or nicad and both need to be
continuously monitored to ensure safe operation. It doesn't mean there's
anything inherently wrong with the technology, but the overall system
design of battery, charger and load profile need to be to be much
better matched and to a tighter spec.

Analysis of this problem is interesting, even if we are stumbling
around in the dark a bit :-)...

Regards,

Chris

grebllaw:

Thanks for the encouragement as well. Interest will probably subside soon,
until more info is available...

Regards,

Chris

saptzae - thank you for your excellent contributions! :D:D:D

The Chicago Tribune reports:

Burnt circuit boards snag Japan Boeing 787 probe


http://i337.photobucket.com/albums/n...SYuasa7871.jpg

The PCB's are large and possess significant monitoring/management capability. As has been stated, the protections failed to prevent thermal runaway.

Let's hope the ANA units are testable and yield good information.

Best outcome may be discovering a bad batch of PCB's.

Lyman:

Perhaps, but let's put it into perspective: After arguably the most serious
level of battery failure possible, the fire was contained within the battery
enclosure, even if the contents did leak out and make a bit of a mess. The
smoke was discharged as per design. Inconvenienced, but no one in any
serious danger, nor lost their life, which is the most important thing..
It's what actually happens, not what might.

These events should provide a lot of new data, even if it is a bit expensive and
whether they were expecting it or not :E...

Regards,

Chris

Thx Machaca.

So,

- "GS Yuasa" is the manufacter of the Li-Ion cells
- "Kanto Aircraft Instrument" deliver the "circuit boards",
- "Securaplane" the chargers (two different models)
- "Thales" put it all together, besides the chargers


The "circuit boards" are very important. To my knowledge, they do the "balancing" and monitoring of each of the 8 cells which is crucial.

Outsourcing at its best....