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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 20th Mar 2013, 22:31
  #1041 (permalink)  
 
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EEngr: Your last sentence is descriptive of 21st century reality. The previous sentence described 20th century reality. Perhaps the recent number of dud aircraft projects could be correlated to the ratio of engineers who go on to get an MBA with respect to the number of MBA's who go on to obtain an engineering qualification.
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Old 22nd Mar 2013, 17:19
  #1042 (permalink)  
 
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Before any test flight(s).

Before ANY test flight(s), as a simple member of the public I think it would be reasonable to see at least something along the following lines.

I understand that two of the first 787 airframes were written off.

I would expect one of these airframes to be fitted out with the new battery containment system, complete with its fully charged battery, with visible and infra-red cameras and video cameras both inside and outside the airframe.

On the ground, bring about the most sudden run-away possible in the battery and let the world see what happens.

Only after that should any test flight be permitted.

Safety is not concerned with any theory of how failure occurs, or with any statistics of probability of failure, but only with the consequences of a run-away, which someday, somewhere, will occur.

What argument could be put forward against this proposal?
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Old 22nd Mar 2013, 17:26
  #1043 (permalink)  
 
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Boeing has already identified 2 airframes for testing...both are recent builds, one already slated for a new engine design testing (??)

Last edited by FlightPathOBN; 22nd Mar 2013 at 17:27.
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Old 22nd Mar 2013, 18:21
  #1044 (permalink)  
 
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See Business jet will have sturdier batteries than Boeing 787 | Business & Technology | The Seattle Times

That is precisely what Cessna have done for the new Citation which will have smaller but similar Li-ion batteries.

A video shows what happened when engineers disabled all the battery’s protective systems, overcharged it and then deliberately ignited the hot chemicals: Nothing more than a few wisps of smoke puffed out of the battery box.
Boeing cannot afford to skimp on these tests
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Old 22nd Mar 2013, 21:37
  #1045 (permalink)  
 
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Ian W Thanks for the link: Boeing cannot afford to skimp on these tests

And the FAA cannot afford to skimp on these tests if they want to retain the confidence of the public.
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Old 23rd Mar 2013, 00:30
  #1046 (permalink)  
 
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On the ground, bring about the most sudden run-away possible in the battery and let the world see what happens.
I believe they have tried this as part of the investigation process, but were unable to achieve any significant run-away.
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Old 23rd Mar 2013, 00:38
  #1047 (permalink)  
 
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I believe they have tried this as part of the investigation process, but were unable

Did they try a short-circuit on the battery?
Wouldn't that release all the stored energy in short order?
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Old 23rd Mar 2013, 02:30
  #1048 (permalink)  
 
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PickyPerkins:
Did they try a short-circuit on the battery?
Wouldn't that release all the stored energy in short order?
I believe that they tried an external short circuit of one cell, and I know they tried an internal short circuit (nail) of one cell.
I think that in both cases the cells were not in closed battery case with other cells when they tried it. It has also been observed since then that because of contact area, contact resistance, and electrode resistance problems a nail may not actually be a worst case internal short. It also immediately vents the cell!
A short circuit of the entire battery might not be a worst case either compared to thermal runaway. To get 900 amps from the battery, you have to use something close to a short circuit anyway.
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Old 23rd Mar 2013, 02:42
  #1049 (permalink)  
 
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Just speculating here............with regard to the original 787 battery design.
Probably the easiest way to overstress a battery cell, without this being detected by the battery monitoring unit would be to lose the electrical contact with one of the plate sets within the battery cell itself.

Seems to me that this would force the remainder of the cell to attempt to carry the load or to accept the charge being applied to the rest of the battery cells during charging.

Basically the same effect as substituting a lesser capacity battery cell in series with the remainder of the battery cells. No way to avoid abusing the 'smaller' cell with the original design if that happened.

Has this been discussed elsewhere?
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Old 23rd Mar 2013, 05:24
  #1050 (permalink)  
 
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Machingbird (I like that one):
Basically the same effect as substituting a lesser capacity battery cell in series with the remainder of the battery cells. No way to avoid abusing the 'smaller' cell with the original design if that happened.

Has this been discussed elsewhere?
Have not seen discussion, other than the argument the the "cell" should be treated and protected as if it were three cells in parallel, or physically or virtually isolated and should be monitored and protected as such. But if the kind of failure you propose happened the result would depend in part on how much adjustment is possible within the battery balancing network. If it is working properly, that cell will not be driven below LVCO because the BMU will tell the charger that it had a problem and the battery itself would open the contractor within the battery if the load or charging did not stop.
Nor will the remaining two sub-cells be forced to take current once their voltage reaches the upper limit. That would have the effect of limiting the whole batteries performance to the low to high capacity range of that partially failed cell. The resistive heating would be slightly larger, but not enough for an thermal cascade or venting.
But that assumes that the balancer wires and pass element for each cell can carry the full ~40 amp charging current, and that is very unlikely given the size of the wires shown.
You may have a good point, and the CAT scans of the APU battery showed some internal collector wire to electrode disconnects within several of the cells. But that was assumed to be a result rather than the cause of the failure. Some of the 1/3 cell rolls did look different from their neighbors.
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Old 23rd Mar 2013, 16:50
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This extract from a Boeing video shows a severe test of the proposed 787 battery containment enclosure. Propane gas ignited inside the box explodes. The 1/8th-inch thick steel walls bulge out but hold fast.
787 Battery Tests | Video | The Seattle Times

Interesting...during the test, look at the expansion, it appears to be contained by the box the test is being run in...

Last edited by FlightPathOBN; 23rd Mar 2013 at 16:54.
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Old 24th Mar 2013, 11:23
  #1052 (permalink)  
 
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Contains an explosion.

Do we think that containing a momentary explosion caused by the introduction of propane gas, proves anything about an effective containment of the thermal runaway reactions of volatile chemicals released as a consequence of the battery failure they experienced twice already?

I don't think it does.

And I do not feel like flying on a 787 while it has that sort of technology which has now proven itself twice to do what it was said to be "impossible" to do - catch fire. I don't ever want to fly on one of those 787's because it looks like it has a potential bomb strapped to the fuselage. I hardly want to be at the same airport as one of those things.

Boeing has really screwed up over outsourcing the risk-sharing and external design parts of that airplane. The ultimate consequent risk-share is the penalties Boeing should have to pay those airlines they promised the benefits too - benefits that now look impossible to materialise in any safe and secure way. How much maintenance is that thing going to take also?

Why any engineer would think that the acceptable way to mitigate the risk of a runaway chemical reaction and consequences is by the containment of that fireball in a stainless steel housing for an hour while the plane attempts to land is beyond me. I won't be on board, that's for sure.

Last edited by LandIT; 24th Mar 2013 at 11:32.
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Old 24th Mar 2013, 19:19
  #1053 (permalink)  
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The steel box is a PR disaster as well as a maintenance headache with its 52 bolts holding the lid on.

The changes in the cell manufacturing process, battery construction that includes ceramic heat shielding, new internal battery electronics and logic and a revised battery charger are the actual fix. The battery is a completely new design, but the containment box ensures that the travelling public - and many uninformed aircrew - will continue to regard the B787 battery as a highly dangerous piece of equipment. (Which the original, in fact, was)
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Old 24th Mar 2013, 19:25
  #1054 (permalink)  
 
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Do we think that containing a momentary explosion caused by the introduction of propane gas, proves anything about an effective containment of the thermal runaway reactions of volatile chemicals released as a consequence of the battery failure they experienced twice already?

I don't think it does.
I am sure neither does the FAA so to assume this was the only test Boeing performed is not logical.

Boeing has really screwed up over outsourcing the risk-sharing and external design parts of that airplane.
I doubt Boeing designed the Ship's and APU batteries for any of their commercial airliners. I expect every one of them had their design and production outsourced to external suppliers.


Blacksheep
The battery is a completely new design, but the containment box ensures that the travelling public - and many uninformed aircrew - will continue to regard the B787 battery as a highly dangerous piece of equipment.
If those changes to the battery stop them from failing, soon enough the traveling public will forget about the containment box just as they have forgotten the nitrogen in the fuel tanks to prevent a fuel-air explosion and the armor in the nacelles to prevent a failed turbine blade from being shot into the fuselage.

Last edited by Kiskaloo; 24th Mar 2013 at 19:28.
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Old 24th Mar 2013, 21:03
  #1055 (permalink)  
 
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Mentioned before in post 1325

With this firebox they mask the real screwup.

Aviationweek, bolding by me.

The battery will also sit on a redesigned frame containing drain holes to allow moisture to escape. In its testing, Vice President and 787 Chief Project Engineer Mike Sinnett says, Boeing found that moisture paths can lead to short circuits in cells which could lead to “stress in the cell,” or the buildup of heat and venting of vaporized electrolytes.
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Old 25th Mar 2013, 08:56
  #1056 (permalink)  
 
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18 kW for 45 seconds

I see the same Aviation Week article says that the batteries will be "... subjected to repeated load tests of 18 kw over periods of 40-45 sec. ...". This looks like the same test originally specified in the Boeing Control Specification Document (SCD) for the battery.
http://www.pprune.org/7745740-post1009.html

The NTSB Interim Factual Report dated March 7, 2013, quoting the manufacturer, said that the operating range of this battery is 20V - 32.2V, with a "nominal voltage" of 29.6V. At this "nominal voltage", 18kW requires 608A from the Yuasa cells whose published current maximum is 375A.

Does anyone know what factor dictates this 375A limit?

Adding to the confusion, the NTSB "MATERIALS LABORATORY FACTUAL REPORT", Report No. 13-013, February 19, 2013, says on page 8-9: "Based on information from the battery manufacturer", nominal specifications for the individual battery cells include a "Maximum discharge capacity" of approximately 1000 A (though typically 450 A for ~45 seconds, when being used for APU start-up, and no greater than three attempts at start up), which is rather different from the published specification for the individual cells of 375A. And on page 4, for the contactor which is inside the battery case, "The battery design incorporates a contactor rated to 400 A", which is a lot less than the 1,000A now claimed as the battery maximum discharge capacity.

Last edited by PickyPerkins; 29th Mar 2013 at 03:09. Reason: Added "nominal" value and NTSB statements.
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Old 25th Mar 2013, 09:20
  #1057 (permalink)  
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My guess would be the internal resistance of the battery, the rated power being the amount that keeps the cell temperature just within it's permitted level.
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Old 25th Mar 2013, 09:20
  #1058 (permalink)  
 
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Condensation

RetiredF4

I think you are on the right track in noting Boeing's remarks on moisture.

From memory, I think the statement was phrased something like, “During testing, we have learned that it is possible for moisture in the form of condensation to form a very high impedance short-circuit between the battery and the case which can lead to stress and venting of cell(s)” Not the exact words but near enough from memory.

Boeing also said that the vaporized cell contents escaping from the box “only caught fire because it came into contact with arcing wires outside the box”. No explanation as to why they were arcing, but I assume it was the ground wire which was found burnt out.

Both these events are very similar to an idea in the link below and earlier links from the same contributor:
http://www.pprune.org/7691355-post785.html

From first-hand experience I know that water will accumulate in any nearly closed container which is subjected to humidity and temperature cycles.
http://www.pprune.org/7746642-post1014.html

The Boston incident aircraft was operating between Narita, which has a daily high of about 50ºF and no dry season, and Boston, which is generally freezing in January. This particular battery had been in airline service for only 18 days and 22 flight cycles before it failed.

So I think that Boeing thinks that moisture was or could have been the likely cause of failure and that if they can keep moisture out that there will be no more early battery failures. The new enclosure is necessary for this purpose as well as to keep oxygen out and debris in.

The proposed enclosure and burst disk certainly has the potential of keeping out moisture if it is installed and maintained properly, e.g. no undetected leaks in the system.

Last edited by PickyPerkins; 25th Mar 2013 at 10:00. Reason: Add to para 6.
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Old 30th Mar 2013, 12:49
  #1059 (permalink)  
 
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Trickle-charging is how the thermal runaway begins

Looking at the CT scans from both investigations you can see puffed out cell cases in the undamaged battery packs. This is an indication of overcharging in Li batteries.
Looking at the Secura patent for battery charging you will notice two things of interest. The method was based upon a Ni-Cd battery charging profile, and the procedure assumes a trickle-charging phase of indefinite end following the fast charge phase.
Looking at the FDR data from the Boston event you can see that the battery charger was indeed holding a constant 32 volts and trickle-charging the APU battery at 1-2 amps for about 17 minutes after the APU start until the fire started.
Looking at the charging process for Lithium batteries from GS Yuasa and every other Li-battery manufacturer in the world and you will find that there is no trickle-charging. Instead you will find a specific charging procedure that consists of a constant current (at ~1C) charging phase up until the cell voltage hits a specified cut-off value, then followed by a constant voltage phase in which the cell is held at this specified voltage until the current drops to C/20, at which point the cell is fully charged, charging is complete, turn off and disconnect the charger.
Looking at the data sheet for the LVP-65 cells from GS Yuasa used in the APU and Main battery packs you will find that they have a nominal capacity rating (1C) of 65 Amp-hrs, nominal voltage of 3.73, maximum charging rate of 1C amps, and the cell characteristic curves use 4.025 volts as the CC/CV specified charging cut-off voltage. The C/20 cut-off charging current is 65/20 = 3.25 amps. There is no procedure called out nor should there be any additional charging below 3.25 amps after reaching the 4.025 volt level.
Attempting to trickle-charge a Lithium battery, especially one that is already fully charged, is a sure-fire way to cause thermal runaway...

Last edited by kenneth house; 30th Mar 2013 at 21:13.
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Old 30th Mar 2013, 20:04
  #1060 (permalink)  
 
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Looking at the Secura patent for battery charging you will notice two things of interest. The method was based upon a Ni-Cd battery charging profile, and the procedure assumes a trickle-charging phase of indefinite end following the fast charge phase.
Interesting. The NiCd 'trickle charge' mode that Boeing employed on previous models was also referred to as the T-R (Transformer-Rectifier) mode. Basically, once the battery was charged, the charger switches to a constant voltage source, both to keep the battery topped off and to supply the few small loads always connected to the battery bus. Without this mode, the battery would cycle between a constant current charge (high speed) and slowly being drained down by these parasitic loads. Since there was a requirement to maintain a high state of charge, this could not be tolerated. The odds that, when the batteries standby capacity was needed it would be at the 'bottom' of one of these cycles, were not acceptable.

Does the 787 system have such parasitic loads? Stupid question actually, because the batteries own built in charge monitoring system is one instance. But then, does the 787 system have such a 'float' mode to avoid this state of charge cycling and the inevitable low points in the capacity curve?

Its possible that the only way to fly with a LiCoO2 is either to provide the float voltage (unsafe) or to accept the lower point on the available charge curve as the 'best' avalable charge for design/certification purposes and thereby have to oversize the battery to allow for it. One would need to peek inside the head of the systems engineer responsible to know how this decision was made, I'm sure.
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