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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 26th Jan 2013, 18:21
  #161 (permalink)  
 
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Machaca:

Thanks for that pic, much easier to get some idea of the topology. There
are a lot of sensing wires, but again, no evidence of individual cell
temperature sensors / monitoring.

Looks a serious amount of analog and digital processing on those boards
and they aren't protected from cell contents in any meaningfull way other
than a possible conformal coating. The connectors look molex'ish and
are exposed as well.

All in all, not impressed. Boards and connectors of that type should
never be located anywhere near cells and their contents...

Regards,

Chris
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Old 26th Jan 2013, 18:24
  #162 (permalink)  
 
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All in all, not impressed. Boards and connectors of that type should
never be located anywhere near cells and their contents...
Exactly!

Even the hobby RC guys doing better....
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Old 26th Jan 2013, 18:31
  #163 (permalink)  
 
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@Chris Scott.
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.
I look at it at the cell level, from an electrical perspective, which I understand and which is the initial trigger of the destructive cycle. I disregard secondary effects of fire sustained by chemical reactions and thermal runaway.

A cell consists of stacks of Anode-plate Separator Cathode-plate. A short circuit occurs when any part of anode touches part of cathode. It could be caused by plates deforming or separator deterioration or contamination due to chemical reactions.

That process starts at a spot, the area of which is a quite small part of total cell area. Short results in a large current density at that spot leading to arcing and local heating. The effect of arcing is increased current flow and more heat. Once the spot is hot enough, it evaporates and the short may weaken. Also, resulting pressure increase may push plates apart and weaken or open the short.

However, the damage has been done and another spot will fail. 200-300 Watt hours is a lot of energy to dissipate within seconds. The short-arc-open cycle goes on until the cell is discharged or ceases to function for lack of electrolyte.

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?
There are two problems on my mind, two separate things to resolve. 1) What caused the initial failure. 2) Was the failure mitigated by disconnecting the battery immediately from all power.

Changing cell chemistry or manufacturer would be effective only if 1) is caused purely by bad cell. IMHO, a cell management issue is much more likely than bad cells. The investigation will figure it out.

If cell management is the problem, "Safer cells" would not make a significant difference. Safer cells mismanaged would break too with quite similar effects due to high energy density.

What makes cells fail is imbalanced charge or load, it does not have to be the charger, it can be the bus, or the interaction of both.

IMHO, other batteries in service, will already show detectable symptoms of impeding failure.

What one could do is to inspect batteries from all over the fleet using CT to look for anomalies such as distortion or contamination and then track back to cell quality or cell management.
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Old 26th Jan 2013, 18:49
  #164 (permalink)  
 
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hetfield:

The assumption they possibly made was that the cells are sealed, but they
are not. With age, vibration and pressure changes from internal heating &
cooling, they will leak vapour which will accumulate within the enclosure.
Vapour meets electroncis = corrosion and it wouldn't need much deposited
on the pcb to cause measurement error in sensitive analog electronics.

I was going to make some comment about consumer electronics quality in a
billion $ a/c project, but I suppose i'd better not ...

Regards,

Chris

Last edited by syseng68k; 30th Jan 2013 at 16:40.
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Old 26th Jan 2013, 18:57
  #165 (permalink)  
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The individual cells of the incident batteries have been subjected to CT scans, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and physical inspection after being disassembled.

CT of the JAL incident battery:

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Old 26th Jan 2013, 19:10
  #166 (permalink)  
 
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The assumption they possibly made was that the cells are sealed, but they
are not. With age, vibration and pressure changes from internal heating &
cooling, they will leak vapour which will accumulate within the enclosure.
Vapour meets electroncis = corrosion and it wouldn't need much deposited
on the pcb to cause measurement error in sensitive analog electronics.
If this is the case, it's just poor engineering.
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Old 26th Jan 2013, 19:19
  #167 (permalink)  
 
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@Machaca
CT of the JAL incident battery
Yes, seen that, I am interested in random in-service batteries.

I believe it is gradual degradation already present elsewhere and want to see whether there is any anomaly such as cell distortion and then track back to what causes it.

Gradual: At least as long as these batteries were in service. 100 hours?
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Old 26th Jan 2013, 19:31
  #168 (permalink)  
 
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saptzae,

Thanks again for that. I now understand that the cell failure involves a complex series of temporary shorts at different spots, progressively destroying the cell.

The fact that the first cell-failure inevitably leads to the application of a higher charging voltage to the remaining 7 cells - unless the monitoring system detcts the problem - represents a potentially fail-hard situation. Not something you want to be happening under the cabin floor...
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Old 26th Jan 2013, 20:22
  #169 (permalink)  
 
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Excellent Report with lots of destructive test data

I highly recommend “Lithium-Ion Batteries Hazard and Use Assessment”+:

http://www.nfpa.org/assets/files/pdf...rieshazard.pdf

for those of you who really want to understand the chemical, electrical, and mechanical aspects of these batteries. This report was mentioned in one of the first dozen or so posts about this problem but, based on the many questions and comments here, I suspect many participants have lost track of the report.

It was published in July 2011 at the request of “The Fire Protection Research Foundation” and was produced by two professional engineers and two PhD engineers. The research group reports they have been studying Li-ion battery failures and conducting destructive testing for over a decade.

The report was not focused only on aviation use of the battery but rather the general fire hazard, fire containment, and fire fighting issues associated with Li-ion batteries. The report does contain a great deal of FAA and UN regulatory data and information. The report is clear, concise, easy to read, and contains a lot of good photographs and detailed test data.

The report tested many Li-ion batteries to total destruction and carefully documents the many aspects of Li-ion battery fire initiation and spread.

Key characteristics of the batteries that I noted in the report that might be important are shown below:

1) The susceptibility to minor point impact on the battery case and the resultant internal damage that leads to individual and then battery wide conflagration. (page 71)

2) Mechanical damage to a cell case “perpendicular to the electrode edge is likely to result in high impedance shorting between electrode layers and initiate thermal runaway.” (page 57)

3) “Mild” mechanical damage can lead to thermal runaway over multiple cell discharge/charge cycles; caused by lithium plating (dendrites?) or mechanical creation of a small internal hole in the electrode. Failure is most likely to occur during charging or just after charging. (page 72)

4) “The vast majority of thermal runaway reactions that occur in the field occur during or shortly after cell charging.” (Page 84)

5) Cells that are below 50% of their full charge will seldom experience thermal runaway (page 84)

6) A single cell that is in thermal runaway mode will vent gas at over 650°C which is far above the Auto-ignition flash temperature of the electrolyte. (page 85)

7) FAA burn tests in an contained space show temperatures of 1000° F to 1400° F at 12” above the burning cells. (page 107)

8) No significant amount of oxygen is found in the gases vented by an overheated cell (page 63) and there must be a sufficient oxygen present from an external (not vented gas) source to sustain combustion (page 66)

9) Multi-cell batteries can be designed to minimize cell-to-cell heat transfer during a single cell thermal runaway by adjusting cell spacing and the orientation of the cell ejection path. (page 67)

10) “Thermal runaway can occur significantly after flame suppression” – a coolant must be applied to prevent subsequent ignition. (page 110)

11) Extensive tests by the US Navy show plain water to be an effective flame suppressant and coolant. (page 112)

12) There are several significant unknowns (“Gap Analysis”) regarding Li-ion fire hazards: Limited understanding of vented gas composition and flammability and Limited data on the effectiveness of suppressants. (page 116)
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Old 26th Jan 2013, 20:25
  #170 (permalink)  
 
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PCB

@Machaca
Thank you for the picture of #158. It shows bypass connections which could be used to divert small part of the charging current across individual cells. Bypass can be used to fully charge all cells and also lower voltage of individual cells. This contributes to explaining why there is lots of stuff on the PCB's.

In the lower right, at the bus bar, there seems to be a breaker used to disconnect the battery from the bus. If so, I wonder whether it was opened and remained open after the initial failure.
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Old 26th Jan 2013, 22:23
  #171 (permalink)  
 
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Parallel charging

Hi,

saptzae:

Parallel charging is the safer way one can charge an stack of critical cells. And it´s easy to design a highly reliable and dependable charger. There are some, IMO important benefits:

1) You fully respect the characteristics of each cell charging it with the proper current to attain the proper charge. Sensing it´s voltage and temperature during the charging allow us to improve the cell reliability keeping them bellow a safe specified temperature. (Remember the failure rate is correlated to temperature.)
2) You can even "help" a given cell to avoid to be transformed in a load (during heavy discharging) maintaining it´s within a safe voltage. (Something possible from energy of the DC bus itself). A dangerous polarity reversal would be practically impossible.
3) You can detect, register and inform on impending cell failures by a better characterization of the cell, instead of just doing this during the discharging. (Batteries for Av. application should have it´s recording capability)

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.
All these factors were seriously considered and weighted before posting. IMO parallel charging is easily justifiable for an improved design.

Electrically, a parallel charge arrangement here would have 1/8th of the efficiency of the current 32V charge arrangement.
No problem with current high efficiency inverters.

Then, these eight chargers must be managed and monitored!
No problem!

Mac

PS

I am here trying to show a way to increase the safety when using these dangerous cells. I yet adopted this approach in a (cost sensitive) design.

PS2

No thanks, KISS

I love the k.I.S.S. design approach. When it´s possible. In this case every reasonable feature to save the reliable use of these dangerous cells are being considered. There are big advantages with Lithium batteries. I´m trying to feel better and more confident when relying (if possible) on her.

PS3

The old approach of charging ("open loop") stacked cells directly from the bus in my mind is being seriously questioned. it´s too simple to be safe with dangerous cells.

PS4

To a better analysis of the 787 Batt/chrgr issue (thread focus) we would go beyond block diagrams (boxes) and must look to schematic diagrams (wiring, etc.)
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Old 26th Jan 2013, 23:09
  #172 (permalink)  
 
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Scary

Hi,

syseng68k:
In digital circuitry could be catastrophic too.

I was going to make some comment about consumer electronics quality in a
billion $ a/c project, but might I suppose i'd better not ...




Bear:


scary

I agree! (organizational problems are potentially more dangerous than Li Ion batteries).
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Old 26th Jan 2013, 23:28
  #173 (permalink)  
 
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Overload

saptzae:

Generally, Li based cell chemistries will tolerate transient overload less than others...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.

Question:

The ANA main batt. geometry mismatch was likely due what kind of abuse? During charging cycles?

(BOS incident had APU start then charging) JAL APU battery had CT possibility?

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Old 26th Jan 2013, 23:41
  #174 (permalink)  
 
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Individual cell temperature monitoring

Hi,

syseng68k:

There are a lot of sensing wires, but again, no evidence of individual cell temperature sensors / monitoring.

There are more connections in some terminals. In the battery terminals you can see 12 (6 in each strip) what is this?
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Old 26th Jan 2013, 23:51
  #175 (permalink)  
 
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saptzae:

Thank you for the picture of #158. It shows bypass connections which
could be used to divert small part of the charging current across
individual cells. Bypass can be used to fully charge all cells and also
lower voltage of individual cells. This contributes to explaining why
there is lots of stuff on the PCB's.
Assuming that they are bypass connections, not just voltage sensing. The
cable thickness is no guide. That technique is used on ups systems
and balances out the differences in individual cell residual leakage
current, which tends to float some cell voltages upwards at the expense
of the others. The question is, how much control range would be needed
under fast charge conditions, arguably the worst case ?.

In the lower right, at the bus bar, there seems to be a breaker used to
disconnect the battery from the bus. If so, I wonder whether it was
opened and remained open after the initial failure.
Well spotted. A breaker of that type would have a normally open contact,
to maximise contact pressure when closed. It may be driven via a
driver and logic level from the charger, or direct. If via a driver on
the logic board and there was an overheat condition, the driver could
fail and the battery remain connected.

Interesting forensics, but we still don't have enough data :-)...

Regards,

Chris
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Old 27th Jan 2013, 00:02
  #176 (permalink)  
 
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Chris:

The fact that the first cell-failure inevitably leads to the application of a higher charging voltage to the remaining 7 cells - unless the monitoring system detcts the problem - represents a potentially fail-hard situation.

During the charging cycle this may be well managed. But there are threats: The internal resistance of the failing cell during the charge (and discharge) may be situated in a range of values capable to generate high heat (Joule effect, like a resistor). And polarity reversal during the discharge is another threat.

Parallel charging eliminates the above problems.

Not something you want to be happening under the cabin floor...
We engineers MUST imagine everything POSSIBLE (not just probable) and provide means to allow the pilots "manage accordingly". Surprises with these dangerous cells nearby PCB´s inside EE bays nearby electronic modules are potentially threatning issues and a surprise to me.

A battery like the one we are seeing seems an absurd. Boeing had luck.
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Old 27th Jan 2013, 00:03
  #177 (permalink)  
 
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RR_NDB:

There are more connections in some terminals. In the battery terminals you
can see 12 (6 in each strip) what is this ?.
No idea, but probably something to do with measurement accuracy. For
example, if you need to measure voltages of a cell in a string of cells in series,
you must use a differential input amplifier from the cell terminals to
cancel out the common mode voltage and any charge/ load current related
effects. Analog stuff often uses kelvin leads or a single common point for this
reason...

Regards,

Chris
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Old 27th Jan 2013, 00:21
  #178 (permalink)  
 
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Guard connections?

syseng68k:

No idea, but probably something to do with measurement accuracy.
Measurement accuracy at this degree with a PCB in the same environment of the cells is

On bypass i am preparing to comment on that.

Question: How many amps the thin white wires could carry without being transformed in fuses? 1 amp? By pass?
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Old 27th Jan 2013, 01:15
  #179 (permalink)  
 
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Bypass? Breaker? (what kind of device?)

saptzae

This contributes to explaining why there is lots of stuff on the PCB's.
Did you see this approach used before? How many amps to be shunted? Voltage control loading a charging (or charged cell) through by pass?

With these thin wires seems just impossible to control cell voltages during the charging. A likely scenario (during charging) JAL BOS fire.

Again, parallel charging is the only way i can imagine to adequately manage cell voltages during charging.

In the lower right, at the bus bar, there seems to be a breaker used to disconnect the battery from the bus. If so, I wonder whether it was opened and remained open after the initial failure.
After loosing control of a given cell voltage, thermal effects will kick in (may be impossible to revert the runaway) and the opening of the main battery connection to the bus may not help at all. I heard on the use of a diode in the main battery circuitry. Anyway means of disconnection of the battery to the bus seems mandatory with Li Ion.
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Old 27th Jan 2013, 01:43
  #180 (permalink)  
 
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Failure scenario

saptzae;

3. Bus voltage remains essentially the same at 32V

Where is the source of information that sez the battery is charged by the bus. I´ve heard of a diode.

The direct connection of a Li Ion stack of cells to a bus seems very dangerous. A much safer approach would be:

1) Batteries being monitored and kept charged.

2) Batteries going to the DC bus only when necessary. A diode in the main battery path can perform the switching.

3) For APU battery it´s a different story. Much simpler issue.

We are needing more details in order to analyze better. Like schematic diagram. Not just block diagrams.
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