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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 11th Feb 2013, 12:14
  #621 (permalink)  
 
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High current (load) transients

Hi,

EEngr:

The 'current spike' brings up another issue which I have found to be often overlooked. That is: the electrical loads on these batteries are not steady state DC. Particularly in the case of the APU, the starter/generator is driven by a controller that draws very high levels of ripple currents from the source.

Batteries, in addition to being an electrochemical voltage source have an equivalent AC model that (overly simplified) can be represented as a series LCR circuit at higher frequencies (tens or hundreds of kHz). If one excites such a circuit near its resonance, it is possible to generate extremely high voltages across the various internal points of this equivalent circuit.

There is the possibility that the various combinations of load ripple and battery AC impedance was not properly characterized when the initial certification analysis was done*. I imagine that subsequent flight tests will be instrumented to capture exactly this kind of data.

*Back in my days at Boeing, I was involved with the 767 static inverter and its adaption to the 747-400. Initially, it had been certified to drive linear AC loads. This was because the smaller loads (typically driven by the standby AC bus) were exempt from limits on harmonic current draw. But, as it turns out, being exempt from a spec requirement doesn't meant that it shouldn't be considered. It turned out that an inverter rated at 1kVA was only capable of delivering about 400 VA to the connected loads before the voltage waveform became so flat-topped that it's output fell out of spec. This new 'all electric' airplane may turn out biting some old school engineers in the a.

I am particularly interested to discuss this issue. I love Circuit theory.

Questions:

1) You consider the possibility of the cell voltages varying outside "safe limits" during transients that as you mentioned occur starting APU?
2) The equivalent LCR of the battery could resonate in this frequency range? (hundreds of KHz)
3) The relay/contactor could generate cell voltage unsafe transients? (opening under high load)
4) The diode module could be an extra factor? (e.g. to cell "integrity" during transients)

Why we hear the inverters noise in HF radio transmission coming from the 767? We know a given call is being made from a 767. EMI/EMC or just filtering issues?
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Old 11th Feb 2013, 13:10
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@RR_NDB

As an alternative path to inducing cell damage, short duration load transients would be tolerable, as long as cells do not drop below minimum voltage.

The APU starter generator is inverter driven. Unlike an old fashioned DC starter motor, it will, at most, pull little more than rated current. Actual startup current will be less than rated current.

I have no data, I'd go for startup current of less than 100A rising to up to 300A. At 300A * 25V = 7.5KW into the inverter, motoring would deliver at least 6KW at the shaft.

Resonant effects can be very damaging, in electrical as well as mechanical systems. Back in the 1920s and 1930s, torsional vibration destroyed plenty of crank shafts and gear boxes of the new twin row radial engines, before it was understood through testing, and well prior to developing the maths behind it.

Resonant currents on the bus would be an ineresting scenario. I experienced load dependent oscillation during charger development.

Last edited by saptzae; 11th Feb 2013 at 13:15.
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Old 11th Feb 2013, 14:31
  #623 (permalink)  
 
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@Lyman

Some here, including myself, like to focus on technical discussion, to understand what really happened.

IMHO, R&N is the better place for "high-level" discussion of great political and strategic significance.
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Old 11th Feb 2013, 14:54
  #624 (permalink)  
 
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Yes, of course.

I think the battery enclosure is certainly a technical issue, as are the certifications the FAA applied to the technology as BOEING interpreted it.

from the photos of the JAL accident, I note some serious charring of the (composite) decking underneath the APU Batt's install site. So that suggests an improvement in separation, battery case from a/c structure might be warranted as well as the isolation of the eight cells within the enclosure. Perhaps an inerting tray, of some non conductive material, perhaps a ceramic.

The space within the E/E bay appears sufficiently large to accomodate a reasonable increase in the space required for an upgraded battery system. Problems of pressure cycling, cell casing, gas production, etc. certainly would impact the discussion?
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Old 11th Feb 2013, 15:13
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The Quantum-Mechanical alternative interpretation ?

If we make the assumption that the chemistry of Lithium-ion batteries is as yet UNSUFFICIENTLY KNOWN ? let me prompt that - food for thought, possibly ? - the electro-chemical reaction liberating free electrons (electrical current) in those Japanese (YUASA) batteries has TWO ALTERNATIVE writings, ONE (standard, well-known, basic) upon which the battery control system engineering has been conceived (and may correctly serve to govern/control/protect) plus ONE OTHER electrochemical ALTERNATIVE REACTION ? with a DIFFERENT WRITING, liberating a much greater number of free electrons in a CHAIN REACTION producing a discharge current of much greater intensity (amperage) for which the present battery protection system has NOT been designed to cope with and of which the Battery Maker is UNAWARE, the inherent assumption here being that the OCCURRANCE of the alternative electro-chemical reaction would be commanded by Quantum-Mechanical principles (RANDOM basis ?) under certain (very particular) catalytic conditions (hypothetically : eg some mechanical vibration of the anode/cathode and electrolyte at given frequencies or otherwise, in presence of PPM traces of catalytic impurities in said anode/cathode/electrolyte ?)
Engineering at Brown University
(see papers n 1 and n 2 indicating that Lithium-ion battery technology is still at the Research & Development stage in 2013, five years after the 787's airworthiness certification ?)
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Old 11th Feb 2013, 15:45
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RR_NDB, 606

Away for a few more days - work still must be done .

Re: The red herring comment, spikes on the bus are way down the list
of possible suspects imho. Although the data sheets don't provide an
ac model for the cell, one would expect the self capacitance to be
very high, the self inductance to be very low, and the esr to be in
the fractions of a milliohm range. All this would provide very good
damping for any transients on the bus. It's still good to discuss it
though, even if everyone doesn't agree - it would be pretty boring
and unproductive if we all did.

For me, it's still down to battery management, at the risk of banging
on about it. Either charge or load related. I still don't see any reason
why those batteries caught fire and the if bms was doing it's job right, it
should have disconnected the battery at the first sign of trouble, which
it should have been able to detect. No excuses please.

Just like the unprotected electronics and harness in the enclosure,
perhaps too many (arrogant ?) assumptions made during the bms design
stage...

Last edited by syseng68k; 11th Feb 2013 at 15:50.
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Old 11th Feb 2013, 16:00
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Hi Chris.

I hope not a stupid question..... Could the battery problem be system related insofar as the battery itself increases its resistance internally?

NTSB is looking at manufacture defects, leading to internal shorting, (on a "microscopic basis"). If this is happening, is there some point at which the battery cannot be controlled externally?

The NTSB isolated the #6 cell as the source of the initial runaway. Working backward, where do you see the failure as initiating? I know it is a general question, but could you contrast physical separation cell/cell, and circuit isolation?

Are these two problems independent? We don't see the ability to separate each cell from the circuit, do we?
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Old 11th Feb 2013, 16:12
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saptzae, #623
Resonant currents on the bus would be an interesting scenario. I experienced
load dependent oscillation during charger development.
Had a the same problem with a one off rotary inverter speed and voltage
control for a lab power supply. With any mechanical devoce in the loop, it
can be quite difficult to model / analyse from basic servo theory and there's usually
some hand tweaking of the lead / lag networks to get it right, especially
if your maths is a bit rusty, like mine. My excuse was / is that too many years
of software have partially dissolved the higher facilities, but ymmv .

Resonant currents might be a possibility, but resonant with what ?. The apu
starter conditions might be the worst case, but the pwm driver for the
starter motor must be well filtered to prevent interference with radio and
other sensitive avionics. Any transient effects will be in the 10's or
hundreds of milliseconds range, not microseconds and you can't have those
effects without considerable inductance in the line. Transmission line theory
applies and all that...
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Old 11th Feb 2013, 16:42
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Any transient effects will be in the 10's or
hundreds of milliseconds range, not microseconds and you can't have those effects without considerable inductance in the line.
One can get a a "resonant/positive feedback" effect if a controller reacts to resistance changes (temp rise from current flow) with a time lag that aproximates the conductors thermal time constant.

The above time constants sound about right for large conductors, especially if one pressumes a slightly faulty connection in the path.

Not knowing the details it is hard to say if this is likely but I do know of instances in other fields where thermal "tails" have caught designers by surprise.

Last edited by MurphyWasRight; 11th Feb 2013 at 17:02. Reason: Spelling and clarity
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Old 11th Feb 2013, 16:57
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Lyman:
The NTSB isolated the #6 cell as the source of the initial runaway. Working
backward, where do you see the failure as initiating?
Imho, the precursor to the initial runaway should have been detected by the bms
as an excessive temp rise or temperature gradient in that cell. If each cell's
temperature is being monitored, detection of an overtemperature condition
should have been part of the process, just as temperature monitoring is critical
to limit charge current to a safe value.

I know there's been some discussion about whether a sensor on each cell would
be fast enough to detect the rise, but the cells do have thermal mass and
unless a short circuit or very high charge current were applied, the temp
rise over time would be such that a sensor should detect it. As the cells have
electrolyte, the thermal resistance from cell core to case should be fairly low.
Temp rise timescales might be in the low minutes under normal operating
conditions. Plenty long enough to be detected by a sensor on the cell casing,
but maybe not so if that sensor were several inches / 10's cm away on the
enclosure wall...
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Old 11th Feb 2013, 17:33
  #631 (permalink)  
 
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RR_NDB

1) You consider the possibility of the cell voltages varying outside "safe limits" during transients that as you mentioned occur starting APU?
This might be more of a steady state phenomena. This APU is an AC device, driven by a variable frequency drive (an three phase inverter). Although there is some input filtering on such units, they do draw continuous ripple current on top of the DC supply. And this ripple current will change in magnitude and frequency as the APU starter accellerates.


2) The equivalent LCR of the battery could resonate in this frequency range? (hundreds of KHz)
It is possible. Some analysis and tests could be done to characterize the batteries' AC response. An LCR circuit might be an over simplification of what is actually going on. For the real RF engineers, it could be more like a lossy transmission line, with standing waves along the battery ribbon and the resulting higher voltages at these nodes. But for simple understanding, the lumped model (as discrete components) will suffice. Here is some more info:

LC circuit - Wikipedia, the free encyclopedia

Two damage modes might be possible: First, the voltage across a series LC circuit at resonance is at a peak across the L and C (but out of phase). The C here is the capacitor created by the battery plates and seperator. If this rises above the designed insulation level of that insulator (3.7 V plus some margin of error), it could punch through and initiate a short in the cell. The other possible mechanism is electrolyte heating due to losses caused by the r.f. voltages. Some capacitors are noted for having high internal losses (dissipation factor for the EEs) and, as a result heat up (and sometimes explode ) when driven with a ripple current/frequency over their rating. Although the total voltages don't exceed the insulation's capability, hot spots can be generated.

3) The relay/contactor could generate cell voltage unsafe transients? (opening under high load)
4) The diode module could be an extra factor? (e.g. to cell "integrity" during transients)
These are possible as well, creating some high voltages due to the "inductive kick" experienced when switching high currents.

This is all guessing on my part. But given the design and certification program of the 787, it is likely that the consequences of load interaction with the battery were not completely tested. When the battery system was being developed, no actual loads (with their AC rippe) were available yet to characterize their effects. Hence the planned flight testing program.

The fortunate thing about these effects is that; should they prove to be the root cause of failres, there are some simple and inexpensive filtering techniques that can be adapted to the battery/load circuits to mitigate them.
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Old 11th Feb 2013, 17:47
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heat

Thank you Chris...

Where I have wanted to go is to Thermal. From your background, would you have in mind discrete heat signatures relative to Chemical decomposition as opposed to resistance generated heat events?

My question has to do with my perhaps faulty image of electrical resistance induced heat, and chemical, and chem/elec combinations?

Would the heat generated by internal fault be more local, and suggest the need
for multiple samples in the cell interior?

Does resistance produced by overcharging lend itself to more local sensors?

Like at the strap, or connectors?

If this technology is allowed into the future, what do you see as the difficulties in monitoring for these internal problems?

very curious....Thank you for your patience, and time.
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Old 11th Feb 2013, 17:52
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saptzae:
So far, there is no hint on availability of any per cell recordings. Sensibly,
the BMS would record (amongst others) things like

Event time stamp
Cell #
Cell voltage
Cell short/over voltage/under-voltage
Charger on/off
Failure notification
Breaker tripped / disconnected
Battery temperature/fire
Very much so. Something that's been bothering me for some time is the almost
obsessive concentration in the reports about the batteries, but still not a word
about the data logs for the battery subsystems. If all the above data is
being recorded, there should be a complete timeline of the events that led up
to the failure.

If they have the above, they have the reason already, unless such data is not
actually being recorded...
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Old 11th Feb 2013, 18:02
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If the certifications were met, and they were, there would be no need for such extensive monitoring.

EXCEPT in a test Programme. Boeing tested and concluded there would be no propagation cell to cell, and any cell failure, any kind, could therefore be effectively ignored.

I think the reason we have not heard about extensive monitoring in these systems is because there is none.

EG: The JAL battery likely went off line immediately when the single heat sensor recorded temperature limit. But by that time, Electrical circuitry was not a concern, because at that point, explosive expansion, flame, and smoke became the issue.

NTSB stated there were no over voltage issues. That means to me, there was a shutdown, but not due to electrical circuitry issues.

Overtemp. Likely due the construction/architecture/ageing?

IOW: The system Worked....By design. Badly.
That may soon change?

Last edited by Lyman; 11th Feb 2013 at 18:14.
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Old 11th Feb 2013, 18:27
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@Lyman,
I think the reason we have not heard about extensive monitoring in these systems is because there is none.
Securaplane,
Our new, innovative battery chargers use advanced DC to DC conversion technology, patented charging algorithms, comprehensive diagnostics and fault isolation.
Mx got the info from somewhere, and knew when to change the units out.
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Old 11th Feb 2013, 18:39
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mm43

Mx got the info from somewhere, and knew when to change the units out.
Yep. Likely a red light on the panel. Doubt Mx inspected the actual battery to get the notice...

And that could be generated from those separate leads, calculating charging response against an indexed standard?

One thinks the accelerated replacement Program got accelerated straight away after the first two or three unscheduled BATT FAULT prompts.

I think the monitoring is visible on the undamaged Batts on display. Because these cells do not tolerate inclusion of foreign material in the cell itself, temperature monitoring got the short stick.I think the "White Wad" might be the only temperature sensing device?

Is there a temperature sensor that melts at a given value, say 120C that then opens up the circuit, stopping all electrical activity? A variation on a "Fusible Link"? If so, that may be the trigger for the ECAM. No current from the battery'

Didn't ANA report a drop to zero voltage concurrent with the smoke event?


Last edited by Lyman; 11th Feb 2013 at 18:51.
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Old 11th Feb 2013, 18:53
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Lyman:
Where I have wanted to go is to Thermal. From your background, would you have
in mind discrete heat signatures relative to Chemical decomposition as opposed
to resistance generated heat events?
It really isn't that complicated, starting from the premise that the cell will
work as advertised if not abused. I'm not a chemist and am quite happy to let
Yuasa deal with that and overall physical construction.

The cell data sheet specifies a maximum temperature, which if exceeded may lead
to thermal runaway. It's the job of the bms to limit the temperature based on the
data sheet values, with a margin of safety. Ie: if the cell is rated at 65C, then
the bms must have a hard limit below this, say 60C. For whatever reason, the
cells should be disconnected from charge or load if that limit is exceeded. The
same applies to min and max cell voltage monitoring, which did appear to work,
judging by the number of batteries that were swapped out due to excessive
discharge...
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Old 11th Feb 2013, 19:06
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same applies to min and max cell voltage monitoring, which did appear to work, judging by the number of batteries that were swapped out due to excessive discharge
Excessive discharge, or "low voltage"? If cells were failing, wouldn't the lowered voltage show as what could be claimed as "discharge"? Wouldn't said low voltage be a "no dispatch"? Creating some real urgency?
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Old 11th Feb 2013, 19:26
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EEngr:
This might be more of a steady state phenomena. This APU is an AC device,
driven by a variable frequency drive (an three phase inverter). Although
there is some input filtering on such units, they do draw continuous ripple
current on top of the DC supply. And this ripple current will change in
magnitude and frequency as the APU starter accellerates.
I'm skeptical, in that the ripple current on the dc line will be directly
related to the inverter o/p frequency, which will be at most, in the low
Khz and more likely the low hundreds of Hz. The input filtering must provide
effective filtering for the pwm carrier, which would be typically in the low Khz
range anyway.

Would the battery have a problem dealing with ripple in the low Khz
range ?. Such ripple would be riding on the nominal dc and wouldn't cause
transients above battery unloaded voltage. You need inductance in the line
to produce significant transients above battery nominal voltage and a few feet
of wire would only produce nanosecond level transients. Insignificant..

Some capacitors are noted for having high internal losses (dissipation
factor for the EEs) and, as a result heat up (and sometimes explode )
when driven with a ripple current/frequency over their rating. Although
the total voltages don't exceed the insulation's capability, hot spots
can be generated.
Agreed, but caps for that sort of service are designed for low esr, have
specified ripple ratings and are designed to tolerate hf / fast rise edges.
I really doubt if the cells would be subjected to anything like that high a
frequency, since there would be considerable rf radiation to other systems
as a result...
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Old 11th Feb 2013, 19:58
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Lyman,
Doubt Mx inspected the actual battery to get the notice...
The Securaplane battery charger carries its own diagnostic tool and records appropriate data. I suspect chargers will be located nearby each of the suspect battery packs, and according to the maker, the earlier BC-1300 series supplies the following:-

Charger faults - Fan speed, CPU Temperature, Voltage, Current, Power, Stored fault history.
Battery faults - Abnormal temperature, Temperature sensor circuit, Cell unbalance sensor unit.
LED display - Eight-character alpha-numeric.
Diagnostics - Self test, Output voltage, Output current, Operating hours, Temperatures, Power.
Time - Date, Software revision.
Charge mode - Bulk charge mode, Topping charge mode, Trickle charge mode.

The likelihood of their flagship B787 charger providing less data than above would IMHO be highly unlikely.

Supposing that the JTSB and NTSB examined the charger records straight up and found nothing untoward except the battery fail event, then it would be reasonable for them to dissect battery cells in order to deduce a reason for that failure.
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