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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 8th Mar 2013, 11:45
  #861 (permalink)  
 
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Quote from TURIN, responding to the sugestion that the APU may contribute some power to APU start even when main electrical power is available:

"This concept of the APUC being powered by the APU battery however, regardless of main bus power does seem odd.
I'm not sure if this is the case on other a/c types. Research required methinks."

I can't comment further on the 787 architecture. But I can repeat an ex-pilot's rough understanding of the fix used by Airbus to solve APU start problems on the A320 - in case it helps as background info.

During an early A320 demo flight in 1988, with the president of France on board, an APU start resulted in a TR failure, and a shedding of electrics. At that time, the system used only one of the two main TRs to provide the DC load of >700 amps. The TR was plainly not man enough for the task.

The chosen fix was to introduce the second main TR into the loop, and let it share the load. However, in later years, it became clear to me that the two main batteries (there's no APU battery on non-ETOPS A320s) were also sharing the load. You could see it quite clearly by selecting the ELEC page on ECAM during the APU start. Cannot detail the mechanism involved, but it seems that once the DC voltage falls below the voltage of each battery, that battery's contactor closes. So the two main batteries' contactors are not inhibited from closing during an APU start. (Whether the system has been changed since I retired, I don't know.)

The 787 has radically different architecture, with a dedicated APU battery and (presumably) much more powerful TRs.

Moving on, I wonder if the number of APU starts using the APU battery alone (on the ground before engine start and with no external power available) has been recorded. The report quotes the JAL captain as stating simply that the APU had previously been "turned on about 30 to 40 minutes before the a/c left the gate at NRT".

Last edited by Chris Scott; 8th Mar 2013 at 11:55. Reason: Last paragraph added.
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Old 8th Mar 2013, 15:34
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After cell failure, 45A went into the battery for 3 seconds and triggered remainder of the mess.

Timely disconnection is key after a cell failure. Within 10-20 milliseconds that is. Faster would be better, it could avoid discarding other cells.

Detection of cell voltage or battery current transients is the only way to detect cell failure fast enough.

Cell temperature rises way too slow and can not contribute to detection of cell failure at all, it is simply too late then.

Last edited by saptzae; 8th Mar 2013 at 16:16. Reason: Clarify
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Old 8th Mar 2013, 15:35
  #863 (permalink)  
 
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Chris Scott
To indulge in speculation, the only other possible triggers that spring to mind might be the changes in pressure associated with the recent descent and landing, or the very recent temperature changes caused by the opening of the rear cargo doors adjacent to the Aft E&E Bay. Does anyone know the temp at BOS?
BOS Jan 7: Min temp 29F Max temp 39F Mean 34F - at 10:10 I would expect it to be around 32F at a guess. So nothing particularly cold.
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Old 8th Mar 2013, 15:39
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I think the BMS did detect a battery fault, as there was an EICAS alert to that effect around the time the APU shut down, according to the report.

Shame that there was nothing it could do to stop the battery bursting into flames. I think the alert was shown as: ''GOODBYE AND GOOD LUCK...''
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Old 8th Mar 2013, 15:57
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In regards to the power distribution panel issues, Boeing traced them back to a bad batch from the supplier. In addition to the three NH events, a UA 787 had the same problem and diverted to MSY. QR also reported a problem with a panel on a 787 delivery flight and AI experienced issues on some of their 787s, as well.

@saptzae
Detection of cell voltage or battery current transients is the only way to detect cell failure fast enough.
Which according to an earlier post on the 787 battery subsystem is something the Battery Management System does on a per-cell basis (in addition to monitoring the cell's temperature).


@FullWings
I think the BMS did detect a battery fault, as there was an EICAS alert to that effect around the time the APU shut down, according to the report.
And I believe I recall reading that the EICAS was alerting the flight crew of NH692 to issues with the voltage and temperature of the Ship's Battery.

Last edited by Kiskaloo; 8th Mar 2013 at 16:42.
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Old 8th Mar 2013, 16:58
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Quote from inetdog:
"For whatever reason, the battery was still being charged a rate of ~45 amps 15 minutes after the APU start. Since there should be no other loads in flight (unless the BOS plane had the same cross-wiring problem as the other), we can safely assume that it at least contributed to the APU starting current."

I think the 45 amps was part of the failure sequence, and the report says it lasted only 4 seconds.

Quote from Ian W:
"BOS Jan 7: Min temp 29F Max temp 39F Mean 34F - at 10:10 I would expect it to be around 32F at a guess. So nothing particularly cold."

Thanks - call it zero Celsius, and cargo doors open 13 mins or less, with Aft E&E bay doors probably still closed. Electronics bays tend to be fairly warm in flight.
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Old 8th Mar 2013, 17:46
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Battery management failure?

Hi,

Events surrounding auxiliary power unit shutdown.

1000:24 Airplane touched down.
1004:10 APU started.
1006:15 Airplane completed turn into parking location.
1006:48 Parking brake set.
1006:52 Engine 1 shut down.
1006:54 Engine 2 shut down.
1021:01 APU battery bus voltage decreased from 32 to 31 volts.
1021:04 APU battery current increased to between 44 and 45 amperes for 4 seconds, indicating
current flowing into the battery.
1021:07 APU battery bus voltage decreased to 30 volts.
1021:09 APU battery bus voltage decreased to 29 volts.
1021:10 APU battery bus voltage increased to 31 volts.
1021:15 EICAS message discrete indicated that the APU battery failed.
1021:27 APU battery bus voltage decreased 1 volt per second during the next 3 seconds until
reaching 28 volts at 1021:30.
1021:37 APU battery bus voltage decreased to zero volts and returned to 28 volts three times, and
APU battery current began to move between zero and -4 to -5 amperes, indicating current
flowing out of the battery.

1021:37 APU controller went offline, and APU shut down.
1021:37 Aft EAFR stopped recording. Forward EAFR continued recording for about 9 minutes
58 seconds.
1021:40 EICAS message discretes indicated that the left and right 1 and 2AC buses became
unpowered.
1021:41 EICAS message discrete showed that the APU battery failure was no longer indicated.
1022:00 EICAS message discrete indicated that the main battery was discharging.
1022:10 APU controller was back online.
1022:53 EICAS message discrete indicated that the main battery power switch was off.
1023:16 Airplane systems providing data to the EAFR had shut down.
1031:35 Forward EAFR stopped recording.
Note: EICAS, engine indicating and crew alerting system. The APU controller is the source of 32 recorded parameters,
includingAPU shaft speed and APU battery bus voltage.


1021:37 APU battery bus voltage decreased to zero volts and returned to 28 volts three times, and APU battery current began to move between zero and -4 to -5 amperes, indicating current flowing out of the battery.
After the battery failure (1021:15 EICAS message discrete indicated that the APU battery failed. ) the (failed) battery was still in the circuit!



1021:01 APU battery bus voltage decreased from 32 to 31 volts.
1021:04 APU battery current increased to between 44 and 45 amperes for 4 seconds, indicating current flowing into the battery.
This is another point:

If (as it seems) batt. was out of circuit and its voltage suddenly drops the algorithm should verify WHY, before trying to charge at this rate.

So, in a first look (still lacks info) we may think the charging system "aggravated" the problem (one cell probably "failing=shorting")
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Old 8th Mar 2013, 17:51
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The 787 APU has two starter/generators each. One powered by the APU battery, one by "something else". If ships power is available, the starter that is powered by ships power starts the APU. Taxiing in the APU battery should not have been used for APU start.

I also read the 45 amps from the report. 45 amps wouldn't come close to starting a 787 APU. They presented the 45 amp flow as a fact, but I don't remember them guessing why.
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Old 8th Mar 2013, 18:27
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APU stops when (APU) battery fails

Hi,

WHy?

1021:37 APU controller went offline, and APU shut down.


Repeating:

Why?
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Old 8th Mar 2013, 18:39
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Hi RR

The APU controller is powered by the APU Batteries. If the Controller shuts down, as a result of battery failure, it would not be good to keep the (uncontrolled) APU powered up?
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Old 8th Mar 2013, 19:01
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Fault tolerance and Graceful degradation

Hi,

Objective should be, must be and can be:

Fault tolerance and Graceful degradation.


Its ABSOLUTELY possible to start it (APU), keep it "well managed" even with the loss of the (APU) battery.

So, simply

I question the design!
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Old 8th Mar 2013, 19:06
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With other aircraft families, if the APU controller goes offline, does the APU continue to run? Or would it shut down, as well?

The APU controller went back online at 1022:10 - in such a scenario, can the 787 APU restart if there was power available (say via the engine generators)? Or must the APU battery be energized and available in order to start the APU on the 787?

Last edited by Kiskaloo; 8th Mar 2013 at 19:11.
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Old 8th Mar 2013, 19:17
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Hi RR..

If one stops to consider the method of construction of the Batteries, then reviews the Failure Analysis Associates link, for spontaneous failures, there are some possibilities remaining to explain the failures.

The construction of each battery requires delicate manual handling, perfectly clean ingredients, and materials that perform within narrow limits, in the interest of performance.

Each very small "shutdown event" increases the internal resistance, and lessens the capacity of the battery. As capacity decreases, demand increases, (as a percentage of "available") exacerbating the failure path.

Ever suspicious, I return often to the "150" Field replaced Batteries. Something was wrong with them, so there was some cause that created the "problem".

Did Boeing replace three batteries per fleet unit without knowing the cause for failure? That would be most troubling.

But if they knew the cause, how is it NTSB does not know it? Even more troubling.....

And why would NTSB use the associated Battery (MAIN) from the incident aircraft as an exemplar?

Budget trouble? ()

cheers
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Old 8th Mar 2013, 19:23
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kiskaloo:
Or must the APU battery be energized and available in order to start the APU on the 787?
According to the description in the report, the controls for the APU are powered only from the APU battery bus.
The battery bus might also be powered from the charger (BCU) if the battery goes open circuit, but apparently when the battery fails in a short circuit mode or high enough level fault alarms go off the battery bus cannot receive external power and so the APU cannot run.

Lyman:
And why would NTSB use the associated Battery (MAIN) from the incident aircraft as an exemplar?
Similar use history and manufacturing batch, maybe?
Easier than searching other aircraft in use to find a qualified exemplar. Or is the manufacturer required to archive one or more sample batteries per batch?

The exemplar has (so far) only been subjected to non-invasive examination.

Last edited by inetdog; 8th Mar 2013 at 19:28.
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Old 8th Mar 2013, 19:29
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The battery bus might also be powered from the charger (BCU) if the battery goes open circuit, but apparently when the battery fails in a short circuit mode or high enough level fault alarms go off the battery bus cannot receive external power and so the APU cannot run.
So if in an emergency, the APU is powering the aircraft, and the APU BATT decides to short circuit, it takes the APU along with?

What?

Also...

The exemplar has (so far) only been subjected to non-invasive examination.
I suppose "Exemplar" could be defined in more than one way. I assume it is used as: Undamaged, unused, for comparison. The Main Battery in JAL is not strictly speaking an exemplar, it is involved in the circuitry intrinsically.

Definition of EXEMPLAR: one that serves as a model or example: as . a: an ideal model . b: a typical or standard specimen
It (JAL008 MAIN) really is not an 'exemplar'.

Right, picky.


Last edited by Lyman; 8th Mar 2013 at 20:03.
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Old 8th Mar 2013, 19:36
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They were warned

Hi,

Did Boeing replace three batteries per fleet unit without knowing the cause for failure? That would be most troubling.

But if they knew the cause, how is it NTSB does not know it? Even more troubling.....
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Old 8th Mar 2013, 19:39
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Fault tolerance and Graceful degradation

Hi,

inetdog

The battery bus might also be powered from the charger (BCU) if the battery goes open circuit, but apparently when the battery fails in a short circuit mode or high enough level fault alarms go off the battery bus cannot receive external power and so the APU cannot run.
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Old 8th Mar 2013, 19:54
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... apparently when the battery fails in a short circuit mode or high enough level fault alarms go off the battery bus cannot receive external power and so the APU cannot run.
If that is the actual design criteria, some heads need more than banging.

The APU once online should be self sufficient, and not beholden to the state of APU battery.
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Old 8th Mar 2013, 20:11
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Fault tolerance and Graceful degradation

Hi,

mm43

The APU once online should be self sufficient, and not beholden to the state of APU battery.

Perfect!
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Old 8th Mar 2013, 20:38
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Two APU Starter/Generators

Quote from USMCProbe:
"The 787 APU has two starter/generators each. One powered by the APU battery, one by "something else". If ships power is available, the starter that is powered by ships power starts the APU. Taxiing in the APU battery should not have been used for APU start."

Thanks, that was the vital factor that was missing in my discussion.
The 45 amps was going into the battery, BTW, 16 minutes after the APU had started, for just 4 seconds; apparently just a symptom of the failure sequence. So, bearing in mind that the battery seems to be uninvolved in the APU start, what made the APU battery fail just 16 mins after the APU was started?

At NRT, was it used to start the APU, or did they have external power? Was it fully charged on APU shutdown at NRT? Does recharging continue from the main electrical system? If not, presumably it would have leaked some of its charge during the 12-hour flight, but not much. It seems possible that the recharge process might have been nearing completion when the failure sequence started.

Last edited by Chris Scott; 8th Mar 2013 at 20:47. Reason: Last para amended.
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