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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 21st Feb 2013, 00:12
  #781 (permalink)  
 
Join Date: Feb 2011
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IFE was rebooting when PAX were exiting the plane in TAK

Hi,

sb_sfo

the IFE will reboot when there is an EE bay smoke alarm.
This is new for me. With multiple threads on the issue this may occur.

Let me understand: The smoke alarm in the FWD EE bay resets IFE?



Do try to keep up, will you?
Considering electricity and electronics are my degree and considering the importance of the 787 grounding issue for aviation i am trying to "keep up". Actually
trying to be proactive.
But the info we have is scarce. Today for the first time i saw the block diagram of the MAIN battery circuit. I had it in my mind only by some info presented by TURIN and today confirmed it was correct.

for the info on reboting. I was trying to image a possible "connection" to the main issue.

Question:

Do you have an idea why flashers were draining APU battery current?

Last edited by RR_NDB; 21st Feb 2013 at 00:14.
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Old 21st Feb 2013, 00:46
  #782 (permalink)  
 
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watch this video of the evac...(the seatback IFE)

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Old 21st Feb 2013, 01:15
  #783 (permalink)  
 
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The a/c was half full, the aisle IFE screens were on, very few of the rest, and the POS lights are powered by APUBatt.

So Captain selected the lights in EMER descent, and left the screens on...

What am I missing?
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Old 21st Feb 2013, 02:46
  #784 (permalink)  
 
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Electricity flow in a 787 'different from design'

Hi,

More precise information from NHK:

Japanese aviation authorities have uncovered a design fault in the electric wiring of a Boeing Dreamliner that made an emergency landing last month.
But officials with the Transport Safety Board say the fault has no direct connection with the aircraft's battery trouble.The 787 aircraft, operated by All Nippon Airways, landed after a battery overheated, sending out smoke.
Experts have focused on electricity flow aboard the plane because some of its lights remained on after their switches were turned off.They found that wiring was installed according to the aircraft's blueprints, but that electricity flowed differently from its intended design.They say current flowed from one battery into a circuit whose switch was supposed to be off. The battery was reportedly not the one that overheated.The officials say they will investigate the problem more closely.

So, another issue.

787 uses high current solid state switches. Same modules are used in F35. These switches (solid state relays) could fail (short) and create "strange" behavior on circuits. For example, the diode module in MAIN battery circuit if shorted needs prompt (protective) reaction from battery management or even crew members in extreme cases. A new project requires IMHO to be more alert than other mature planes. The degree of innovation in 787 is a factor to be considered with attention. Some (due components) failures could be very difficult to quick understand even for a (circuit) designer. The above news may be related to a shorted solid state relay.
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Old 21st Feb 2013, 05:20
  #785 (permalink)  
 
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Secondary failure

@RR_NDB
So, another issue
A direct connection, feeding current into a battery with a failed cell, would drive good cells into runaway by overvoltage.

There should be no paralleling of battery to battery or powered bus to battery without protective means such as BDM.

A MOSFet based BDM would have to be switched off quickly to prevent current reversal into the battery.

Last edited by saptzae; 21st Feb 2013 at 09:22. Reason: Spelling
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Old 21st Feb 2013, 09:03
  #786 (permalink)  
 
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I've been thinking about the APU battery feeding the nav lights during towing and such.... presumably the nav lights are normally fed from another source. This raises the possibility of current from that other source finding its way to the APU battery - without a diode - if the switching fails...
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Old 21st Feb 2013, 09:48
  #787 (permalink)  
 
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@fizz57

Concur, by connecting to NAV lights from two sources, such as main bat bus and APU battery, there may be a parasitic current path into APU battery, bypassing the intent of the design.

When any cell is fully charged, a very small net current (mA!) into the cell, would lead to overvoltage of the cell, unless the BMU balancing manages to act against it and compensates it out, to keep cell voltage down. I am not sure if parasitic bypass current would be high enough to cause overvoltage. Depends on the bypass voltage, which would have to be well above 32V, and also on how good BMU is at balancing.

Any overvoltage will deteriorate cell, thus lead up to Primary failure.

Once a cell shorts (Primary failure), and battery voltage drops, the parasitic current path allows larger current to flow (a few A), which also would be too large to be balanced out by the BMU.

That would set battery up for Secondary failure of multiple cells by overvoltage.

When multiple cells fail, Tertiary failure is just a matter of time.

It may be a lead, its not all of it. A parasitic current path into main battery remains to be found.

Last edited by saptzae; 21st Feb 2013 at 16:09. Reason: Clarify
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Old 21st Feb 2013, 15:30
  #788 (permalink)  
 
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Once a cell shorts (Primary failure), and battery voltage drops, the parasitic current path allows larger current to flow (a few A), which also would be too large to be balanced out by the BMU.
Once a cell shorts, the input to the BMU for that particular cell will show a short circuit. How the BMU responds to that will depend on what it thinks is the cause of the short circuit.
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Old 21st Feb 2013, 15:55
  #789 (permalink)  
 
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@SoS
Once a cell shorts, the input to the BMU for that particular cell will show a short circuit. How the BMU responds to that will depend on what it thinks is the cause of the short circuit.
Right, and the BMU will hopefully disable the charger.

For this scenario, the parasitic source - main bat bus -> switch -> nav light -> switch -> APU bat - remains, feeding the APU bat.

Another possible parasitic connection reported is direct connection of Main bat and APU bat. If so, scenario applies to Main bat as well.

Scenario is
  • APU bat cell short
  • BMU detects cell short
  • BMU shuts charger down and alerts
  • APU bat voltage is reduced by up to 4V
  • But parasitic source remains at above 28V (4V x 7)
  • Source voltage to be taken up by the remaining 7 cells
  • Ideally (if BMU not already disabled), the BMU trys to balance against parasitic charge current to prevent 7 cell overvoltage.
  • BMU fails because current too large (> 3 A)
  • Multi-cell overvoltage
  • Cascading cell failures
  • Runaway
Edit
As I explained in earlier posts, short may be transient, and hard to detect, unless BMU sees cell voltage transients.

Last edited by saptzae; 21st Feb 2013 at 16:12. Reason: clarify
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Old 21st Feb 2013, 15:55
  #790 (permalink)  
 
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Sorry, guys (RR & FPO)

I was a little hasty in my reply last night, and I couldn't get the right smiley to stick to my post. The info on the IFE was a month old(!) now.

When there is an EE bay smoke alarm, the IFE load is shed automatically. The Panasonic IFE will take about 20 minutes to spool back up, and you will get uneven screens through the cabin depending on the pathways in the system.
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Old 21st Feb 2013, 22:22
  #791 (permalink)  
 
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sb,

no worries..this thread is very long, and the other thread has parallel, same, and different information...

Concur, by connecting to NAV lights from two sources, such as main bat bus and APU battery, there may be a parasitic current path into APU battery, bypassing the intent of the design.
I think in wiring we call that a double fault...

Last edited by FlightPathOBN; 21st Feb 2013 at 22:26.
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Old 21st Feb 2013, 23:09
  #792 (permalink)  
 
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B787 battery

don't know the solid state relays used but the units I've seen all have a protection diode which will pass current in the reverse & care is always needed where they are used. i.e. useful only where one DC source is involved or any other component or wiring failure cannot profide a DC backfeed source.

Regards

Last edited by bill good; 21st Feb 2013 at 23:20. Reason: ps
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Old 21st Feb 2013, 23:34
  #793 (permalink)  
 
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IF the ground was blown as shown in the JTSB report...there is a very deep rooted problem with the electrical flow diagram in the Firebird...
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Old 22nd Feb 2013, 00:01
  #794 (permalink)  
 
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IF the ground was blown as shown in the JTSB report...
Kind of looks as if there is some unforeseen commonality via the DC buss and the Main and APU battery packs. The on ground services supplied by each battery may well be the link.
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Old 22nd Feb 2013, 03:00
  #795 (permalink)  
 
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Ground wire fused

Hi,

The ground wire (MAIN batt. ANA) was fused by internal short. (model)

How without a (internal) short to ground fuse this thick wire?

Posted earlier the equivalent circuit. Will think if there is another way to blow the ground wire.

fact is, lack of continuous fuselage ground return pose some difficulties and a broken return path (seems not the case) creates very strange behaviors difficult to understand quickly. Even with DC.
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Old 22nd Feb 2013, 15:51
  #796 (permalink)  
 
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I am not allowed to pull quotes from the R/N thread (I am banned), so I will paraphrase a recent post.

The "cells" are in series. Lose one "cell" lose the "battery".

It is not for nothing the emphasis in the press has been on "cells", instead of "batteries".

Perception is everything. Fire is the dramatic issue, as it should be. The base line is the regulations. Any problem with one "cell" fails the entire SYSTEM.

It is an odd thing to emphasize, but important. Fire is not the problem.

The problem is the certificate, and loss of a safety critical system more often than the regs allow.

Eight batteries in SERIES, Not eight cells.

In protecting against "FIRE", the eyeballs are off the problem.

How many of the replaced batteries had one bad cell? Does it matter? The question is about dispatch, and reliability.

Cannot launch without both batteries at a specific SOC. The rest is spilt milk.
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Old 22nd Feb 2013, 16:06
  #797 (permalink)  
 
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Todays news is saying that the Japanese investigation found the APU had been mis-wired to the forward EE battery....

perhaps that is why even with the forward battery ground gone, the IFE was still powered up

Last edited by FlightPathOBN; 22nd Feb 2013 at 16:07.
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Old 22nd Feb 2013, 16:14
  #798 (permalink)  
 
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How did they start the APU? Ground Fault?

Last edited by Lyman; 22nd Feb 2013 at 16:23.
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Old 22nd Feb 2013, 16:55
  #799 (permalink)  
 
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How did they start the APU? Ground Fault?
One would assume that the main busses were still powered, therefore APU would be started from power available and not the APU bat.

How many of the replaced batteries had one bad cell? Does it matter? The question is about dispatch, and reliability.

Cannot launch without both batteries at a specific SOC. The rest is spilt milk.
Didn't we cover this already? MEL allows dispatch with APU inop. I assume that includes APU bat inop.
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Old 22nd Feb 2013, 17:29
  #800 (permalink)  
 
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TURIN

MEL allows dispatch with APU inop. I assume that includes APU bat inop.
But not on fire. Even if it's in a titanium box, I doubt they'll push back and go.

What will the procedures be for a fire in flight (in the new box again, of course). In all probability, immediate diversion, landing and evacuation. The battery box just improves the survivability of an in-flight fire.
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