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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 30th Jan 2013, 19:48
  #321 (permalink)  
 
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APU management and ETOPS

Some time ago I argued that, if special restrictions were put on the discharge and charge of the APU battery in advance of a complete fix, ETOPS flights requiring the APU would still be possible if the APU was started - perhaps in flight - prior to the beginning of the ETOPS leg, using the main electrical system, as a pre-emptive measure. I pointed out, however, that starting it in flight would be a hostage to fortune, in that cold-soak MIGHT (though should not) inhibit APU start. A more reliable tactic would be to start it before departure, as an idling APU uses little fuel, particularly at cruise altitude.

Quote from cockney steve:
"WRT charge/discharge.....APU should be started off bus fed by 4 main gen's before shut-down....APU feeds all demands in conjunction with main Gen's until engines are shut down.
IF Apu is shut down for ANY reason, the aircraft is then dependent on an outside GPU for the next half-hour ,before the APU can be started again. As soon as the main engines are restarted (GPU?) the power is restored, but temperature/time limitation means that the APU will be inop, under these conditions, for takeoff."

What are these temperature/time limitations? Not something I've ever found a problem on other types, and a typical long-haul turnround time is about 90 minutes or more.
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Old 30th Jan 2013, 20:00
  #322 (permalink)  
 
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@rottenray

You're on the right track.
Originally Posted by mm43
#215
I suspect the reason for the present arrangement is to keep the battery management system intrinsic with the cells that it controls. Additional and unquantifiable ohmic losses could present a significant problem for remote monitoring.
My point all along. Logging is separate issue, and that I agree can be done in a separate E/E rack, with each battery pack logged in with its own IP type address, i.e. a Plug 'n Play arrangement, to avoid loss of valuable data.
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Old 30th Jan 2013, 20:02
  #323 (permalink)  
 
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Sorry,

it's not about logging.

It's about controlling/charging.
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Old 30th Jan 2013, 22:10
  #324 (permalink)  
 
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rottenray:

First, according to the securaplane website, battery history is recorded
by the chargers - not the BMS co-located with the battery cells.
I had a look at the securaplane website few days ago and couldn't find
any detailed info re: where the logging is done. This may depend on
particular system requirements. Can you post a link to the relevant page
for the actual 787 charger system ?.

To summarise the discussion re: battery data logging:

1) In the charger, which I originally suggested, but that leaves the
problem of what happened to that data ?. It should provide an audit
trail of the events leading up to the failure and would expect it to
be the first thing examined. Yet, no word about this.

Also, it makes the exchange of either battery or charger more inconvenient
since the logged data no longer matches that for the battery. That is,
there needs to be a process to download, store the data from the charger
and ensure that it's tagged to the battery that's been removed. Hence,
the suggestion that the data may be stored in the enclosure. There's
a lot of electronics there and while they may just have made a meal of
it, it does suggest further functionality.

2) Internal to the battery enclosure, which makes more sense from a
maintenance pov, but such data would be unlikely to survive a fire
using the present layout.

It's worth noting that this, too, apparently worked as advertised as there
was a "drop to near zero" recorded on the batt of the ANA aircraft.
Sorry, but a drop where ?. The associated dc bus, in the fdr, or what ?.

Putting the BMS inside the battery case makes it easier to calibrate and
more reliable. You are dealing with DC levels into the millivolts, and
at these low voltages even the best connectors can add undesired and
unpredictable resistance.
What was being discussed in context was the data logging: where it takes
place, what parameters are stored (timeline ?) and where that data is
now ?...

Regards,

Chris
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Old 30th Jan 2013, 22:47
  #325 (permalink)  
 
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mm43:

My point all along. Logging is separate issue, and that I agree can be done in a separate E/E rack, with each battery pack logged in with its own IP type address, i.e. a Plug 'n Play arrangement, to avoid loss of valuable data.
That is another option and had heard that Boeing were using ethernet transport for
non critical data to reduce cost, but for critical subsystems such as this ?.

If they did, let's hope they made a better job of it than Windows. Actually not that
bad these days, but it usually takes until SP3 before they thrash out *most* of the bugs...

Regards,

Chris

Last edited by syseng68k; 30th Jan 2013 at 22:51.
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Old 30th Jan 2013, 23:23
  #326 (permalink)  
 
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Quote:
First, according to the securaplane website, battery history is recorded by the chargers - not the BMS co-located with the battery cells.
I had a look at the securaplane website few days ago and couldn't find any detailed info re: where the logging is done. This may depend on particular system requirements. Can you post a link to the relevant page for the actual 787 charger system ?.
The 3rd paragraph, just above the images, on this page: Innovative Inverter Technologies and Main Ship Battery Chargers for Power Conversion

Securaplane battery chargers store every fault including battery over-temperature, cell unbalance, defective temperature sensors, defective charger/battery connection and GMT time/date of fault period. Our chargers possess extensive diagnostics such as charger microprocessor status and permanent memory of faults with readout to the integrated 8-character alphanumeric display.
It also mentions it in their product brochures.


Quote:
It's worth noting that this, too, apparently worked as advertised as there
was a "drop to near zero" recorded on the batt of the ANA aircraft.
Sorry, but a drop where ?. The associated dc bus, in the fdr, or what ?.
Neither article was that specific, but both IIRC mentioned the battery dropping to "near zero voltage."


Sorry,

it's not about logging.

It's about controlling/charging.
What? The circuits in the battery box? Finding the problem?
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Old 30th Jan 2013, 23:49
  #327 (permalink)  
 
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rottenray:

Securaplane battery chargers store every fault including battery over-temperature,
cell unbalance, defective temperature sensors, defective charger/battery
connection and GMT time/date of fault period. Our chargers possess extensive
diagnostics such as charger microprocessor status and permanent memory of faults
with readout to the integrated 8-character alphanumeric display.
Thanks for that and clears up one mystery. Obviously didn't look hard
enough..

From the above, it seems like there should be a complete timeline of the
events that lead up to the failure. Will be interesting to see what it says,
but still curious as to why this wasn't the first thing examined...

Regards,

Chris
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Old 31st Jan 2013, 05:58
  #328 (permalink)  
 
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Chris Scott.
Temp/Time limits on the APU are due to a rotor bow problem caused by excess heat soak after shutdown.. The quick fix is to leave the APU inlet door open to allow sufficient ventilation of the APU powerplant. Long term fix will be hardware/software changes.


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Old 31st Jan 2013, 08:16
  #329 (permalink)  
 
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@TURIN
Long term fix will be hardware/software changes.
Cute, a software patch to de-bow the rotor.

Seriously, it should be possible to use the inverter powered starter, reprogrammed to support it after shutdown, at very low speed (5-10%), to ventilate for a few minutes. Would not pull much power really, 32V/10-20A on battery for 10 minutes would be minor.
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Old 31st Jan 2013, 08:20
  #330 (permalink)  
 
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I'm sure it's fine but looking at the photo posted a few pages back..

What's the clearance between the two output bus bars and the metal brace that runs under them? Look where the black sleeving ends. This clearly isn't the cause of the current problem or the brace would be melted.


Last edited by cwatters; 31st Jan 2013 at 08:23.
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Old 31st Jan 2013, 08:39
  #331 (permalink)  
 
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@cwatters
What's the clearance
Hard to judge. I'd say 5mm+

I was thinking, assembling and wiring these boxes, with all cells at least at minimum charge, must be really interesting.
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Old 31st Jan 2013, 10:11
  #332 (permalink)  
 
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It's amazing how proper balancing is done with these tiny wires.
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Old 31st Jan 2013, 10:43
  #333 (permalink)  
 
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Saptzae.
Re: Motoring the apu after shutdown.
Unfortunately, the APU starter motor controllers are liquid cooled and would need the PECS (Power Equipment Cooling System) to function after shutdown. Which is fine if you are on ground power but not if you are shutting the a/c down.


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Old 31st Jan 2013, 12:02
  #334 (permalink)  
 
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Sorry I misspoke. I did mean I was surprised that the batteries were not charged in parallel, individually. Including monitoring individual current. I did not mean I was surprised they were discharged in series.

I don't design or work on airplanes but fly them. I have done a lot of automotive electrical stuff, professionally. I was shocked at the pictures of the BMS wiring.

A bunch of little wires with crimped (and probably soldered as well) eyes being held in place by little screws seems archaic. I am sure the wire and terminals are the best money can buy, but you just don't see that anymore in higher volume production stuff. Maybe Jaguar (Lucas) still does it that way? Did Thales buy Lucas?
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Old 31st Jan 2013, 12:16
  #335 (permalink)  
 
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Sorry I wrote it wrong again. I thought lithium batteries were charged individually in an application like this. Discharged in series is a given.


When I read the NYT article about the ANA battery fire, they said the crew noted a normal voltage, then zero voltage. I believe they referred to the zero voltage as a short circuit, not that the battery charger had stopped charging "as designed". If the battery charger had stopped charging, the voltage would have dropped slightly to actual open cell voltage, assuming no load on the battery. Airbus puts a little connecting line on the EICAS to show that the battery charger is connected, but you still look at the voltage across the terminals. It has been 8 years since I have flown a Boeing, I believe they all just show battery voltage. Above 28 volts the charger is working.
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Old 31st Jan 2013, 13:00
  #336 (permalink)  
 
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saptzae:
Seriously, it should be possible to use the inverter powered starter,
reprogrammed to support it after shutdown, at very low speed (5-10%),
to ventilate for a few minutes.
Good idea, the starter / generator is inverter driven, so may only need a software
change to implement that.

Not a new idea either: 1994 and maybe earlier Audis had a small run on water pump to keep coolant
circulating around the turbo to prevent heat soak damage...

Regards,

Chris

Last edited by syseng68k; 31st Jan 2013 at 14:47.
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Old 31st Jan 2013, 13:12
  #337 (permalink)  
 
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A couple of thoughts....

Boeing's insistence on retaining the lithium batteries make me think that they weren't chosen just for their size and weight, but that some electrical characteristic (such a low internal resistance) make them central to the design of the electrical system. It's been said here that the diode essentially isolates the battery from the bus in normal operation, but this isn't strictly true: any undervoltage, even for a time measured in milliseconds, will cause the diode to conduct and the battery to attempt to prop up the bus. The 787 electrical system is huge by aircraft standards and may be subject to many transient effects as the various loads start and stop, within a time frame that the main generating system may not be able to compensate for. So the battery may have a more active function in the system than just a backup supply.

Secondly, I find the present focus on the batteries and chargers as isolated systems to be pretty naive. The engineers at the various plants involved can be assumed to be reasonably competent and that they would have picked up on any design fault that results in an MTBF of only a few months (or one that will be identified by outside, non-expert inspectors). Rather, the problem is likely to be a result of battery operation within the complete aircraft system, whereby electrical or environmental loads are being placed on the battery that were not envisaged in the specification or tested for during construction.

Edited: fixed typo

Last edited by fizz57; 31st Jan 2013 at 18:26.
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Old 31st Jan 2013, 14:05
  #338 (permalink)  
 
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Hi,

fizz57 @ (#337)

Indeed, high current transients due (negative) voltage spikes in DC busses may "stress" the battery (due itīs low internal resistance).

Questions:

1) This condition was not analyzed and tested by Yuasa?
2) This spykes (voltage and current) were recorded? (FDR has resolution to "see"?, battery charger recorded it?)
3) Failure modes are correlated to this "non thermal" stimuli?

You introduced a good point. I commented earlier on batteries normaly working as "capacitors" when directly connected to a DC bus. With a diode (you never apply positive spykes to the battery) but you make the battery "go to the bus" immediately if the bus voltage falls below, ~ 30 V as i put in an earlier post.
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Old 31st Jan 2013, 15:19
  #339 (permalink)  
 
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Duty cycle of batteries

Hi,

Questions:

Typically in the 787 commercial operation the APU and MAIN batteries how long probably operated supplying current? I.e. :
1) APU battery started occurred frequently? Certainly not. Towing? Yes, but at low amps and few minutes. Low discharge.
2) MAIN battery (probably only on ground (tarmac, towing and hangar) operated (estimated) how many hours ? (each plane)

I suppose in both uses (APU and MAIN) the use (time) was low. (compared to their capacity).

So we could expect they were most of time being charged (most time being "trickle charged" or charged with high nominal charge current during few minutes)

I think we may say say these batteries exhibited a low MTBF despite a light use. (MAIN battery is "out of the bus" if BUS voltage is "normal" (gennies ok).

Exceptions: Possible spikes as suggested by fizz57 @ (#337) (probably much larger than 1C (65 Amps)

This "electrical environment" aspect is perhaps the last one we may imagine (characteristic of the 787) if chargers, BMS algorithms or defective components are to be discarded as causing the low MTBF.

I aasume the designers put a way to not allow discharge of ANY cell below a "safe" voltage. Remember you CANīT depend on these batteries too long at ground. GPU, APU or engine gennies are essential for (prolonged) prolonged use. And a "automatic" disconnection shoul be adopted. (due a limitation of cells).

This may explain part of the low battery MTBF of the fleet.

Last edited by RR_NDB; 31st Jan 2013 at 17:37. Reason: Spikes insteat spykes
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Old 31st Jan 2013, 15:58
  #340 (permalink)  
 
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Square peg, round hole. Except in this case, a round battery into a square case.

I think the reality of the technology is catching up to Boeing, and it is sad.

"Innovation" is like "sophistication"....

If you can't pull it off, you look silly.

Last edited by Lyman; 31st Jan 2013 at 16:00.
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