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787 Batteries and Chargers - Part 1

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787 Batteries and Chargers - Part 1

Old 5th Feb 2013, 22:46
  #481 (permalink)  
 
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And are you saying, that the battery changes have nothing to do with the known two incidents from ANA and JAL?
No, but it's certainly a thought.
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Old 6th Feb 2013, 00:25
  #482 (permalink)  
 
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B787 battery

The whole installation has shown how the best of design can go wrong on so many levels. Internal shorts within the cells to case then via shielded cable to other components bypassing the disconnect circuits? One cell failure which lead to destroying other cells? I have never seen a more compelling case to remove the battery & replace with a super cap pac. (made in the USA)
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Old 6th Feb 2013, 00:47
  #483 (permalink)  
 
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The only photo I have seen that shows what you describe is the one seen at this link.
If I had to guess what happened, based on the nut being left behind while the connector is mostly missing, is that the connector melted rather than being explosively destroyed. But I suppose it might be possible for a jet of superheated gas to destroy the connector body and lug but leave the nut behind.
Another possibility is that internal elements of the battery were displaced enough that the connector plate formed a bridge between the plates themselves and got melted down or vaporized. With no kinetic energy involved, that could have left the nut behind.
I have seen pictures of wrenches and other tools that have accidentally shorted battery terminals, and what I see here is consistent with that too.
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Old 6th Feb 2013, 03:54
  #484 (permalink)  
 
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The "frayed and broken earthing wire" looks like it was mechanically destroyed. I.e. somebody wanted to take the battery out in a hurry and didn't have the right tools. It is not burned/welded/melted. Somebody took a pair of pliers to it. The firemen did it.

I haven't seen any good pictures or scans of the ANA battery. I thought it was burned so bad that they couldn't really make a valid conclusion. The JAL battery was "lucky", in that they stopped the fire before complete destruction and they got a good look at the failure mode.

150 battery replacements does seem high at first, but 787's have flown thousandsof flights. Most of the CB's are "virtual" on the cockpit displays. It would not surprise me that unfamiliar ground crew would leave things powered by accident or lack of education. Especially during the day. At night you see the lights on. During the day, maybe not.
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Old 6th Feb 2013, 07:20
  #485 (permalink)  
 
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Hetfield - the subcells appear to be in parallel with each other to make one cell. That makes balancing less essential and more difficult to do.
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Old 6th Feb 2013, 07:55
  #486 (permalink)  
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USMCProbe:
somebody wanted to take the battery out in a hurry and didn't have the right tools

The fire crew working the JAL 787 at Boston not only cut the wires, they ripped out some of the aluminum support and tore the battery box from its mounts:

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Old 6th Feb 2013, 08:53
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I'll attempt to simplify things a bit wrt to series/parallel battery configurations.

It appears that the battery pack is made up of 8 individual batteries (NOT 8 cells!). Note this difference in terminology. Each of those 8 batteries appears to contains 6 individual cells in parallel. So 3.7V @ ~11 Ah per cell. All of the data (pictures, datasheets) I have seen points to each of the 8 batteries being monitored but not each cell in those batteries; the BMS "sees" qty 8 - 3.7 Volt batteries each with a current rating in the 65 Ah range.

Here is where it is possible for things to get interesting. Lets say a single cell (1/48 of the battery pack; 1/6 of an individual battery) goes bad. It is very difficult for the BMS to detect this because the other 5 cells are still putting out the correct voltage and accepting charge current. This can cause problems both during charging and discharging; however lets focus on charging. The BMS can be doing everything right and limiting charge current/voltage to the battery, however with 1 dead cell the other 5 are actually getting overcharged by 17%. That overcharging / discharging will heavily tax the remaining 5 cells which can induce further cell failures in the same battery. My **guess** (I haven't done math on this) is that 1 dead cell wouldn't cause thermal run-away / venting, however 2 or more dead cells within the same battery...

This scenario also fits with what has been said from the beginning. Several sources were pretty adamant that the batteries hadn't been overcharged. Based on the data they had from the electrical logs, that could well be correct. As a whole, the battery pack never received too much voltage / current. With a series-parallel design lacking individual cell monitoring, it is possible for individual cells to over charge/discharge while the pack as a whole looks fine.

I can see two possible "simple" solutions that wouldn't require much re-engineering beyond the battery itself (because their electrical interface to the aircraft could remain the same). Have a look at the LVP10 (same datasheets). There is a good chance that it is a single cell from the same mfg. Either monitor all 48 cells or monitor 6 strings of 8 series-connected cells. Either option would provide the BMS with a much better overview of individual cell problems and possibly the ability to remove a single string of cells from the circuit without taking the entire battery offline.

On a separate but related note: Unless we don't have the right datasheet (which is possible), how is it that the battery is only rated for -18C? Doesn't the certification process require for extreme cold start (at least -40 F/C) operations?

Vince (1st time poster; long time reader)
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Old 6th Feb 2013, 09:16
  #488 (permalink)  
 
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@vince

Welcome!

Each cell contains several paralleled sub-cells in the same housing. Please lets use battery, as it is common practice, for an assembly of one or more cells.

From a BMS perspective, battery consists of 8 cells. How many sub-cells per cell is of no concern to BMS.

however with 1 dead cell the other 5 are actually getting overcharged by 17%
Absolutely not! Charge is voltage limited, and _not_ volume limited, one dead sub-cell out of six would just reduce cell capacity (and thereby usable battery capacity) by 1/6th.

One can't really comment on what to change, prior to understanding what caused the failures.

Yes, these cells are rated to -18C only.

Last edited by saptzae; 6th Feb 2013 at 09:19. Reason: paralleled, -18C
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Old 6th Feb 2013, 09:40
  #489 (permalink)  
 
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@vince

Welcome

I really appreciate your support.

About overcharging a single cell (1/48 cells), I'm with saptzae, it's voltage limited.

About a bad single cell (short) I'm fully with you!

Let's say one cell out of six in one of the eight "cells" goes short.
The remaining 5 cells will deliver max current in that short until they are dead or the short cell is on fire. The BMS CAN'T stop this. It's an internal process out of reach for the BMS/Charger.
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Old 6th Feb 2013, 09:56
  #490 (permalink)  
 
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Regarding low temp limit of -18c.

Both batteries are situated in pressurised/air conditioned areas.
The only time these batteries should be subject to extreme cold is if the aircraft is parked up, for long periods, with no power....er, during the winter.....oh!
Bugger!
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Old 6th Feb 2013, 10:04
  #491 (permalink)  
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As the electric car guy said, switch to round, true single-cell batteries, this would overcome the possibility of 'sub-cell' failure.
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Old 6th Feb 2013, 10:11
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saptzae / hetfield: Thanks for the warm welcome

I respectfully disagree regarding voltage based charging being a sufficient safeguard. For "old" technology (Pb / NiCad / Ni-Mh) cells, voltage based charging is a somewhat sufficient both as a charge strategy and as a detection method. Li-Ion is a totally different animal.

Have a look at a charge graph for Li-Ion / LiCO2 battery charging. Here is an example:

Charging Lithium-Ion Batteries

Charge is current limited (constant current) for a good portion of the charging cycle before transitioning to voltage-based charging. This information can be confirmed from various Li-Ion mfg spec sheets.

If a discharged Li-Ion cell is connected to a safe charge voltage (< 4 .2volts) without current limiting, it will overheat and may enter thermal run-away.

So I'll stand by my previous statement that the possibility exists to overcharge parallel cells in a Li-Ion battery without the BMS detecting it.

The only time these batteries should be subject to extreme cold is if the aircraft is parked up, for long periods, with no power....er, during the winter.....oh!
Bugger!
That's what I'm talking about. I distinctly remember watching the A380 documentary and they covered the part where they flew to the arctic and the aircraft has to "overnight" in ~ -40. It then had to start without abnormal assistance.

Just something that caught my eye looking at that datasheet.
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Old 6th Feb 2013, 10:18
  #493 (permalink)  
 
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USMCProbe stated:
I haven't seen any good pictures or scans of the ANA battery. I thought it was burned so bad that they couldn't really make a valid conclusion. The JAL battery was "lucky", in that they stopped the fire before complete destruction and they got a good look at the failure mode.
From the couple of pictures that I have seen (sitting on a wooden pallet), I would say that the ANA battery was actually in better shape than the JAL battery.
Pictures shown in some of the articles about the ANA battery were actually NTSB photos of the JAL battery (on a clean plastic pad with dimension calibration tags next to the battery.)
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Old 6th Feb 2013, 10:19
  #494 (permalink)  
 
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@hetfield
Let's say one cell out of six in one of the eight "cells" goes short.
The remaining 5 cells will deliver max current in that short until they are dead or the short cell is on fire. The BMS CAN'T stop this. It's an internal process out of reach for the BMS/Charger.
Fully concur, and I believe this is what actually happened to both ANA and JAL batteries.

I try to focus on what happened and want to stay away from what to change until failure is understood.

- There is no indication as of yet that the cells were bad ex GS Yuasa.
- To short, cell(s) must have deteriorated quickly
- What caused this deterioration
- Was deterioration during integration
- Was deterioration during operation (BMS)

- When a [email protected] shorts, the BMS _must_ detect this and take the battery off-line. That did not work either. Clarify: The BMS must detect a cell short, which, at the initial event, will be a sub-cell short. How: By detecting cell voltage transients.

However, I can see merit in parallelizing 24 standard 3Ah cells via a (30A) fuse per cell. A single cell short would simply remove the shorted cell from the others, with much less energy dissipation and collateral damage.

Edit: A123 went broke after several batteries, consisting of small cells, went up in flames... A123 Replacing Batteries That Led to Fisker Karma Shutdown - Bloomberg

Last edited by Jetdriver; 7th Feb 2013 at 12:43.
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Old 6th Feb 2013, 10:19
  #495 (permalink)  
 
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The Canadian regs require cert to -40c. The FAA do not.

Plus you can do the cert test and remove the battery during cold soak, only refitting just before you power up. But this should be SOP for all airlines if certified this way.
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Old 6th Feb 2013, 10:27
  #496 (permalink)  
 
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Originally Posted by green granite View Post
As the electric car guy said, switch to round, true single-cell batteries, this would overcome the possibility of 'sub-cell' failure.
According to Tesla Roadster - Wikipedia, the free encyclopedia their battery pack has 53 kWh capacity at 375 V and weighs 450 kg. 787 battery has 2.2 kWh at 29.6 V and weighs 28.5 kg. So as soon as Tesla's battery gets downscaled 24-times while handling different voltage and the currents for starting APU and gets certified for aircraft it can be readily used. 2015, maybe?
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Old 6th Feb 2013, 11:17
  #497 (permalink)  
 
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Plus you can do the cert test and remove the battery during cold soak, only refitting just before you power up. But this should be SOP for all airlines if certified this way.
It is SOP on present day aircraft (A320/B737).
They all have procedures for parking in cold weather, and at Minus 20 or so that includes battery removal to a warm office.
It catches us out once in a while. We always put a hot air heater on the aircraft, and leave the batteries in and the outflow valves open. Then a pilot will read the FCOM and do something off his own bat.
It is amazing how much force there is with a heater hose connected, and the ditching button pressed on an A320, and you open the pax door. The FO caught the Capt as he flew across the jetty!
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Old 6th Feb 2013, 11:58
  #498 (permalink)  
 
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USMCProbe
The "frayed and broken earthing wire" looks like it was mechanically destroyed. I.e. somebody wanted to take the battery out in a hurry and didn't have the right tools. It is not burned/welded/melted. Somebody took a pair of pliers to it. The firemen did it.

I haven't seen any good pictures or scans of the ANA battery. I thought it was burned so bad that they couldn't really make a valid conclusion. The JAL battery was "lucky", in that they stopped the fire before complete destruction and they got a good look at the failure mode.
out of this reference from today Japantoday

Photos distributed by the Japanese investigators show severe charring of six of the eight cells in the ANA 787s battery and a frayed and broken earthing wire meant to minimize the risk of electric shock.
There are some photos as well in the above reference.

Last edited by RetiredF4; 6th Feb 2013 at 12:00.
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Old 6th Feb 2013, 13:32
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From Machaca' picture of the JAL accident....

The aluminum structure to the right of the photo shows evidence of being torn away from similar structure.

I don't think the firefighter could have accomplished that kind of damage, the Aluminum is torn. i think that could be evidence of an explosive force prior to arrival of the FF crew, an explosion.

Also there is damage in the adjacent bay to the right, fire damage, it looks like....Although early press was specific in claiming no damage to equipment "within twenty inches of the battary case", that seems to have been incorrect.

I think the twenty inch distance is a requirement of the 'special waiver' given to this LithiumIon chemistry...

Last edited by Lyman; 6th Feb 2013 at 13:42.
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Old 6th Feb 2013, 13:45
  #500 (permalink)  
 
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@hetfield

I though more about the practical side of detecting, in a timely manner, a short of a sub-cell. At that time, the BMS can't stop the cell failure, but it must protect the other cells by taking the battery off-line.

A short will not be static, it starts at a spot, current, temperature and pressure increase, it may even "burn out", but damage done will lead to further shorts nearby, propagate and short resistance will be transient with a reducing trend.

As you pointed out, a shorting sub-cell will "fight" five good sub-cells.

Thus, the the voltage variations seen by the BMS may be quite small and it may not respond in a timely manner.

What is timely? I am not sure, a few milliseconds at the most, below 100 microseconds would be better. Such short response time is difficult to accomplish.

Summing up, your concern about managing sub-cell shorting is valid, and i see your interpretation as a practical explanation for why a single cell short propagated to other cells.

What has to be done, is to make the BMS more sensitive to voltage transients on individual cells and shut charging down very quickly.

Last edited by saptzae; 6th Feb 2013 at 13:46.
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