Two posters have suggested that the solution would be to remove the battery chargers and fly the batteries as "primary" cells that would be recharged only on the ground. That won't solve the problem.
The problem is "thermal runaway". As the battery gets hot, the internals expand, getting closer to each other, and thus increasing the likelihood of a short-circuit. The more often they are cycled, the greater the likelihood of a short-circuit.
The batteries get hot because of the current passing through them. While this happens to a certain extent during charge (at 2C which is about 130 amps for these cells) it is much worse during discharge (when the APU battery flows more than 1,000 amps for up to 30 seconds while winding up the APU). If it's going to catch fire, the problem is most likely to begin just after you "use" it. Removing the charger doesn't solve that.
The onset of thermal runaway happens at about twice the boiling point of water: this research shows these batteries are well into "coffin corner" by 200 degrees Celcius: Modeling Thermal Runaway for Safer Lithium Ion Batteries
As Lomapaseo points out, hot fires are successfully contained by metal cans every day: in things called "engines". I suggest that Lithium Ion batteries, with a known propensity to spontaneously combust, should be contained in a fire-proof box whenever they fly, whether they are "in use" or not. We know these things are going to burn occasionally: we should ensure the tail (or the cockpit!) doesn't fall off the aircraft when they do. Either would cause the flight crew to think unkindly of the designer/manufacturer.
My greater concern is not that these batteries catch fire, but that they fail! On a fly-by-wire aircraft, that gives a whole new meaning to the term "dead stick". Yes, I know almost everything else has to fail before this matters. But consider Sully's celebrated ditching:
He lost both engines. His APU was not running. If his batteries had failed, and his aircraft had been a 787, he would have had no flight controls. At all.
I think the pilots on this forum would be more than anxious to have this possibility designed out. Before they have to fly a 787 again...
Here, I have sorted the data by "Watt/Hours per Kilo":
Type Energy density
(Wh/kg) Lithium-air (organic)[7] 2000 Lithium sulfur[10] 400 Lithium-ion 200 Molten salt 180 Lithium-ion polymer 165 Sodium-sulfur 150 Silver-oxide 130 Lithium iron phosphate 100 Lithium–titanate 90 Alkaline 85 Zinc bromide 80 Nickel–hydrogen 75 Nickel–zinc 60 Nickel–metal hydride 55 Nickel–iron 50 Nickel–cadmium 50 Lead–acid 35 Vanadium redox 30 Sodium-ion[13] 0 Thin film lithium 0
Sorry, the PPRuNe website doesn't like tables, but you can get the idea. Leaving aside the two outliers at the top that are not on-sale yet, Lithium Ion comes in third. Molten salt is a little aviation-unfriendly.
But Lithium iron phosphate would be my pick. Twice the energy density of NiCad, and already approved for aviation service in an application I know about. OK, they are electrically "fragile", which means you have to pay very close attention to the design of the charger: one spike and they are trash. But that's probably what happened to the 787 batteries.
The benefit of the LiFePo battery is that it does not catch fire when it fails. Another benefit is that it has a very low internal resistance, which enables it to withstand truly prodigious discharge rates (APU starting...) without suffering. And they have around 5 times the in-service life.
Hi Tech, I agree with your analysis. but as a commercial pilot of 36yrs & 20K hrs I thought I would look more deeply at what is still a fine a/c. Multiple failures of systems are a pilots worst nightmare & the Hudson river incident would not have been successful if the Capt had not pre-empted the cck list & started the Apu. I see the flying controls have 3 hydraulic systems, with the centre electrically powered, so with a double engine failure it is essential to start the APU, (from the battery)? I appreciate the windmilling engines should provide some output of both electrical & hydraulics but again the pilot is faced with pressurisation problems (High Techs) and how to restart engines at 40000ft without electrical power. I note that the a/c needs 2 GPUs to provide engine starting, I have been to numerous outstations where you are lucky to get even one doubtful unit. Boeing have said this a/c is a game changer, I agree, but it is very complex, and as such I can see lots of down line problems.
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poorjohn
Quote:
even after two fires and an OAG (sic) fleet the best way forward is still to leave everything as unchanged as possible, and put a steel box around it.
I think there's a lot more involved in the proposed modification than just putting a box around it. There is talk of cell re-design and battery modification to isolate cells thermally.
Walnut
Quote:
so with a double engine failure it is essential to start the APU, (from the battery)? I appreciate the windmilling engines should provide some output of both electrical & hydraulics but again the pilot is faced with pressurisation problems
The 787 starts the APU automatically in the case of a double engine failure, yes from the battery, but it would be assisted by windmilling engines and the RAT.
The pressurisation problems are no worse than any other twin when faced with a double engine failure. You're going down anyway, so the most important action for loss of pressurisation is already initiated!
You can do some WYSIWIG formatting of tables by wrapping the table text inside CODE tags (The # format button in the toolbar).
Code:
Type Energy density
(Wh/kg)
Lithium-air (organic)[7] 2000
Lithium sulfur[10] 400
Lithium-ion 200
Molten salt 180
Lithium-ion polymer 165
Sodium-sulfur 150
Silver-oxide 130
Lithium iron phosphate 100
Lithium–titanate 90
Alkaline 85
Zinc bromide 80
Nickel–hydrogen 75
Nickel–zinc 60
Nickel–metal hydride 55
Nickel–iron 50
Nickel–cadmium 50
Lead–acid 35
Vanadium redox 30
Sodium-ion[13] 0
Thin film lithium 0
This will work best if you have formatted the table in a straight text editor and not a word processor and have used a monospace font such as Courier. Try to avoid using TABs if possible.
I note that the a/c needs 2 GPUs to provide engine starting, I have been to numerous outstations where you are lucky to get even one doubtful unit.
I believe this is one reason that the APU can be started by either battery or ground power. The APU can then start the engines, one at a time. If there is no fuel on board for the APU, the main battery will power the fuel transfer system to allow fuel to be added and made available to the APU.
Quote:
... but again the pilot is faced with pressurisation problems...
I also believe that since pressurization is provided by electrically powered compressors rather than air bleed, minimal cabin pressurization can be powered from the APU or even just the RAT.
The RAT has absoulutely nothing to do with pressurizing the aircraft. It simply does not have the capability to power the CACs and thus is limited to only the most essential items needed such as hydraulics and flight instruments.
My greater concern is not that these batteries catch fire, but that they fail! On a fly-by-wire aircraft, that gives a whole new meaning to the term "dead stick". Yes, I know almost everything else has to fail before this matters. But consider Sully's celebrated ditching:
He lost both engines. His APU was not running. If his batteries had failed, and his aircraft had been a 787, he would have had no flight controls. At all.
The 787 also has individual back up batteries for the three ACEs (Actuator Control Electronics-part of the fly by wire system) they are also Li-Ion. The 777 is similar.
Quote:
I note that the a/c needs 2 GPUs to provide engine starting, I have been to numerous outstations where you are lucky to get even one doubtful unit. Boeing have said this a/c is a game changer, I agree, but it is very complex, and as such I can see lots of down line problems.
It's worse than that. Two GPUs is the absolute minimum-engine start cycle can be up to 3 mins due to the lack of power. For optimum engine starting a third GPU is required-plugged in at the aft EE bay area.
As has been said, getting two 90Kva GPU supplies is hard enough, but three? FORGET IT!
B777 is the only type I can think of with a pneumatic starter (in addition to electric). Are there others? A380 perhaps?
A380 has no pneumatic starter. No other aircraft I know of has one.
Have there been many (any?) total electrical power failures that haven't been caused by loss of the engines too? I can't think of any.
Only major failure when all power was lost on B787 was in the Loerdo incident, when there was a total loss of power and the RAT deployed. As they were on approach at that time they continued in that condition. Details of that incident are sketchy. There was a major fire in the central Electronics bay, where all power panels are located. I don't think NTSB got involved as the aircraft was still in the test phase. The reason for the power loss and fire according to Boeing was FOD !!! Then there was the United incident and diversion to New Orleans, where power loss from one Gen was not properly restored from the other good generators. When you have 6 Gens loss of one Gen should be a piece of cake. Again the details are sketchy. The same problem occurred again on a new Qatar B787 after delivery flight. The details are once again limited. the out burst of it's Chairman against Boeing is well documented. Boeing has mentioned that these problems are now sorted out.
During my time in aviation there were some incidents of total power loss on different aircraft, but did not result in any accident. In one instance the aircraft was B777 that was dispatched with one gen inoperative, and other side engine failed for an unconnected reason. But as the APU was already running, power was automatically restored. One remarkable incident was on a A300-600, belonging to a charter airline, that was positioning to an MRO. It was dispatched with one Gen inoperative from far east and when over India the other Gen also failed. The flight continued all the way to France with just APU. It was a ferry with just 2 crew. Still it was remarkable that the crew had faith on the APU for 6-7 hours. When I met the crew after landing, both Mexican, I had to tell them that they were bravest crew I have ever met to have faith in the APU for 7 hours. On airbus there is detector called Avionic smoke Detector, which can trigger a warning (Most instance false), the crew have to kill most of the power and land ASAP. This has happened several times, a recent one is on Air Canada 320 at Edmonton. Incident: Air Canada A320 near Edmonton on Aug 18th 2012, avionics smoke indication
Airbus knows when it potentially has a tool to beat its competitor over the head with. Not in an overt way mind you - there will be subtle reminders...
Leahy already jumped on it, in a not so subtle way (quoting from CONF iture's link):
When Airbus redesigned the A350 seven years ago to create the all-new XWB, Leahy admits that he pushed the engineers to follow Boeing's lead on all-electric architecture. But he is pleased he was overruled. Airbus engineers went "back and forth" three times about whether to equip the XWB with electric brakes before deciding to stick with conventional hydraulic architecture. "I'm guilty as the commercial guy for pounding the table saying 'look [the 787's] all-electric - it's game-changing'," Leahy admits. But trade-off studies by Airbus engineers could not justify adopting the technology. Leahy says: "They told me: 'You're not going to like the reliability - it's going to be complex, heavy, and hard to maintain'."
Quote: I see the flying controls have 3 hydraulic systems, with the centre electrically powered, so with a double engine failure it is essential to start the APU, (from the battery)?
My original post was just to high light how Boeing are in a pickle by doing away with engine bleed. Though a bit old fashioned, engine bleed was reliable power source. Without bleed we need huge power source from generators and batteries etc, and has actually created a problem on basic electric systems which normally do not have any serious problem on most aircraft these days.
Last edited by Hi_Tech; 28th Feb 2013 at 12:36.
Reason: Typos
The Interflug Il-62 crashed back then because of a leaky bleed air pipe crossing the little known cargo compartment between the engines burned through insulation and some electrical wiring which ignited some barrel of deicing fluid that was carried there (not permitted per regs). And all that led to a fire, loss of control, inflight breakup and loss of all lives. The type received modifications afterwards.
Agreed, in that Boeing don't design batteries. They would have looked at the energy density values and at the design stage, Lithium-Ion would have been a no-brainer.
Yuasa's expertise in battery design is beyond reproach, however at this moment it's uncertain what caused the battery pack to ignite. While it may have been a faulty battery, we don't know if the cell spacing exacerbated the problem or if the charge/discharge algorithms were faulty or if it was an as yet unknown factor.
What is clear is that Boeing pretty much went all in on 'Electric plane' concept and this made the Lithium battery a commitment rather than a contribution.
I'm still slightly surprised that there were no indications of potential battery problems during the testing/flight testing regime, these normally take each system to the edge of the envelope, yet no issues reported?
Agreed, in that Boeing don't design batteries. They would have looked at the energy density values and at the design stage, Lithium-Ion would have been a no-brainer.
Yuasa's expertise in battery design is beyond reproach, however at this moment it's uncertain what caused the battery pack to ignite. While it may have been a faulty battery, we don't know if the cell spacing exacerbated the problem or if the charge/discharge algorithms were faulty or if it was an as yet unknown factor.
What is clear is that Boeing pretty much went all in on 'Electric plane' concept and this made the Lithium battery a commitment rather than a contribution.
I'm still slightly surprised that there were no indications of potential battery problems during the testing/flight testing regime, these normally take each system to the edge of the envelope, yet no issues reported?
Hang on a minute shouldn't the design be "Fail Safe"? I wouldn't fancy flying 8 hours with a fire in the cabin. Just redesign the battery.
I'm still slightly surprised that there were no indications of potential battery problems during the testing/flight testing regime, these normally take each system to the edge of the envelope, yet no issues reported?
