Discussion in 'Electric Propulsion' started by Bille Floyd, Nov 5, 2019.
That is a miniscule possibility.
This is the path I intend to take with my hybrid for a very light plane. My math says it is practical. But the weight of the battery(s) I plan to use is small compared to a pure electric plane that most people will fly.
Engineers and pilots are not going to be the ones that make the final determination weather dropping a flaming battery pack is socially acceptable or not. I'm of the opinion that after the first fire that makes the news from a dropped battery, not only will dropping them become illegal, the scarred masses will try to ban any electric flight over any 'sensitive' areas. Over reaction is normal behavior for humans.
I'm not very lucky.
Three fires that we know of. Only one could be traced to a battery pack that had been damaged. All appear to be related to manufacturing processes that allowed metallic swarf to migrate down among the pouch cells and eventually penetrate them. They have changed their processes to prevent recurrences, but it is still a point of concern.
There are four main reasons why battery can burn.
1) fire can be caused by production fault (or during cell production or during solding when battery packs are produced) as mentionned by BoKu.
2) fire can be caused by mechanical damage
3) fire can be caused by too high discharge rate
4) fire can be caused by too high re-charge rate
Beside of that can fire start somewhere in wiring (short circuit), but that is another issue.
Point 1) Producer of airplane have to buy battery from reliable and experienced supplier of cells or battery packs. Some companies are already using sofisticated system of production checks.
Point 2) This is real problem of replaceable battery packs. If some technician or user will drops battery packs and he will not report this accident it can easily cause fire. "G" sensor installed inside battery pack can at least show if battery received some non-reported impact.
Some homebuilders are thinking to buy cheap used battery from crashed electric cars. What to recommend? Well... make a lot of ground tests before first flight and measure temp. carefully.
Point 3) and 4) In modern electric car is used battery temperature management. Good solution, but this increase the installation weight = problem for electric cairplanes. Solution? It is necessary to measure temp. of battery on multiple points and keep "C" rate as low as possible. Easiest (but not cheap and light) way is to use as large as possible battery.
You have an airplane with requested 60kW of power for safe TO.
With 30 kWh battery you need 2C to get 60 kW of power. With 10 kWh battery you need 6C to get 60 kW... etc. More "C" = higher loaded battery = high temp. increasing = lower lifetime (cycles)...
And same situation is with re-charge. Well designed system should decrease charge power when battery temp. will increase to protect it and precede fire danger. So it means that without temperature management you will re-charge with high power only limited time and than charge power decrease till battery temp. is lower again.
Sounds complicated and too dangerous?
When combustion engines started it was for first users difficult to imagine, that highly flamable liquid will be easily available everywhere. Now we have high pressure LPG, CNG, maybe hydrogen in future... So I believe that with more electric cars will be this technology well known and we will handle it.
Afer 9 years of experience I am carefully optimistic
It is no relevant, nor cooling istself. Battery is not ideal, it have some internal resistance. And by taking 100 amps from it, it will have some i^2*r losses. And some internal layers - can be hotter and will burn it...
So - it have to be designed, to avoid overheating.
I have used an aluminium silicita "plate" for heat insulation..
Excellent, enlightening, electric post.
Great to have some one with considerable experience to join in the fray.
Sophisticated management systems are a definite requirement for safety, longevity and efficiency. Same with naturally the wiring, and controller must be high quality, the motor suited to the complete system etc. And ability to manage airflow to the batteries ideally would all help greatly in temp management. Cells from cars is another matter. In a ideal world the chances of fire would be extremely low like a gasoline fire in a properly developed modern aircraft. Like many others, I see a very great future for electric aircraft.
What is your perspective on a potential runaway battery fire- ie when the worst case scenario quickly happens?
We know it can happen and has sadly, so we are looking at the design needs to maximise the potential for the pilot and maybe passenger to get down in one piece in this nasty situation.
Think of it like a ballistic parachute design wise- only to used when everything goes FUBAR. With the exception the last thing you want to do is stay with a fiercely burning machine slowly coming to earth.
We assume the pilot is not just bailing out with a personal chute- not worn by most pilots of potential aircraft anyway. And its a real downer introducing a friend to the delight of electric flying, and then hand her a parachute.
What are your thoughts?
Superstition is a very poor aircraft design strategy.
I have been facing problem with fire in fligt. It didn't burn the battery but the motor. It was one of prototypes produced by us before we bougth motor from Rotex. One magnet has been not glued properly, it has been crushed into powder and the friction caused fire. Well... the only solution I had on my mind was quick emergency landing. So I opened air brakes to achieve max. sink rate and I landed.
I agree that use of balistic parashute is in this case probably not the best idea. But I also cannot imagine to "throw away" the battery from airplane in flight - you never know what can be below. And quick change of CG would be probably criticall. Like with fire caused by fuel - you have to try to keep smoke out from crew to give pilot time for emergency landing.
So far is probably best way to keep batteries in non flamable containers to slow down effects of fire - again to give some time for pilot to make emergency landing.
I received info that they are existing special very high pressure tubes able to cool down the cells with some chemical process.
If this will work, could be interesting. However if this system will remove danger of fire I don´t know.
Apparently , you didn't see the post ; "before" the edit job ?
Kind of yoke - lets build it in a glider with meat bag on board, and a remote controller tug with battery.
Personally, I think the best way to avoid a thermal runaway is to choose an appropriate battery chemistry and make sure you have good thermal management and voltage regulation. Tesla's and many other electric car batteries are going to have some sort of magnesium or cobalt oxide cathode material, which is great for energy density, but needs to be carefully managed to prevent a thermal runaway event. Many of these packs have active water cooling as well to keep the temperatures within safe limits, along with allowing them to operate efficiently.
For something like a motor glider, I think your best bet will be with using a battery with iron phosphate as a cathode material, as these are among the safest lithium packs in terms of thermal safety. You sacrifice energy density of course, but you'll drastically reduce the chances of a fire. Couple that with a good battery regulator should make things much safer as well.
I already used , most of my 9 lives ; if it is a safer alternative, then
i'm still open to E-power . What would the weight penalty be ?
LiFe packs weigh more and lower density (so you need more for same range) there's some great writeups and calculators over on endless planet.
But theres still more to it... A lot of the packs that go up are technically unknown cause...
In the rc world we knoe after repeated teardowns the 18650 (and 28650 etc) cells the wrap wears. Sames been seen on the other packs. When you look at vibration and then collection of dust/rub/detritus that accumulates in any closed area... That's often what many think leads to a short that eventually takes out the pack.
I still go back to those baggage pods feel ideal for this scenario right now -side bonus of being able to swap pods military style and keep flying if its a nice day.
The dust buildup causing shorts seems to point to the need for a good air filter if forced air cooling is used... and a way of keeping dust from getting to the pack if no forced cooling.
In case of fire, jettison the pod. Instead of a parachute, have the pod deploy a hot air balloon envelope so it will stay aloft until it cools down, or is over an ocean and can be dropped.
I like this outside the box thinking.
Plenty of energy to keep the balloon aloft from the burning pack. Maybe start with a helium balloon for a quicker deployment? Equip the unit with it's own ADS-b/ELT ping and everyone should be happy?
You guys like stressing over things for days longer than needed?
No I'm in the straight down camp. But i fly over rural canada mostly
That plan certainly wouldn't work for anyone flying electric aircraft in Ozzie at the moment. Actually one could probably have to add a fire ban check to ones pre-flight planning and just be prepared not to fly in summer.
I used Estes rockets as a kid. They were made with extremely hot rocket fuel packed in an 1/8" thick cardboard tube.
It takes time to burn through cardboard, probably several minutes. The rocket burn was probably 5 seconds.
So place the cells in a cardboard tube vented away and rearward. Problem solved.
There isn't time to eject after you notice your airplane is on fire. You want to vent the flames in the manner as if you had installed any other sort of certified rocket assist.
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