Is electric flying safe today ?

Discussion in 'Hangar Flying' started by Speedboat100, Jul 26, 2019.

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  1. Jul 26, 2019 #1

    Speedboat100

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  2. Jul 26, 2019 #2

    MadRocketScientist

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    Could possibly be a failure of the monitoring electronics designed to keep the batteries in balance etc. The electric bike guys seem to call the BMS the "Battery Murder Suspect" :D
     
  3. Jul 26, 2019 #3

    Derswede

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    LiPobatteries are very susceptible to thermal runaway. Have had a few incidences myself with smaller batteries that were LiPo. I now use LiFe batteries. Have not had any thermal problems with that chemistry.

    Derswede
     
  4. Jul 26, 2019 #4

    MadRocketScientist

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    I have been using LiPo batteries for radio controlled planes and helicopters for over a decade and haven't had anything get close to catching fire. I always make sure the batteries get plenty of cooling. It does seem strange that such large packs have no cooling gaps between the cells. If you look at the Tesla car batteries, they have cooling spaces between all of the cells and forced cooling to keep them at the optimum temperature. Given the overall space inside the shell of an aircraft, I can't see any reason for not having cooling space in large packs.
     
  5. Jul 26, 2019 #5

    pictsidhe

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    Many if these systems are poorly engineered. Dunning-Kruger and lithium is a bad combination. The Kokams are particularly high performance. Thermal runaway is a suspect for me. Does the BMS monitor pack temps?

    Charging below freezing point can plate out metallic lithium. The batteries will seem fine afterwards, but are then an accident waiting for somewhere to happen.
     
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  6. Jul 26, 2019 #6

    FritzW

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  7. Jul 26, 2019 #7

    Cy V

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    I recently read an article about a new company that has found a way to make a battery with a solid state polymer. Their batteries are supposed to be fireproof.

    https://ionicmaterials.com/
     
  8. Jul 26, 2019 #8

    12notes

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    The article only lists battery fires, not batteries in electric powered planes. Plenty of piston and turbine powered aircraft have batteries.

    The increase could easily be from the increase in LiPo battery use, especially in the proliferation of cheaply made batteries advertised as lightweight, drop in replacements for lead acid batteries. There's probably some people swapping them out, charging them with a automotive battery charger and not understanding the problem.

    The article is about two fires, a year apart, on the same model of electric motor glider. I suspect a design issue with this particular battery & charging setup, not an issue with the safety of electric flying in general, akin to a fuel system design that eventually leaks on a specific piston aircraft.
     
    Last edited: Jul 26, 2019
  9. Jul 26, 2019 #9

    Hephaestus

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  10. Jul 26, 2019 #10

    FritzW

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    ...good point
     
  11. Jul 26, 2019 #11

    Speedboat100

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    Peter Sripol always touches the batteries after a flight and comments...these aren't even warm etc....!
     
  12. Jul 28, 2019 #12

    Speedboat100

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    Should the batteries be close to you so that you can check them by hand or in a fireproof compartment and temperature checked with a meter ?
     
  13. Jul 28, 2019 #13

    pictsidhe

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    The BMS should monitor temps and disconnect and alter charge/discharge as appropriate. Relying in a human to do that is why many of these fires happen . If you can build a proper BMS, it wouldn't be hard to add a blinking red light on the dash.
     
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  14. Jul 28, 2019 #14

    bmcj

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    There is a huge difference in demand and duration between models and man carrying.
     
  15. Jul 29, 2019 #15

    MadRocketScientist

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    I understand that, however a lithium battery isn't going to end well if its discharge rate exceeded. Often models fly for around 10 mins for a 6C average discharge, If you want a light aircraft to fly for an hour the average discharge rate would only be around 1C.

    Also, from memory a lot of the aircraft lithium battery fires (787 included) have all the cells directly against each other with no cooling spaces between them. The battery packs in Tesla cars have forced cooling (and heating) and separate fuses to each cell from the bus bars. While none of these things stops the packs catching fire, having cells hard against each other in a brick, doesn't help the middle ones shed any heat.
     
  16. Jul 29, 2019 #16

    BoKu

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  17. Jul 29, 2019 #17

    pictsidhe

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    It doesn't matter how you build the packs. What matters is keeping every cell within it's safe operating area and throwing it away after any excursions. That is absolutely vital. These things will merrilly burn to a crisp if you step outside their happy zone. If that means that your BMS suddenly cuts the power during a go-around with looming poerlines, well, fly under them! How many people will replace a $2k battery that has stepped outside it's safe zone briefly but seems 'fine'? Dig into a battery fire, you'll find in just about every case, the cells left their happy zone, then went FWOOM! Either straight away, or some time later. This is why those who do keep their cells in the safe zone for 10+ years don't understand why there is all the fuss about fires. These cells are made in huge numbers. the manufacturers have established under what conditions they can be used. Step outside that zone at great peril. Cooling will help you run cells harder but still safely.

    Overdischarge, overcharge Too high or too low a temp during charge, discharge or storage. Physical damage. Just some of the things that can damage cells and turn them into a ticking bomb. Many BMS's don't cover all the possible damage scenarios. I've not come across a single WWW circuit that prevents charge below freezing. That's unsafe with Li-ion. If you can't be sure they are within their safe parameters, you are playing russian roulette.

    Now, how many li-ion power tool fires are there? Those are high discharge li-ion systems that take considerable abuse. 5C+ from full charge to flat is not unheard. Hell, I've put a paint stirrer in several and run them hard until flat. But they are engineered to just shut down when they shouldn't be charged or discharged. They are made by the million. They work safely.
     
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  18. Jul 29, 2019 #18

    TFF

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    I have seen a couple of RC planes burn. One was a new 2m pattern plane. It’s not often now and most people have the right system to do ok. RC models generally use between 20-40C draw. 50% drawdown of capacity is about the limit for long pack life. Using a timer is critical if you don’t like buying new packs. 120F is about as hot as you want a pack. It will start venting and puff. Life will be lower and then of course fire if you really are not paying attention over and over. Tools have bigger safety controls for the average consumer. They will shut off and prevent charging if things have gone wrong. It has all been designed in. RC does not have the same safeties, the set up is the safety to get performance. I think the bigger RC electric ducted fan engines are pulling 100C.
     
  19. Jul 29, 2019 #19

    litespeed

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    I think it all comes down to design the system to meet your needs with sufficient headroom to allow for high discharge and charge rates. No good having low c rate cells, that is a recipe for disaster.

    Allow the cells to be managed by a full on bms with temp controls in a well designed ventilated case. A well designed BMS could also allow emergency power for a short period when needed even when the cells are getting low or warm.
    Buy the quality cells, you get what you pay for.

    Think of it like a fuel system, you design it poorly and you may burn. And also like a engine- run it at full power all the time and it is very short lived esp with poor cooling.

    There is a reason aircraft engines are not massive revs or power compared to car engines. We are not after rocket grunt just reliable power that gets us up and back down safely.

    I have said this before and will repeat it-

    IF possible design to have the cell pack over the C of G and under/behind the pilot. IF you carry 200lbs of battery and it all goes wrong- you can eject it and have it parachute down or airbag it. You get a much greater glide range, a much lower stall and much less inertia in the event of a hard impact. And naturally have left the flammable mass far away from the aircraft.

    I know peeps don't like the idea of a eject button but it makes a great deal of sense for safety. An additional benefit is it also allows for the battery to be quickly swapped if you wish.

    If we just grab old designs and electrify them we are missing a lot of advantages a clean sheet electric can provide- think outside the old square box.
     
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  20. Jul 29, 2019 #20

    Speedboat100

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    I agree 100 % with you....and the last is the most important..there are millions of new ways now as the e-engine is 5-10 x lighter that IC engine...and batteries can be placed where ever you need them to be. You also must redesign as battreries will never have the same endurance as the IC systems..to gain at leat 50% better or more suited aerodynamics and a structure for an e-type of flying machine.
     

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