thermal runaway - help
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thermal runaway - help
Can anyone out there, give me an easy explanation of thermal runaway. (I have almost no understanding of electricity- I do know that knives and toasters don't mix) Any info would be great, thanks
As a twenty year memory serves me, thermal runaways can happen with nickel-cadmium batteries. As I recall, the battery resistance drops to near zero, sucks everything out of the generator and starts getting very hot. There may also be other failures in the voltage control circuitry.. .. .This situation can happen where all current must pass through the battery -- systems where you can disconnect the battery offer an extra degree of safety.. .. .In the article I read on a thermal runaway incident, the a/c eventually caught fire on the ground.
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Thermal runaway???. .. .Is'nt that what you do when it sets on fire???. .. .Seriously though. When current passes through a resistive medium it generates heat. The heat alters the resistivity of the medium, as a consequence more current passes than before so even more heat is generated and around the circle we go. Iv'e used the word medium as this can happen in resistors, transistors, ICs and yes batteries.
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Yep, fun to watch, from a distance. . .. .Had a Batt melt in an HH-1N once. Bad Juju ... Crash Crew got it put out in about 10 min, but with extensive damage to the nose of the aircraft.
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Thermal runaway only happens on nicads, it is most likely to happen when a battery is discharged to much then somebody starts the aircraft using external power and the generators are then putting to much current through the batteries whilst charging them. Large amounts of heat generated and the interesting thing that then happens is that one of the cells which are joined in serial to each other to make up the battery reverses polarity and feeds current to the adjacent cells without any current ie circuit external to the battery required hence the term runaway. The only remedy is to isolate the battery from the Bus and let it run it's course ,do not move it cool it or interfere with it in anyway. It can explode. Once it has cooled down it is now unserviceable and needs sending to a battery shop where all the cells will be discharged to zero by resistors then shorting to ensure they are all equal then recharged in stages using a specialist charger.
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In more general terms, thermal runaway happens when you have something that makes more heat as it gets hotter.. .. .One of the most common examples of thermal runaway is the common or garden household lightbulb. Over time, material evaporates from the filament, evenly to start with, but there are always some places where a little extra vapourises. Those bits then get thinner than the rest of the filament, have less metal in them to conduct the lectrikery and have a slightly higher resistance. By the wonders of Ohm's Law -- voltage equals current times resistance -- these thinner segments get a higher voltage across them than the filament next to them, and tend to get hotter still. Thus, even more filament gunk evaporates, the thin bit gets thinner still and... eventually, there's a bright flash as the last bit goes out in a blaze of glory.. .. .R
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As has been previously stated, Thermal runaway occurs only min NiCad batteries. It can happen due to high charge current due to battery being fairly low in charge to start with, or having a very large load placed on it (too many starts tried in short succession). These high current will cause the battery to get hot, and in some cases can cause hotspots within individual cells. . .The hot spots cuase their own mini battery to form (for want of an easier explanation), and allow for local high current discharge, cusing more heat, cusing more hotspots and so on (Thus the term Thermal Runaway).. .. .Ther is no way in an aircraft to stop this once strarted, so the only thing to do ids turn off power and go away for a few hours. (not much that can be done out of the aircraft either for that matter). . .. .Do not try to remove the battery from the aircraft, as it can be extremely hot and will cause great bodily damage. It can also explode wich will cause even more bodily damage and spray hot metal , plastic and potassium hydroxide ( burny stuff even when cold) every where. . .. .If absolutely neccessary, move the aircraft away from other aircraft, people and buildings and let sit. Check it tomorrow, and replace with a serv battery. the over heated battery will in all probability need extensive cell replacement (and expensive) due to heat damage to adjacent cells. (assuming that the battery and plane are still in one piece.) <img border="0" title="" alt="[Eek!]" src="eek.gif" />
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exaac,. .. .After a thermal runaway we don't do any servicing. The battery is scrap. Normal maintenance for a NiCad battery includes discharging the battery while monitoring each individual cell then shorting each cell as it falls below 1 volt. After all the cells are shorted the battery is left to stand for 24 hours, then the links are removed and the battery is charged up again. This process is referred to as cell balancing and it plays an important part in preventing in-service cell reversals and overheating. These days most NiCad batteries have thermal sensors that can operate overheat warning lights or open the battery circuit breaker in the event of an overheat. The commonest cause of battery overheating is low water levels in the cells. With regular inspection and maintenance a NiCad battery can provide good service for years. A replacement for a typical aircraft 25AH NiCad will set you back more than US$3,500 so it pays to keep your eye on the water level and do regular cell balancing.. .. .**********************************. .Through difficulties to the cinema
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Water levels??? In a NiCad???. .. .New to me I thought you only had water (Acid) levels in flooded Lead acid cells. NiCads are a solid state battery usually and in my experience not used that much in engines.. .. . <img border="0" title="" alt="[Confused]" src="confused.gif" /> <img border="0" title="" alt="[Confused]" src="confused.gif" /> <img border="0" title="" alt="[Confused]" src="confused.gif" />
Yes, even Nicads have water (a water\dissolved salt electrolyte, actually) in them. Large Nicads (we had 'em on some merchant ships) have a Potassium Hydroxide solution as the electrolyte and they will electrolyse water (turn it into gaseous hydrogen and oxygen) if overcharged, leading to a need to top up with distilled water. . .. .So what's the difference between these and the good old lead-acid we all know so well from our cars etc?. .. .First, the electrolyte is a strong alkaline one, not acidic. It'll still burn you though, and the stuff is very corrosive so don't get it splashed onto anything/anyone you care about.. .. .Second, unlike lead-acid cells, the specific gravity of a Nicad's electrolyte changes little throughout the charge-discharge cycle so measuring it doesn't tell you where you are on the discharge curve.. .. .They are a very efficient energy storage medium on a cost-per-unit-weight basis, but require specialist care if they are to give maximum service life. Treat them with care! If in doubt about them, seek out a battery technician and ask his expert advice. They will be happy to be asked and tell you all you need to know.
Here is the theory that I tend to go along with (for what it is worth!) One of the properties of a semi conducting material is that the resistance of the material drops as it's temp increases (As opposed to any other material where the resistance increases with temperature).. .Therefore when you charge the battery too fast, it heats up and it's resistance drops and so a larger current flows. The heat produced in the battery is given by Power = Voltage * Current or more importantly Power = Resistance * (Current Squared) so although the drop in resistance would reduce the heating effect this is more than countered by the effect of the increased current (because it is dependent on the current squared).. .As the Semi-conductor heats up further, so the resistance drops - more current - more heat - less resistance - more current and so on ad-infinitum until it goes bang or you disconnect the battery. (In retrospect can you go on ad-infinitum until something happens?)
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I like the steam plumes from the battery vents that accompany a thermal runaway. You can almost hear the airplane cry to you "Call the fire department and order some servicable batteries". Some unimaginative types have gotten the batteries out of the plane under these conditions, but I'd hold out for some good protective gear first before I tackled the problem.. .. .As Blacksheep implied, the battery turns into a molten mess inside its metal case. Makes a good audio/visual training aid, after it cools down.
