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Small battery fire


Nik Harrison
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Today at the flying site I had a small fire with a 4.8V battery pack.  It was in my plastic box with various other bits and pieces - some of which were metallic.  Thankfully I spotted it before too much damage done.  The reason I'm mentioning it is I've often left this type of battery on a shelf in my hut without considering what it could come in contact with.  From now on I'm going to be a great deal more careful handling what I previous though of as "small" batteries.

 

Melted_ Battery.JPG

Melted01.jpg

Melted02.jpg

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4 hours ago, Simon Chaddock said:

Nick

Not suite sure it was a battery 'fire' so much as some burning caused by a shorted battery.

I don't believe NiMh chemistry will self combust as can a LiPo but you are quite correct care is needed with any high capacity, high discharge rate cell. 

 

Agreed, it does look like the exposed part of the plug shorted.

A nimh will cause cable to glow, and burn the insulation

 

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Yes indeed, Pat, it certainly looked a bit odd straightaway; I thought at first glance it was a servo lead plug. Not sure of any reason for that third wire unless it had been added later for a specific non standard purpose surely? Also the wires still attached to the pack appear to be in unusually good health too, in my experience of examining these faults in the past the whole length of the wire tends to burn up. A mate nearly burnt his house down shorting a 4 cell rx nicad years ago so I did some experimenting at the time to find out exactly what did go on under these conditions.

    

All in all, something of a little mystery…

 

PB

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What Peter said. I had a not too dissimilar incident when charging a 4 cell rx pack many years ago. I was using one of those HiTec CG350 chargers, which had spring loaded positive and negative battery ports, into which you put bare wire ends and an appropriate socket. The charger beeped and terminated the charge and instead of disconnecting the rx pack plug from the socket, I inexplicably operated the two spring contacts and took the whole thing off the charger. Of course in half a second the bare wire ends touched the wire glowed bright, melted the insulation and the plastic coating on the pack welded itself to the palm of my hand. I was shaking my arm frantically trying to disloge it and managed to do so. The pack was completely wrecked.

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1 hour ago, Peter Beeney said:

Yes indeed, Pat, it certainly looked a bit odd straightaway; I thought at first glance it was a servo lead plug. Not sure of any reason for that third wire unless it had been added later for a specific non standard purpose surely? Also the wires still attached to the pack appear to be in unusually good health too, in my experience of examining these faults in the past the whole length of the wire tends to burn up. A mate nearly burnt his house down shorting a 4 cell rx nicad years ago so I did some experimenting at the time to find out exactly what did go on under these conditions.

    

All in all, something of a little mystery…

 

PB

I suspect that there's either a plug & socket joint between the battery & damaged wire (possibly a switch) or a soldered joint to extend the length.

If whatever caused the relatively short circuit was still high enough resistance it may have confined the highest temperature to the area of the plug. Even the damage to the wires seems mainly to be melted insulation, however it does appear as if the black wire has suffered significantly more than the red. Given that both wires will have been passing the same current it suggests that there was "black wire" corrosion present which would have increased the resistance in that wire particularly close to the plug. In fact that in itself that may have been what confined the heat to the affected area - limiting the current & acting like a fuse.

 

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The reason there is a white lead still on the plug is because I used an old servo lead to connect to the battery.  It is completely redundant.  The battery is not a flight battery, it is used to drive my servo tester.  I never said the battery was effected apart from the heavy current drawn.  The smoke was released from the wires because a piece of metal in the same box connected the power leads on the side of the plug.  This probably last seconds but the result was startling.  The point I was trying to make was that although the exposed metal parts on the side of the plug are small they can still be shorted.  There was no "black wire" involved in this post. Lol.

 

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Nik, I didn't make any assumptions that's why I asked the question in my first post & you've confirmed the first option of what I said in the first sentence of my previous post.

Unless you have removed some of the black wire's insulation before taking the photos I stand by what I said in answer to Peter's surprise that the damage to the wires wasn't more extensive. On the evidence of the last pair of close up photos posted the black wire insulation appears to have melted much more than the red. This suggests that the black wire had a higher resistance than the red, the most likely cause of this is black wire corrosion.

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Thanks Nik, that now makes good sense of it all.  Actually I think you may have got away with it quite lightly perhaps, maybe in some small measure due to the longer lead. In my colleague’s case he’d made two aileron servo extension leads for a glider wing and had somehow put a full short on both. He never would tell me what he’d done but he’d obviously managed to bolt neg and pos quite close to each other somewhere along the line. Without checking anything he put it together and plugged the battery straight into the receiver to try it; and being a nicad it was probably a bit pokey anyway, i. e. a nice low resistance itself as well. Instantly he’d got liquid insulation, well alight, dripping everywhere, mostly on his carpet and for a few moments he said the flames seemed to be springing up around his feet faster than he could stamp them out. Then the wire burnt through and he was just left with some serious explaining to do.

 

When I tried it I found it took a current of around 70 - 80 amps to get this sort of result in these battery sized wires. At 5 volts that’s a total impedance of around 70 down to 60 milli ohms, fairly low I’d have thought. I also read a letter in the BMFA News a long time ago written by someone with a similar story, although his was a short on the rx as I remember. But he’d been tinkering too and he also had the experience of the burning insulation spraying around….


PB

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PatMc, In the same vain as your "black wire" argument you could say that the proximity of the white lead acted as a heat sink and therefore reduced the damage on the red lead.  All this misses the point that the exposed metal of the sockets on the side of the plug is vulnerable to shorting.  I have repaired the lead and now put heat shrink round the plug as I do not require it to fit into a female receptacle.  I do not have a simple solution to the plugs that are required to fit into a receptacle.  Any suggestions?

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50 minutes ago, Nik Harrison said:

  All this misses the point that the exposed metal of the sockets on the side of the plug is vulnerable to shorting.  I have repaired the lead and now put heat shrink round the plug as I do not require it to fit into a female receptacle.  I do not have a simple solution to the plugs that are required to fit into a receptacle.  Any suggestions?

 

I get it Nik, and have shorted those exposed pins briefly before, while threading or fitting the pack into odd parts of models.

I bet you short continued, as the offending sliver of metal became welded across the terminals.

Although a faff to do it, have melted a little hot glue into my " desk" pack, and trimmed this flat, due to your advice

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Maybe one way of protecting the battery plug is to cut the socket off a servo extension lead by say an inch, fold the wire back on itself and then make a good job of heat shrinking it all down tight. Use that as a cover when the battery is not in use. Then with perhaps the added precaution of making sure the battery doesn’t ever get involved in a melee in a container particularly with any rough players, very especially metallic types, that should do it for you…  

 

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Nik, A pleasure Sir, but I have admit it was but just a simple thought, I’ve never considered it before. A ‘de luxe’ version I might do would be to stagger the cut a sniffer thus ensuring slightly wider wire end separation.

 

I like simple, I find it the easiest way to do anything.  Indeed, probably the only way of doing anything these days…

 

Good luck.

 

PB 

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  • 2 weeks later...

As per title " small battery fire " most fires start small . Lucky you were able able stop it from developing. It's amazing how things can , against very high odds come together and cuase a problem. Sods law, Murphy's law whatever you want to call it , it happens regularly. 

I had a large glow powered boat some years ago. After starting the engine I took off the glow stick and dropped it the  pit box. Went and had a few circuits of the lake then smelt burning. The glow stick had landed on a screw that had found its way into the box and shorted out . It was glowing bright orange and the box was smoking! Luckily it was away from the fuel container that was in a separate part of the box. The glow stick ,a large type with "D" cell nicad just kept on going, in fact I still have that glow stick and it still holds a charge.  Moral of tale ? Keep pit box tidy and batteries in a proper compartment.

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