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Lipo Bags Rule OK!


ChrisB
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I had an email from a club mate today. He woke up in the middle of the night and thought he could smell a bbq as it was a hot night and the windows were open, he looked around the house and found nothing so went back to sleep.

The next day he went into his modelling room and found his Lipo bag shredded and several lipos melted. All lipos had deans connectors and hadn't been touched for several weeks due to him being unwell. The Lipo bag saved the day....just!!

I've been to the local army surplus store today and invested in some ammo boxes and will also be making sure my lipo bags are up to scratch!!

lipo fire 1.jpg

lipo fire 2.jpg

lipo fire 3.jpg

Edited By ChrisB on 02/07/2017 21:07:03

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Posted by John Bisset on 02/07/2017 22:33:05:

Ouch. What does your friend think started it?

I use a stainless steel box about 30cm eachs ide (old surplus item), with the Lipo bag inside. I wondered if that was OTT - I think I've decided it is not.

John

No, he doesn't know. None of them were puffed and he'd left them as they were, without any checking of capacity/charge.

I think a lipo bag inside a box is a good idea! Ideally each lipo should be individual such that is can't spread to its neighbour but that's not realistic when you have more than ten or a dozen.

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Posted by ChrisB on 02/07/2017 21:04:01:

and hadn't been touched for several weeks due to him being unwell.

That's the aspect that concerns me most. If we believe it to be literally true, then why did the fire occur then (and not earlier or later)?

What changed?

The only possible provocation must be temperature change, surely?

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Its difficult to know. They are the only lipos he had. Never abused through charging or impact etc. Only used in his glider a few times a year. It looks as though the fire started on the right hand side of the photo above, that looks the most charred.

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Good post. I keep mine in my workshop shed away from the house. I have two separate steel boxes and use LIPO bags inside. Separating each LIPO into separate bags and boxes would be impractical as I have quite a few. I think one possible reason for a fire is if a soldered connection on a deans plug breaks. The deans plugs don't have proper sleeves for the two wires and rely on shrink wrap. If a solder joint fails and there is any uncovered portion on the plug there is a chance of shorting. I have seen some club members wrap elastic bands round the battery and leads when putting them away to keep the leads from moving . Maybe that helps?

Edited By Timothy Harris 1 on 03/07/2017 10:32:09

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The fact that model shops are not burning down left right and centre underlines that LiPo's don't just spontaneously self-destruct for no reason.

Model shops stock them, lots of them. But they don't handle them, so failure modes like broken wires or shorted connectors would not be likely in that case...

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Yep it's seems to be handling that's the danger. A solder connector popped off one of my charging leads yesterday, that gets a lot of handling. It was a "bought " multilead so not my handiwork 😊, so I guess the same can happen on a battery's lead, but luckily not often.

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Posted by ChrisB on 03/07/2017 09:01:52:
Posted by John Bisset on 02/07/2017 22:33:05:

Ouch. What does your friend think started it?

I use a stainless steel box about 30cm eachs ide (old surplus item), with the Lipo bag inside. I wondered if that was OTT - I think I've decided it is not.

John

No, he doesn't know. None of them were puffed and he'd left them as they were, without any checking of capacity/charge.

I think a lipo bag inside a box is a good idea! Ideally each lipo should be individual such that is can't spread to its neighbour but that's not realistic when you have more than ten or a dozen.

Posted by The Wright Stuff on 03/07/2017 09:13:21:

That's the aspect that concerns me most. If we believe it to be literally true, then why did the fire occur then (and not earlier or later)?

What changed?

The only possible provocation must be temperature change, surely?

No, not necessarily. Whilst extremely rare lipos can internally short due to dendrite growth - if the dendrites get long enough the anode and cathode meet. This normally requires long term storage of the pack at elevated (>4V/cell) voltages throughout the life of the pack, though. Another good reason for storing them at storage (~3.7-3.8V/cell) voltage!

Edited By MattyB on 03/07/2017 18:26:42

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If storing in ammo boxes then I have seen some info and videos that this can be dangerous if a lipo burns because it builds up pressure. I believe the recommended thing to do is either drill some vent holes or remove some of the rubber gasket. I have removed some of the rubber gasket from mine as I both charge and store in ammo boxes. Remember, lipo bags do not seal, they will let the hot gasses vent, but hopefully keep flame inside.

Another caution about ammo boxes, make sure you do not shut the lid on the balance lead, that is one for the embarrassing moments award. Try and solve one problem to create another.

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I can see I need to get an/some ammo boxes. There hasn't been an ex army surplus type store near me for 40 years or more. I've seen some on t'internet but would ask if anyone can point me at a particularly good buy - both for practicality and price.

Thanks. Ian

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I highly recommend looking at the bat-safe boxes. They are well constructed and make life easy for storing and charging lipos. Check out their web site. I know they're a little more expensive but for me that is money well spent. It's bought me real peace of mind that any alternative just couldn't achieve. I'm away often and the thought of a house fire from my batteries frankly scared the hell out of me. Not any more.

TP

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IanR try Denbigh Army Supplies www.denbigharmysupplies.co.uk/ they have a good selection of different sizes of ammo boxes and the prices are OK. Their service is excellent. My boxes from them were in almost new condition. Other club members have also used Denbigh with equally good results.

As has previously been said it is best to remove some of the rubber seal from the lid., do not take the seal from the side where the catch is as the boxes may not close/lock properly. I removed the seal from the long sides and rear hinge which allows sufficient venting.

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Posted by MattyB on 03/07/2017 18:12:05:

The only possible provocation must be temperature change, surely?

No, not necessarily. Whilst extremely rare lipos can internally short due to dendrite growth - if the dendrites get long enough the anode and cathode meet. This normally requires long term storage of the pack at elevated (>4V/cell) voltages throughout the life of the pack, though. Another good reason for storing them at storage (~3.7-3.8V/cell) voltage!

Thanks Matty - we can rely on you to provide a detailed and balanced explanation!

Out of interest, how does the risk profile look when storing cells below storage charge (e.g. 20%-30%)? I realise this is bad for the cells for other reasons, but does the risk of dendrite growth continue to go down, or does it start to climb again?

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Posted by The Wright Stuff on 04/07/2017 08:00:38:
Posted by MattyB on 03/07/2017 18:12:05:

The only possible provocation must be temperature change, surely?

No, not necessarily. Whilst extremely rare lipos can internally short due to dendrite growth - if the dendrites get long enough the anode and cathode meet. This normally requires long term storage of the pack at elevated (>4V/cell) voltages throughout the life of the pack, though. Another good reason for storing them at storage (~3.7-3.8V/cell) voltage!

Thanks Matty - we can rely on you to provide a detailed and balanced explanation!

Out of interest, how does the risk profile look when storing cells below storage charge (e.g. 20%-30%)? I realise this is bad for the cells for other reasons, but does the risk of dendrite growth continue to go down, or does it start to climb again?

It is only bad if the cells self discharge down to a voltage where they would be damaged (generally considered to be 3-3.2V/cell). Self discharge is normally very slow with lipos though, so providing you are using your batteries reasonably regularly (say once a month) it should not be an issue to store them at ~3.6V/cell.

However, whislt there is plenty of evidence that storing at an elevated SOC increases the rate at which capacity reduces and IR increases, I've not seen anything that scientifically describes the benefits of storing below 3.7V/cell. I think there are probably other factors that would dominate over a tiny reduction in storage voltage (i.e. variable QC in the manufacturing process and usage patterns for two), so my recommendation would be to store at 3.7-3.8V/cell unless flying again within 48 hrs and feel safe in the knowledge your packs have the best chance of a long and happy life!

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