Revisiting RC Lipos - Safety box to limit fire/smoke.

[neptronix]

I recently saw a bat-safe on hobbyking's website when revisiting the idea of using Lipos in 2021.

https://hobbyking.com/en_us/bat-safe-lipo-battery-charging-safe-box.html

Company video: [youtube]ZCLHDU_dOCA[/youtube]

It got me thinking about a post on ES that theorized that a sealed box with a carbon filter vent could be used to both limit the intake of oxygen ( necessary for a battery fire to get super intense ) and also filter the toxic smoke being emitted from the battery. This sounds like a super cool solution but i have never seen it tested.

I came across several inspiring battery box build videos i'd like to share.

[youtube]Rv_3vwSZmzA[/youtube]

[youtube]Ubc_Sub36E0[/youtube]

[youtube]CnNId0mDnBo[/youtube]

In the last video, we see that an ammo can with it's rubber seal removed performs the best. No flames are emitted, just lots of smoke. The downside is that the ammo box gets EXTREMELY hot. Therefore, you really need some kind of insulating material lining the ammo box. But even this subpar box performs well.

I suspect that a sealed ammo box with the right diameter/restriction carbon filter as well as an insulating lining would allow a 1kw-hr lipo battery to blow up without creating excess heat/smoke/flame. A box like this could save you from any serious property damage.. aside from your garage smelling like a meth lab.

The burning question.. ha ha.. is.. how could a box like this be made in a manner that is light so that it could be used as a permanent battery box on a bike. A small steel ammo can is 4lbs at the lightest, before any insulating material is added. The resulting box may weigh 7lbs.. and that's quite the weight penalty, considering 1kwhr of lipo weighs about 12lbs.

An aluminum box of equivalent size would likely come in at 2lbs.. so we're talking a 5lb box carrying 12lb of lipo.. which gives us slightly greater than lifepo4 energy density.

A better option to reduce the weight of these boxes is to split the battery into 4 individual units and use a thinner lining in each box.. each box would only need to handle 1/4th of the thermal intensity.. even if the boxes are the same weight.. you cut down on the damage a single pack can produce by 1/4th.

Anyone considered building such a box/boxes?

 

[Chalo]

I wonder whether finely divided, high surface area carbon might not be a lot more of a liability than a help in case of fire.

 

[neptronix]

I wonder whether finely divided, high surface area carbon might not be a lot more of a liability than a help in case of fire.

Very possible.
I was thinking of using a fairly narrow diameter but long ( 1 foot at least ) exhaust pipe where the carbon filter is at the end of it so that an internal fire would have to travel quite a distance to be able to touch the carbon. The right amount of air restriction should result in a smoke emission instead of a fire emission, and therefore the only thing that could ignite the carbon is heat.

A casual web browse shows that depending on the form of carbon, we are looking at between 200C and 500C before ignition.

A temperature measured on the outside of a flaming lipo canister is over 800c.. so it's obviously hotter inside.. but what is the temp of a lipo that's oxygen restricted and billowing smoke?

I think it's high time i order a FLIR and start running some experiments....

 

[Chalo]

My impression is that lipo fires are due in part to the presence of oxidizer inside the cells. If there's any surplus of oxidizer in the fumes, the combination of heat, oxidizer, and free radicals could touch off the carbon even without flames impinging on it directly.

I guess it comes down to whether the vented gas is fuel rich or oxidizer rich.

 

[neptronix]

It would be good to figure out what the constituents of Lico battery smoke are, exactly.

I know that carbon does an exceedingly good job of capturing volatile organic compounds... because i have a big air filter unit with 9lbs of carbon + zeolite in it. Works 100x better than a hepa filter on my local ozone/VOC emissions.
Zeolite can capture heat and is absorbent too.

But yes, that absorbed VOC can still catch fire, i am sure.

You gave me an idea.

Here's some vids of lithium batteries burning inside water:

[youtube]x7PmuKalY1k[/youtube]

[youtube]wUtOIo3BXn4[/youtube]

So here is a goofy idea.. how about a little box of water on top of the enclosure.. and what separates the water and the battery is a plastic divider that would melt and then submerge the battery..



This would help cool the thermal reaction too... but now the problem is that you have water vapor coming out of the exhaust tube... is the carbon really effective in that condition.. my guess is no, because it's too busy soaking up water

The water would help buffer heat transfer to the case and likely prevent the case from becoming too hot while also suppressing fire. This may be a more lightweight method than using liners to achieve this. And this wouldn't require much if any precision in the diameter of the vent.

 

[neptronix]

Lithium Battery Thermal Runaway Vent Gas Analysis - 2016

https://www.fire.tc.faa.gov/pdf/TC-15-59.pdf

Lithium-ion and lithium-metal cells in thermal runaway vent a variety of flammable gases. For
the lithium-ion cells, the quantity and composition of the gases vary with SOC. Higher SOC
produces a greater volume of gas in the cells tested along with a wider flammability range.
Hydrogen, hydrocarbons, carbon monoxide, and carbon dioxide were the most abundant gases
produced.
At a higher SOC, the battery electrolyte breaks down further into smaller, more lightweight
molecules.
The small-scale tests showed that cells at a higher SOC not only produced a greater volume of
vent gas, but that the vent gas was also more flammable.

Ah.. this is why overcharge tests show the most spectacular of explosions.

 

[Hillhater]

This is a recurring topic, so forgive me for reposting a previous answer.
First,... not all lipo is the same, and their reaction to over charging, damage, over discharge, etc etc can be very different.
Somewhere there is a YouTube of a series of tests on different makes of RC lipo packs being massively overcharged , damaged, etc.
The results varies from basicly NOTHING..no reaction ?....to seriously powerful EXPLOSION !......with a range of smoke only, steady slow burn, fierce flame like a blowtorch, etc.
Different brands, different results
So maybe you either need to KNOW your lipo (?) ...or prepare for the worst !
You need a LOT of water to be effective against the bad lipo fires.

And again i post my ultimate “safe storeage” and charge facility...
Lipo in a plastic bag ,suspended over a swim pool ( 90,000L) on a plastic string with a smoke alarm attached !
If anything gets too hot, the smoke alarm triggers,the bag melts, and the lipo takes a swim !
.....and i sleep easy .

 

[neptronix]

I've never seen a video of a RC lipo NOT exploding. But i'm sure there's some which are safer than others.

Anyway, the point of the thread is to create a practical case which you can have fitted to your bike permanently.
This would ensure that there's no 'i forgot to take the battery off and put it in my special case that night' situation.

IE something practical for everyday use that doesn't have a huge weight or size penalty.

 

[Hillhater]

Nep... you know that is impractical to have a “safe”, on bike, lipo battery container for those bad fires.!
I think it is widely agreed that the only effective way of suppressing a lipo melt down is with water ....a LOT of water. !
..That would imply carrying multiple times the weight of lipo as water.

 

[neptronix]

Well, as seen in the early part of the thread, the fire aspect can be mitigated by limiting oxygen.

Why would you need to carry multiple times the weight of lipo as water? look at how little water is turned into vapor in the other videos. Practically none.

Please read/watch and i would like to hear your comments afterwards about how a safe battery enclosure could be made.

 

[Hillhater]

Those tests above, Ammo cans, and even the big commercial “safe” storage boxes... were only testing with power tool batteries .18650’s ?...5.0Ah ?..100Wh or so,..... but the OP was thinking of 1 kWh lipo type pack...IE 10x the capacity,..10x the problem those tests showed. !
My 1 kW kettle boils 1 litre of water in less than a minute... so you can estimate what 1 kWh might do if shorted or goes thermal and discharges that 1 kWh in a minute or two ?
As was mentioned earlier, many of these lipo’s do not require oxygen to burn, they generate their own , especially those which practically explode.
Let us know if you find a practical fire/heat containment/suppression system that can be fitted to an ebike .

 

[neptronix]

That's why we use proper measuring tools and do tests to see how much the solution needs to be scaled up. So far i have not seen any 'youtube scientists' do this yet.

Air restriction clearly reduces the amount of heat produced by preventing flame. However, the containing box needs insulating material otherwise it will reach insane temps. This material is very heavy. But we are not sure how much insulating material is really even needed.
Water submersion in combination with air restriction probably cools the reaction even more, and likely eliminates the need for insulation and could possibly be lighter than insulation. We don't know.

If too much water is required then we can consider the idea of running the battery explosion a fail because it is not practical.

I want to know if it is practical. If so, i'm building a battery bong for my bike. :lol:

 

[Hillhater]

......
Air restriction clearly reduces the amount of heat produced by preventing flame. ....."

Flame or no flames, a 1kWh pack in thermal runaway is going to release best part of 1kWh of heat...because we know that the reaction doesnt stop until the energy is all spent....with or without oxygen .
If that happens all in say 2 mins , that would be like trying to insulate a 30 kW electrical heater within a “bike sized” container !
Serious redesign of the pack construction.. single cell fuses, thermal separation of cells or groups, individual venting for cells/groups,..etc etc may help, but if you are trying to contain the heat inside the pack,..it is going to be complicated and difficult to thermally isolate individual sections.
Maybe NASA like explosive retaining bolts to eject the pack from the bike if it overheats ?? :wink:

The physics...
1 kWh is 1000x60x60 = 3,600,000 joules.
At 4.2j per deg C, per gm... that 1 kWh could take 10+ litres of water from 20C to 100C
...or it could totally boil, and evaporate over 5 litres of cold water !
That is a lot of boiling water and steam...with some nasty contaminants in it !