PaulD's Experiments with Endothermic Potting Compounds

[jonescg]

We used to make gunpowder (KNO3, S, C) and rocket toffee (KNO3, Sucrose) using coffee grinders and ball mills. The ball mill ended up being best for the gunpowder, and the grinder for the rocket fuel.

As for the long cooldown with PCMs, yeah I reckon the solution is to ensure there's still a heat-shedding surface available. The thermal conductivity of solid paraffin is pretty bad, but still better than air. To be honest the main reason for considering it is so the wax behaves like a potting compound, while weighing significantly less than a potting compound. And it contributes to some corrosion resistance too.

 

[liveforphysics]

Fantastic testing Paul! I've never tried washing soda, but it looks very promising from your data! How exciting!

 

[CONSIDERABLE SHOUTING]

I had a client that was building batteries at the same factory in Taiwan as Gogoro about 6 years ago. I saw the machine that injected PCM it into their packs at the end of the assembly line. I can't remember a whole lot, I never knew what material it was, but the rumor was that it was wax that was actually flammable. Hope they changed that.
Allcell is the other company that comes to mind when I think of PCM. Maybe check out their patents. I think it was some kind of graphite matrix with wax in it, similar to what Amberwolf mentioned.
The downside to PCM is that long cool-down time. I was surprised that a battery share service like Gogoro would use it for that reason, but maybe it just never got so hot as to need cool down time before recharging for the next user.

If it helps, I know EGO cells use PCM on each cell along with a fan for forced air cooling, which is partially how they have such a robust lifespan. I tried doing research into what the material they were using was and if it came from a contracted company or not, but the best I got was a general description of it and my trail went cold.

I also remember in a different thread, that LFP above said potting compounds were used more for corrosion resistance before being used as a heat sink of some kind, which would make perfect sense for a company like Gogoro who's business model is trying to get as many people using their swappable batteries as possible in scooters. Still, any material that saps heat from the anodes/cathodes to distribute it more equally throughout the pack is gonna be better than one without.

 

[amberwolf]

We used to make gunpowder (KNO3, S, C) and rocket toffee (KNO3, Sucrose) using coffee grinders and ball mills. The ball mill ended up being best for the gunpowder, and the grinder for the rocket fuel.

Is that because the surface area to volume (not sure I have the terms right) of the finer powder (presumably) made by the ball mill let it burn faster, vs larger grains with less surface area to volume burning slower?

 

[jonescg]

Is that because the surface area to volume (not sure I have the terms right) of the finer powder (presumably) made by the ball mill let it burn faster, vs larger grains with less surface area to volume burning slower?

Without derailing the thread toooo much
The gunpowder needed to be ultra fine for it to burn as quickly as possible (aka explode) whereas the toffee rocket fuel was OK being a bit bigger and slower burning, but you melted it all together anyway so there wasn't a lot of point in making it super fine.
When the gunpowder was in the coffee grinder (high speed blades and a brushed DC motor sparking in the cavity below :O ) it mostly just made a mess, but was probably on the riskier side of things.

 

[PaulD]

Fantastic testing Paul! I've never tried washing soda, but it looks very promising from your data! How exciting!

Well, I re-did the tests, and things look a little different now. So, the changes in my 3rd test are:

• Blended washing soda and epsom salt in the Vitamix. This decreased the settling considerably. I had much more homogeneous castings.
• Increased heater power to 22W from 20W. This was an accident on the first one, so I continued on the remaining samples.
• Secured the heater cartridge with a screw to prevent it from being ejected.

I managed to re-use my 3d printed fixtures (silicone doesn't stick to anything very well, except more silicone) , and drilled and tapped holes for an M3 screw:

And here are the results (no control this round, to save some time):

Notes:

• So, washing soda is not looking nearly as good as last time. In the V2 test in the previous post, the heater cartridge got ejected and messed up the results.
• Epsom salt looks almost exactly the same as baking soda. I think the "noise" in the results are a combination of the endothermic reaction and the MgS04 dissolving in the water creating a conductive solution. This messes with the uninsulated thermocouples
• Baking soda is looking solid. It's the easiest to buy, mix and doesn't produce anything toxic. So, I think it's a winner.

Next steps:

• Begin construction of a pack thermal runaway chamber. I have a few ideas that I'll post in the coming weeks.
• Obtain some old packs and prep them with thermocouples and a nail penetration rig.

 

[CONSIDERABLE SHOUTING]

Next steps:

• Begin construction of a pack thermal runaway chamber. I have a few ideas that I'll post in the coming weeks.
• Obtain some old packs and prep them with thermocouples and a nail penetration rig.

I'll try to find some way to have patience

 

[Jrbe]

Thanks for sharing your research PaulD!

... For instance, if the silicone over time absorbs enough moisture from the environment for it to migrate thru the silicone and up against the metals the silicone is bonded to, it could allow corrosion of the metals.

To add to amberwolf's post, if the additives are not well encapsulated it could make a salt bath for the battery to quickly corrode.
Might be worth considering some thermal cycling (with humidity) with a clean and dry piece of steel embedded in your mix to keep an eye on.

 

[CONSIDERABLE SHOUTING]

Next steps:

• Begin construction of a pack thermal runaway chamber. I have a few ideas that I'll post in the coming weeks.
• Obtain some old packs and prep them with thermocouples and a nail penetration rig.

Any updates Paul?

 

[Jrbe]

Magnesium hydroxide powder (like milk of magnesia) might also work well. Should be cheap and readily available.

 

[PaulD]

Magnesium hydroxide powder (like milk of magnesia) might also work well. Should be cheap and readily available.

Magnesium Hydroxide undergoes thermal decomposition at 330C, which is too high a temp for preventing thermal runaway. Higher temp materials like that may be useful at some point to slow down TR, but at that temp stopping TR is too late.

No update yet, I am hoping to at least have some test packs built in the next few weeks.

 

[PaulD]

Progress has been slow, but I do have my TR chamber ready to go. I want to try 2 different cell triggers. Both are heaters. One is a cartridge heater tied against the side of the can. The other is nichrome wire wrapped around the can.

I'll do some cell only tests soon. Probably once air quality in due to fires gets bad enough that no one will notice a little extra smoke.
The TR chamber is from a wood fired hot tub which my wife wanted for her birthday a few years ago. It's basically just a stainless steel box with chimney and some pipes going through it that helped transfer heat to the water. I found that feeding it with wood for 8 hours to enjoy a soak was not worth the time, so I converted to electric. Now I have a perfect TR chamber! I added mounting points inside for a light and camera. I still haven't finished a pass thru for wires. I do plan on building a scrubber into the chimney to reduce the smoke. This will definitely be a phase II project.

Next up, I'll build up some test packs with salvaged cells to get a feel for how this will work.

 

[hallkbrdz]

Interested in your research using solids as a simpler heat transfer method.

I'm currently working on liquid immersion cooling, which is more complex due to container requirements (leak proof and vented) and various material incompatibilities, such as ABS, ASA, or any other styrene compound.

 

[PaulD]

Well, both cell triggers work. The heater cartridge seems to work much more quickly at 100W than the Nichrome wire wrap. Not sure why. This is the heater cartridge:

Both tests resulted in side-ruptures. The cells are Sanyo GA at 4.2V. Starting temp about 35C (it was a hot day).

 

[PaulD]

Some post mortem pictures of the heater cartridge cell:

Nichrome wire wrap


The goal here is to choose the trigger that best simulates a real cell thermal event while least affecting the test cell.
Some observations:

• The rupture on the heater cartridge test was right underneath the heater. I was slow to turn off the heater after the explosion, but I don't think it would have made any difference.
• The nichrome wire seems to have basically held the cell together in the middle, as there are ruptures on either side of it.
• The nichrome wire test too about 5 times as long as the cartridge. This is 5X the heat input

Because of the above, the heater cartridge is the better choice. Certainly not perfect, but it seems like it has a lesser impact on the end result. The real professionals use super expensive special cells with triggers built into them. Not an option here for many reasons.

 

[CONSIDERABLE SHOUTING]

• The nichrome wire seems to have basically held the cell together in the middle, as there are ruptures on either side of it.

It's probably an additive affect; I can't remember the name of it, but there's a specific term for like, when roadies would tape down connections and wiring and find that two parallel lengths of tape with a slight overlap hold better than two on top of each other, or are parallel and don't support each other like that.
There's also another fact- that it swelled throughout the cell but was limited there by the coils, but then that stressed the regions above it and that's where it burst. My bet is that has something to do with it as well.

 

[Hardergamer]

For a more accurate test, what about using some 18mm copper water pipe and put the heater cartridge inside of it, it would also make it easier to read the temperature with a diode on the outside of the cell/pipe.