Wax cell fixture

[slomobile]

TLDR Can I apply some kind of wax to my cells to get then to stick together, to reduce abrasion, fill space so they cannot swell, and absorb/release heat with a phase change like moon rover batteries did? What kind of wax would be best? How to apply it? I have no experience with wax.

My first thought was canning paraffin because it is in the grocery store and pure. But I thought it might put too much heat into the cells if applied molten, and might not be sticky enough, and wont remelt at low enough temp to absorb enough heat when charged or discharged aggressively..
Maybe body wax, the kind for hair removal would be better, low temp, sticky, workable, cheap melting pots are available. But I'm not sure if the composition is maybe corrosive or conductive. Maybe ski or surfboard wax? Car wax? Soy? Carnuba?

For technique I'm guessing slather it on in a goopy, barely melted state, wait for it to harden, then scrape it level with the edges of each individual cell. At assembly time, apply a heat gun briefly to both sides to soften, press together for a few seconds till bonded. Repeat. Then wrap the pack in tape so it cant expand too much. Install in battery box. Any better ideas? Caveats?

My new prismatic LFP cells have fairly deep depressions on their largest sides and a reputation for swelling at the drop of a hat. They came with really nice modular top and bottom plastic spacers which are nice for cooling, but not preventing swelling. Also, I have a very close fitting battery box without room for the spacers anyway. The fit is so tight in fact that I need a sheet spacer on the bottom to raise the pack lower edges above the battery box bend radius and cannot have more than a single layer of 8" wide kapton tape on the pack sides without any overlap. It is a 200AH 2P 8S LFP pack made from 16 100AH cells each rated to charge at up to 50A and discharge at up to 100A. I'll probably actually charge around 40A with a 240V supply charger and discharge real close to 200A for up to 8 seconds hovering around 60-80A most of the time.

One nagging concern with this method is the lightweighting holes in the side of the battery box. If this works as intended, the wax will eventually melt and push on the kapton tape, expanding into the lightweighting holes, locking the pack in the vehicle, perhaps permanently. Thats good for vibration prevention, but bad if I ever need to remove the pack for service. If it lasts the spec'd cycles, I'll never need to. But when has anything ever gone to plan.

Discuss. It would require much more wax to pot cylindrical cells like this but should work. Pouch cells could be dipped in the pot like a gang of candles to build up a shell on each one, eventually bonding them all in a single mass. Or just fill up a pelican case one side at a time with molten wax and submerge your cells for a hardened and waterproof pack.

 

[lnanek]

There are some long threads already on phase change material like that, although not specifically wax. Might be worth searching them up.

 

[slomobile]

Something like this Phase change material panel concept
I typed "phase change" in the search thread box directly above right and it only returned the thread we are in. Guess I already used up my one good brain cell today.

 

[BlueSwordM]

TLDR Can I apply some kind of wax to my cells to get then to stick together, to reduce abrasion, fill space so they cannot swell, and absorb/release heat with a phase change like moon rover batteries did? What kind of wax would be best? How to apply it? I have no experience with wax.

My first thought was canning paraffin because it is in the grocery store and pure. But I thought it might put too much heat into the cells if applied molten, and might not be sticky enough, and wont remelt at low enough temp to absorb enough heat when charged or discharged aggressively..
Maybe body wax, the kind for hair removal would be better, low temp, sticky, workable, cheap melting pots are available. But I'm not sure if the composition is maybe corrosive or conductive. Maybe ski or surfboard wax? Car wax? Soy? Carnuba?

For technique I'm guessing slather it on in a goopy, barely melted state, wait for it to harden, then scrape it level with the edges of each individual cell. At assembly time, apply a heat gun briefly to both sides to soften, press together for a few seconds till bonded. Repeat. Then wrap the pack in tape so it cant expand too much. Install in battery box. Any better ideas? Caveats?

My new prismatic LFP cells have fairly deep depressions on their largest sides and a reputation for swelling at the drop of a hat. They came with really nice modular top and bottom plastic spacers which are nice for cooling, but not preventing swelling. Also, I have a very close fitting battery box without room for the spacers anyway. The fit is so tight in fact that I need a sheet spacer on the bottom to raise the pack lower edges above the battery box bend radius and cannot have more than a single layer of 8" wide kapton tape on the pack sides without any overlap. It is a 200AH 2P 8S LFP pack made from 16 100AH cells each rated to charge at up to 50A and discharge at up to 100A. I'll probably actually charge around 40A with a 240V supply charger and discharge real close to 200A for up to 8 seconds hovering around 60-80A most of the time.

One nagging concern with this method is the lightweighting holes in the side of the battery box. If this works as intended, the wax will eventually melt and push on the kapton tape, expanding into the lightweighting holes, locking the pack in the vehicle, perhaps permanently. Thats good for vibration prevention, but bad if I ever need to remove the pack for service. If it lasts the spec'd cycles, I'll never need to. But when has anything ever gone to plan.

Discuss. It would require much more wax to pot cylindrical cells like this but should work. Pouch cells could be dipped in the pot like a gang of candles to build up a shell on each one, eventually bonding them all in a single mass. Or just fill up a pelican case one side at a time with molten wax and submerge your cells for a hardened and waterproof pack.

Yes!

I've been working a lot of this with a few friends of mine and a chemist, but with cylindrical cells.

I recommend starting with a fully refined paraffin wax mix (like canning wax) and 1-10% zinc oxide, a surfactant and some stearic acid.

Lives for about 5 cycles until the zinc oxide settles out, but since the wax can absorb enormous amounts of heat in phase change and melts at around 60-63C, that's plenty of headroom for prismatic cells that can't really get all that hot anyway.

My goal for my final wax blend is 70C, since my max working cell temp is ideally 60C, with headroom for thermal emergencies and 0 cycling. It also has the advantage of being dirt cheap thermal potting.

The problem is that unlike cylindrical cells, you don't have an easy area to shove wax on the bottom of the cells, since it's less conductive.

 

[amberwolf]

fill space so they cannot swell

No, on that part. Anything that is deformable will just deform out of the way of the swelling cells.

To prevent swelling for prismatic or pouches, you need a pack design that has compression plates on the ends of the cell stack. If there is more than one cell stack, you need a compression plate fixture on the ends of each stack. To perform the compression of the cells it must be very rigid, and each of the celll stack's end plates affixed to each other in tension. YOu should check the datasheet for the specific cell being used to find out the minimum pressure-per-area required for the compression.

If you look around at my posts on this topic there are a number of them with images of various packs designed with this function to give you ideas of how to build the fixture.



If they're cylindrical and swelling, they're either defective or failing or being used beyond their capabilities, because the can should be doing the cell compression that's required.

 

[amberwolf]

Something like this Phase change material panel concept
I typed "phase change" in the search thread box directly above right and it only returned the thread we are in. Guess I already used up my one good brain cell today.

Google search results

endless-sphere.com

There's others but not found in just a simple thread title search.

 

[slomobile]

Thank you, that helps a lot. https://www.youtube.com/@Nighthawkinlight helped me understand what zinc oxide and stearic acid do as well as some possible application techniques.

Regarding fixation vs compression, because the prismatic cells are already concave, compression would just tend to deform the outer edges of the cell aluminum case, where it is already strong and not likely to expand because there is no electrolyte there. The center of the cell, where it is most likely to swell, is also where it has the most space to expand into, even when compressed by flat endplates, because of the initial concavity. The idea is to fill the space between the cells with a moldable incompressible solid material to eliminate any volume which the cell might expand into.

The cells naturally swell a little when in a charged state. By building the pack in a discharged state, wrapping it in something resistant to stretch (fiber reinforced tape, kapton, straps, end plates w/compression rods), under just a small amount of tension, when the pack charges, the natural swelling provides the pressure increase to meet the pressure specs. What is the benefit of compression? Additional cycle life. I won't live to see the end the pack's cycle life even if it is lowered by 1000 cycles. Even if we cant prevent swelling, we can limit it in the highest volume area at the center of cell face. I think I'll add thinner strips of kapton to the top junction between cells, to limit egress of melted wax out the top.

If this is done right, the wax should never become fully liquid. The latent heat of melting is absorbed while the wax is solid without raising the wax temperature. It only melts (with concurrent temperature increase) once the latent heat threshold for the specific material is reached. However, not all parts heat equally and some areas will soften before others. The tape will make a liquid tight seal bottom and sides, but wax could still force out the top. Hopefully the aligned aluminum edges of the cell housings will prevent that. If not, wax could potentially cover cell vents. Which could delay or maybe prevent proper venting. I don't think this is likely, but must concede the possibility.

I think this is ok to experiment with after I test how wax and the property modifying ingredients react with the polyolefin shrink insulation around the cells. Being similar chemically sometimes means things don't react, sometimes means they react a lot. But I am certainly no chemist. I'd hate for the wax to soften the insulation.

 

[BatteryMooch]

If this is done right, the wax should never become fully liquid. The latent heat of melting is absorbed while the wax is solid without raising the wax temperature. It only melts (with concurrent temperature increase) once the latent heat threshold for the specific material is reached. However, not all parts heat equally and some areas will soften before others.

It was my understanding that the temp of the solid wax will immediately start rising (if cell temp rises) and only when the wax reaches its melting point will its temp hold steady. Once the wax has fully melted (assuming good heat transfer to all the wax) the wax temp would start rising again.

 

[slomobile]

Correct BatteryMooch. I think we were saying the same thing but I started at the moment "when the wax reaches its melting point ... (its temp will) hold steady." You started before that point, while the battery and wax heats up like any other solid with thermal mass.
I think we agree. Melting doesn't happen all at once, and while it is happening, the already melted parts trying to heat up the mixture are balanced by the heat absorbing bits trying to hold it steady. If rate of heat input remains greater than dissipation rate for long, eventually all the wax will melt, temps rise and bad things. That situation should be avoided/designed out by sufficiently raising the melt point. If the rate of incoming heat ON AVERAGE is less than dissipation rate, it stays mostly solid. Brief surges might create hot spots and melt a little, then resolidify. This is just a damage buffer for when I do dumb things. Not extra capacity.

 

[amberwolf]

Regarding fixation vs compression, because the prismatic cells are already concave, compression would just tend to deform the outer edges of the cell aluminum case, where it is already strong and not likely to expand because there is no electrolyte there. The center of the cell, where it is most likely to swell, is also where it has the most space to expand into, even when compressed by flat endplates, because of the initial concavity. The idea is to fill the space between the cells with a moldable incompressible solid material to eliminate any volume which the cell might expand into.

That will require something that cant' deform, melt, etc. So if you want something moldable, it would have to be something that you mold into place that then hardens to remain that hard even at the highest temperature that could ever be reached in the environments it will be used in.

Waxes, etc., won't work for this. They will flow out of the way and cannot prevent compression.

If you have something that is a literally incompressible liquid, material, etc., under whatever pressure was needed, and filled the space in a way that it could not move out of the way, it might work, but you would have to have each of the individual concavities sealed with a form-fitting fixture to contain the material regardless of pressure applied to the fixture (the fixture itself must not deform or it allows the material inside it to deform out of the way and that then allows the cell to expand).



If the cells are concave enough to be visibly so, that makes it difficult to create a simple compression / constraint system, and requires more complexity than should be required for such a thin piece.


If the expansion of the cells and the resulting potential damage to cell layers (outgassing causing deformation / delamination causing loss of current-passing capabilty and capacity) doesn't matter, then don't worry about it. But if that does matter, and you prefer not to risk the damage, I'd recommend coming up with something that does prevent this expansion.