potting compound for batteries

can anyone share what they are doing to pot a battery?
I use cell holders so I can fill the cavities easily enough
do you need to seal the positive end of the 18650 with silicone first?
I was thinking of just taping off the sides with electrical tape, putting it in a mold and pouring epoxy over the entire battery but I don't know if that's a dangerous stupid thing to do

I would not want to use anything permanent.

Obviously dielectric is most important.

Lightweight.

At high C-rates thermally conducting rather insulating.

Otherwise just a medium density expanding polyurethane foam would be fine I think.

some existing discussions

https://endless-sphere.com/forums/search.php?keywords=pot*+batter*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

there's otherse not in that list you can find with more poking around

I read the entire thread and my take away on it is that if you want to take something to market and ship it anywhere it has to be fool proof, cant be blowing up planes and killing people.
im just looking for waterproof, plasti dip or liquid tape might be useful. I know polyester resin isn't entirely waterproof but could be used I think, I don't believe its an insulator.
I want to see if I can build a mountable, waterproof battery for less than $50 by the new year, ill keep you posted

john61ct said:

I would not want to use anything permanent.

Obviously dielectric is most important.

Lightweight.

At high C-rates thermally conducting rather insulating.

Otherwise just a medium density expanding polyurethane foam would be fine I think.

Click to expand...

a medium expanding polyurethane foam, do you mean spray foam? I think that's a bad idea, its a closed cell foam, full of air pockets, ive seen quite a bit of it get water logged, it cracks then its a sponge and as it expands it creates psi and starts moving things.

I want to get rid of all the air so a liquid that flows but dries hard, waterproof would be nice but resistant will work. heat, I can use tin foil. so glue with tinfoil inserted in the crack between each of the 4p would be easiest/cheapest but might take forever to dry so most likely polyester resin for fiberglassing mixed coolish, tough to say, the tin foil will shed the heat have to watch it. im either just going to silicone all the positive ends of 18650 or liquid tape them and then plasti dip the sides.once im at that point I can do anything to it
im just heading out to see what I can find

You can find boron nitride fairly easily online, I think it's the thermal conductor they mostly use for commercial potting mix. Also recipes online probably. https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR3.TRC0.A0.H0.Xboron+nitride.TRS0&_nkw=boron+nitride&_sacat=0

here's a vid of some pro's https://www.youtube.com/watch?v=ymNWq1wGUgU

whatever you use, if it doesn't expand and contract with temperature at the same rate as the cells and interconnects and external coverings do, it will develop fractures over time, which will be ways for water to get inside. it might not be much but it could be enough to corrode things, and being potted you could not see this happening.

additionally a number of materials are hygroscopic, which means they attract and soak up water over time, even though they are not visibly porous. so you must check that the material you wish to use is not one of these. silicone, for instance, seems perfect because it is used to caulk and seal things, but it actually absorbs water.

if you didn't get what you wanted out of any of the other threads i linked in that list, i'd recommend looking at things like the grin phaserunner and ligo threads and the grin site / videos to see if they say what they used in theirs, as those have already been time tested in the field.

If I was going to pot a battery I would separate the cells using thin walled aluminum tubes (arrow shafts would work well). Plug the end with wax then drill them out after the potting material cured. You could probably 3D print in a channel for the tubes to keep them suspended away from the cells and with the smallest size wouldn't add much size to the pack.
This would allow very effective heat transfer.

Grantmac said:

If I was going to pot a battery I would separate the cells using thin walled aluminum tubes (arrow shafts would work well). Plug the end with wax then drill them out after the potting material cured. You could probably 3D print in a channel for the tubes to keep them suspended away from the cells and with the smallest size wouldn't add much size to the pack.
This would allow very effective heat transfer.

Click to expand...

I'm having a hard time visualizing all that, seems overly complex.

Better to just use potting compound designed to transfer heat, if that's such a concern, use aluminum for the case.

Anything's better at that compared to air in an unventilated case.

I had seen that youtube, I think they use vacuum for the epoxy and grin is pneumatic pressure? I will look at their site to see what they did.i don't have those tools, I noticed in some marine below water line epoxy repairs they add micro-balloons to the epoxy so it will flow.

heres the sticker shock, 850ml of polyester resin is $25, a gallon $80. the last time I bought resin I was paying $110 for 5 gallons so I don't know what epoxy will be. 500ml plasti dip $25 plus tax and env fee will be $30.
the local building supply is going out of business so everything was 40-60 percent off. 850ml resin (3m brand) was $16 so I bought 2, extra 11ml hardner $4.50 and 8 sqft of fiberglass matt so after taxes, environment fee and discount was $32. I can probably do 3 batteries with that. the worker told me where to go for cheap Plasti-dip, $15 in cloverdale, he brings it in from the states. hopefully he has liquid tape but that will be a tomorrow trip.
amberwolf, yes I agree the resin will probably crack and that will be game over im going to see what micro balloons are and maybe add to resin.
I believe in letting things breathe so its very tempting to just plasti dip the battery ends, wrap it in fiberglass and have a drain hole, I think of a 26650 cell, holds hot or cold too long. now im building a solid brick so that's why I was thinking tinfoil. I forgot to pick up some galvanised metal strapping, something like 2 pieces of 2 or 3 inch wide by 24 inch long.

come to think of it I might just do a solid plasti dip battery with 18 gauge copper or smaller gauge wire( I think cat5 cable is thin gauge solid copper wire) to act as cooling rods instead of the polyester resin and tin foil, waterproof, flexible and will shed the heat from inside the battery. then I just strap the battery so I have 2 mounting points at the end, wrap it in fiberglass and be done

john61ct said:

Grantmac said:

If I was going to pot a battery I would separate the cells using thin walled aluminum tubes (arrow shafts would work well). Plug the end with wax then drill them out after the potting material cured. You could probably 3D print in a channel for the tubes to keep them suspended away from the cells and with the smallest size wouldn't add much size to the pack.
This would allow very effective heat transfer.

Click to expand...

I'm having a hard time visualizing all that, seems overly complex.

Better to just use potting compound designed to transfer heat, if that's such a concern, use aluminum for the case.

Anything's better at that compared to air in an unventilated case.

Click to expand...

The tubes run parallel to the cells creating both a heat sink and air flow. Plus greatly reducing the amount of resin needed.

You want a thermally conductive, electrically insulating resin with a modest Shore hardness. Something with a hardness roughly between that of a pencil eraser and roller-blade wheel. This way it's less likely to suffer any cracks due to vibration. On larger battery packs it's less of an issue, but for e-bikes and e-motos they get thrashed fairly hard.

The thermal conductivity needs to be as high as practical, but for most compounds it's about 1-2 W/m.K and for reference, aluminium is about 200 W/m.K and copper is over 300 W/m.K so it's nothing flash. But remember, air is about 0.05 W/m.K and most plastics are about 0.4 W/m.K.

Conducting heat is one thing, but moving it out of the battery and into some kind of exchange medium is important - so make sure the outside of the battery pack is made of aluminium with a Kapton lining for electrical safety.

parallel to the cells, but the cell holders will be in the way no? I was thinking air movement is better, and upward air flow remember those coffee stir sticks from mcdonalds, thin plastic sort of shaped like an 8 or even plastic straws.

jonescg said:

You want a thermally conductive, electrically insulating resin with a modest Shore hardness. Something with a hardness roughly between that of a pencil eraser and roller-blade wheel. This way it's less likely to suffer any cracks due to vibration. On larger battery packs it's less of an issue, but for e-bikes and e-motos they get thrashed fairly hard.

The thermal conductivity needs to be as high as practical, but for most compounds it's about 1-2 W/m.K and for reference, aluminium is about 200 W/m.K and copper is over 300 W/m.K so it's nothing flash. But remember, air is about 0.05 W/m.K and most plastics are about 0.4 W/m.K.

Conducting heat is one thing, but moving it out of the battery and into some kind of exchange medium is important - so make sure the outside of the battery pack is made of aluminium with a Kapton lining for electrical safety.

Click to expand...

are you saying copper would expel the heat faster out of the pack than air tubes?

Heat transfer of forced air cooling is much greater than simple conduction through still air. But yes, a copper heatsink around the cells would work great. You wouldn't want one though as it'd be very heavy and dangerously electrically conductive.

It is one reason why a lot of cell cooling can be done via the electrical connections on cell terminals (preferably copper bus bars).

Only if the copper is leading somewhere cooler. If not, you aren't achieving anything. The goal is to take the heat from where you don't want it, and move it to somewhere else. Forced air through convection can remove a surprisingly large amount of heat, but it needs to go somewhere outside the battery pack. This is fine, but it makes it no longer waterproof.

So the next best thing would be a short conductive path made of thermally conductive resin, leading to an aluminium enclosure. The outside of the enclosure is exposed to ambient airflow. The delta-T from inside to out is what drives the heat flow, while the air absorbs the heat removed from the battery.

goatman said:

parallel to the cells, but the cell holders will be in the way no? I was thinking air movement is better, and upward air flow remember those coffee stir sticks from mcdonalds, thin plastic sort of shaped like an 8 or even plastic straws.

Click to expand...

In my idea they would replace cell holders or else print holders which also mount the tubes. Ideally the tubes would be in contact with the cells but that has the potential for turning into a short if the cell wrap were to rub through.
Somehow thermally bridging to an aluminum case has a lot to say for it.

jonescg said:

You want a thermally conductive

Click to expand...

For low C-rate applications where there is little internally generated heat gain, not really a concern, in fact if freezing ambients are involved thermally insulating is better.

Fire suppression is another good property, as would be non-toxic fumes for indoor use cases.

im starting to get an idea of what im going to do but I don't want an aluminum shell as a heatsink, might have to.

a piece of tin foil will stop and reflect an enormous amount of heat away. I wonder if wrapping the battery with high temperature aluminum tape would absorb the heat of the battery and reflect it out through a fiberglass or carbon fiber shell. as your riding air flows through the battery and the foil reflects heat away. im off to get some plasti dip and plastic straws

What I've done on a couple of batteries is just pour epoxy potting compound over the top of the cell holders without any sort of mold. This was over 4 years ago and the batteries have all done fine and it seems it will hold until the cells no longer have useful capacity/IR. The sides of the cells are still exposed but the terminals are all potted.

I got the idea from forum member okashira in Justin' thread "Potting Batteries, Thoughts and Experiments".

district9prawn, I saw that in justins thread, I think it would be good in a battery box. im trying to make a waterproof battery mounted to the frame fully exposed with just an xt60 connection encased or an xt60 pigtail.

it needs to hang upside down from 2 mounting points and I haven't seen a battery like that

got the plasti dip, 2-429ml cans, ace hardware $38 cad.

make it on a budget with parts you can get from the neighbourhood and it needs to be safe.

off to the dollar store to find plastic coffee stir sticks and possibly really long plastic straws, stir sticks for vertical and long straws for horizontal ventilation

john61ct said:

Grantmac said:

If I was going to pot a battery I would separate the cells using thin walled aluminum tubes (arrow shafts would work well). Plug the end with wax then drill them out after the potting material cured. You could probably 3D print in a channel for the tubes to keep them suspended away from the cells and with the smallest size wouldn't add much size to the pack.
This would allow very effective heat transfer.

Click to expand...

I'm having a hard time visualizing all that, seems overly complex.

Better to just use potting compound designed to transfer heat, if that's such a concern, use aluminum for the case.

Anything's better at that compared to air in an unventilated case.

Click to expand...

Here are some applicable documents with real info. This evidence agrees with your initial thought on thermal management. The issue is how to manufacture this to not cost a fortune. They are rolling mica tubes then billet aluminum block of a substrate if that word works here. Nice design but expensive. I did find some mica paper that may work, to prevent the aluminum shorting to the battery, but the aluminum would be quite expensive. I do have some other ideas that I'm not ready to share, but these NASA docs maybe you'll find interesting food for thought to explore mentally from your initial assertion.

https://ec.europa.eu/jrc/sites/jrcsh/files/eric-darcy-nasa-lessons-learned-passive-thermal-runaway-propagation-resistant-designs-spacecraft-batteries.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170001655.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160014008.pdf

Simply wrapping Kapton down the arrow shafts before cutting should work. The potting will prevent any vibration that could wear through.

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plastic straws are 3/16 inch I.D., they slide right through pack running horizontal and vertical and the straw is shaped about 1/16inch x 1/4 inch inbetween cells. the straws will go through every layer. a plasti dip brick, 2 metal straps and a fiberglass shell.

I have to go grab the guinea pig battery tomorrow and start building this.

I guess you missed that air is a very poor medium for thermal transfer.

Unless you intend to get a fan sucking through the straws?

Just find the right potting compound. . .