sillicone layer over cell tops

[flippy]

as the topic implies,

i am trying to find a nice solution for using a large 18650 battery in a quite moist/humid (mobile) application.

my idea is to build the pack as usual and after welding it make a 1cm high edge from tape around the battery that i then pour a nice layer of sillicone over it to perfectly shield the positive terminals of each cell from any moistiure.

i know covering the PTC might pose an issue but my reasoning is that if the PTC's of cells trigger you got bigger problems to worry about as the battery is probably on fire at that point.

 

[john61ct]

Anything but silicone.

Research thermally conductive dielectric potting compounds.

 

[999zip999]

As I remember Justin at ebikes.ca did some threads on waterproofing a battery.

 

[fechter]

I don't think it will be a problem. Fully potted packs seem to be better during a "thermal event". I've used silicone before and had no issues other than regular silicone is too thick to fully cover the parts. They make runny stuff but it's expensive. Epoxy or two part polyurethane are other options.

 

[flippy]

Anything but silicone.

Research thermally conductive dielectric potting compounds.

why? please support your statement.
to be clear: it is only to protect the ends from mostiure and other crap, the middle section of the cells are spaced 1mm apart with a standard cell holder.
thermal is not an issue as the pack is properly sized and specced for its application. so heating is a non-issue. that is a benefit when you are not running packs on the bloody edge like most people here.

 

[john61ct]

Silicone is really nasty stuff, there are very few use cases where it's OK.

If the potting compound isn't designed to be thermally conductive, then it is likely to be thermally insulating.

Of course your rig your choice. . .

 

[john61ct]

Note rather than full potting, another option is a thin conformal coating, plenty of acrylic, polyurethane and epoxy solutions there as well.

 

[BotoXbz]

I love the thermal argument that keeps on being brought up here.
Air is a much better insulator than silicone.
The foam stuff that a lot of people wrap their batteries in is a much better insulator than silicone.
And you really shouldn't be running your pack at such crazy levels that heat is an issue. If it is then you need a special pack design anyways, though your cells will quickly degrade either way, the inside of the cell is a lot of plastic foil - also an insulator.
I've siliconed a 1mm silicone sheet to my battery ends and the cells themselves are inside wood. No issues with that.

 

[flippy]

Note rather than full potting, another option is a thin conformal coating, plenty of acrylic, polyurethane and epoxy solutions there as well.

I dont like non flexible solutions as it will develop cracks when stuff shrinks or expands or flexes. Please note the surface area is fairly large at 30x40cm.

Silicone is really nasty stuff, there are very few use cases where it's OK.

If the potting compound isn't designed to be thermally conductive, then it is likely to be thermally insulating.

Of course your rig your choice. . .

as stated before: this pack is properly sized for the load and will not heat up in any meaningful way apart from normal temperature cycles from being outside.

So if there is no thermal reasons then what reason is there to not use sillicone that i am missing? Stuff like expoxy us too stiff and will crack over time.

 

[jonescg]

Yeah, I'd say build a close-fitting container or even a bag and pot the whole thing. Lots of thermally conductive electrically isolating compounds out there.

 

[wturber]

Depending on space and mounting needs, consider using a pelican-type waterproof case. That's what I'm using even though I don't have much of a need for serious waterproofing here in the desert.

 

[john61ct]

I dont like non flexible solutions as it will develop cracks when stuff shrinks or expands or flexes.
..
Stuff like expoxy us too stiff and will crack over time.

The datasheets on the compounds specifically designed to perform well in either use case show flexibility after curing.

Protection against shock and vibration is one of the main reasons product developers use them!

Look for the **very** common attributes that show up in the first 10 seconds of googling the topic "High Elongation. Non-Cracking. No Shrinkage."

I'm not talking about just grabbing something off the shelf at a hardware store.

_____
My ban on silicone-based compounds has nothing to do with the thermal issue.

Silicone is a **contaminant** of any surface it touches, nothing is likely to ever adhere to it again, including more silicone.

I would recommend floating your bank in an oil bath - or even using the old tar / pitch + animal hair mixtures the Vikings used to seal their boats in the 8th century - over silicon.

Avoid it like the plague!

 

[john61ct]

If you want to fully pot inside a thermally insulating material, (why again?)

and want to avoid researching compounds designed specifically by industry to protect and encapsulate electronic components from harsh environmental conditions, (why again?)

then 2-part pourable polyurethane foam is cheap and easy

http://www.uscomposites.com/foam.html

Note I am **not** recommending that approach.