Battery waterproofing

I’m looking for a lightweight method to seal a small 6s3p battery for a foil drive system. Shrinkwrap doesn’t seem like it would be very waterproof, as water could seep in at the ends, and glue likely wouldn’t stick to the shrinkwrap.

Has anyone tried using a water based liquid polyurethane waterproofing membrane, such as bostik dampfix gold? I was thinking of applying it over the top of a fully kapton wrapped battery pack, so that it doesn’t soak into all the gaps between the cells. One issue will be the smallest size you can get this liquid membrane, which seems to be 4L, and it has a short shelf life of a few months once opened.

Any other suggestions for a lightweight and thin flexible membrane, either liquid or adhesive backed, to seal my battery packs? Plastidip, polyurethane pcb conformal coating?

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Great question… I’m looking for similar. Did you end up having success with a particular product? I’m looking into plastidip.

Inert gas to save your plastidip

You can look for potting compounds with some thermal properties. I have bought this one but didn’t use it yet:

You just have to learn swedish to order it :laughing:

Interesting topic.

I’m curious about why you feel you need an increased level of water “protection” for this battery pack?

Waterproof pockets can be made out of 100g/sqm TPU used in kitesurf bladders.
The proper way to seal TPU sheets to make tubes is using a High frequency welding machine.
As DiYers, we can use:
1- special TPU glue like or Aquasure.
2- A domestic cloth iron set to low temperature + baking paper to spread the heat (testing required)
Self adhesive valve + sealant as a resistant waterproof cable pass through…


An interesting possibility to get a reusable watertight bag is to copy waterproof tube bags ie seal/weld one end and roll up the other :

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You can also heat weld with a vacuum heat sealer. I have done this with success on my packs. I make a 3 sided bag and then seal the final end with window mastic from a roll. The wires coming out are easily sealed with it as well. I then put heat shrink over it all to protect the tpu. You can also pressurize the bag and seal the mastic and see if it stays under pressure. Its a but fiddly but seems to work well.

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The right tool for this job, efficient and DiY budget friendly.

I normally use hot melt adhesive to seal my vacuum bags when doing composites, it works surprisingly well.

The adhesive makes the plastic film melt just a bit locally which provides a great bond. It’s probably the easiest way of getting a vacuum tight seal for odd shaped bags and certainly the cheapest :smiley:

Butyl rubber sealing strip is another option commonly used for vacuum bags but this is more expensive and does not provide a seal with any pull strength.

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A question then, if battery can be sealed when it produces gaz while operating?

If you have in mind a balloon effect, I don’t think LiIion cells produce any gaz while operating.

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I think only if they in some failure mode where they can poof.

@SoEFoil @windego
Yes, you are right, they don’t produce gases in normal use.
But in case they are sealed or water proofed somehow, then considerations must be made to conduct heat to outside.

Maybe silicone thermal padding to fill air gaps and allow heat conduction? I purchased a second hand battery last month. It included silicone padding about 5mm thick resting on the top off all the cells.

I had considered putting a two way pressure relief valve (or breather valve) on the side of my other battery box. I was concerned about a warm battery and a pressure differential developing with the air in the box. I ended up skipping this because I haven’t seen anyone else include these in a battery build, it seemed like an extra failure point, and I also don’t expect my battery cells to warm up too much.

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I rode last season mostly with a 12s6p 18650 samsung 30q pack I made. I sealed it watertight and in shrink wrap. After a 1/2hr session battery is depleted but the pack was barely 30C. I think if you expect to get a lot hotter than that your battery pack is not really in its happy place. 12s12p of same setup was even lower temperature and that is a more typical efopil size pack.

One of the tests that I found on a LG data sheet indicates they place a cell in a thermal chamber and slowly raise temp at 5C/min up to 130C and leave it for 10 minutes with no explosion or fire so I can’t see your pack getting anywhere close to that temp unless it’s on fire😀


Interesting, but that is actually a different sort of test since it just subjects charged cells to heat from an external source, or the environment. I guess it’s more of a test about how robust a cell is if another adjacent cell is throwing off lots of heat. Not really about heat generated by a loaded cell.
“No explode, No fire.” Hopeful words to live by!

You seemed concerned about heat buildup which I don’t think occurs with efoils. Lots of
posts here about overheated VESC but I don’t recall anyone having a breakdown as a result of the batteries getting too hot.

I appreciate a lab test doesn’t relate directly to real life use but any quality cell can handle the temps generated from foiling imo.

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If your batteries get hot the design is wrong. You can look at the specs of your batteries internal resitance and estimate the losses and determine the watts. But following this forum you see the setups that work and battery heat is not a concern if you chose the cells suggested.