Potting compound - way to go, thoughts/suggestions?

[cyberfed]

Hello brilliant minds,

I'm working on a prototype for providing a lot more power than what current dive lights provide for cave diving. It's clear they are using a 4S Li-ion setup that lasts about 5 hours. The price is crazy for what it is. I wanted to see if I could build my own pack with a way longer runtime. My question of course is around water!

So I'm seeking suggestions on the best way to keep water out 100% always. I was thinking as a prototype to use some of the black PVC pipe and seal it. But then I also go to thinking about the batteries of which I haven't decided what I would use. I was thinking of encasing the whole battery pack once built in potting compound, but I dont know if that is a good idea or not, and I actually dont know where to get it and what brands are good. Also what would be a good recommendation for a type of power cable that can run from the lamp to the pack in the open water. Looking for the best highest quality durable cable possible. Cave divers are always scraping everything on rocks and the cable needs to be tough as nails. I would imagine that black pvc would have no problem with depths of 300ft but the pressure changes encountered inside concerns me about the batteries and electronics.

Lastly in the event of a flood of the compartment how can I protect the diver from injury?

I know it's somehow possible because not only myself but my fellow divers have flooded their lithium packs before and there was no reaction whatsoever and the batteries DID were completely submerged for extended periods of time upwards of 30-45mins.

Sadly I haven't caved dived in a long time and got rid of my gear, so I can reverse engineer anything. This is more of a "I wanna see if I can build a safe pack" kinda project for fun. Curious how someone experienced with packs and water specifically have to say.

Cheers,

 

[hwy17]

The first concern that I see is that potting rechargeable would block their vent port, in case they popped.

I would think lithium primary cells would be be more ideal for the application? But I guess for regular divers they may be cost conscious of buying a new battery for every dive.
I was thinking energizer AA's but I found this thread and they mention that wireless water meters use potted lithium primary cells in long term water submersion:

Potting Batteries, Thoughts and Experiments

Background. Last year we had a number of custom batteries made up in a flat 10s X 1p cell layout arrangement for 2 reasons. #1 is so that we'd have convenient low profile 18650 batteries for mounting on powered skateboard projects (see here...

endless-sphere.com

 

[Warpspeed]

I have from time to time been asked to repair electronic equipment used in the deep mining industry.
Deep mines are often very wet places, and subject to complete flooding sometimes for extended periods.
Electronic equipment must be very rugged and also be made totally waterproof when fully submerged, sometimes at high static water pressure.

They use some kind of clear jelly for potting. I don't know what that stuff is, its kind of soft rubbery in texture, but with a bit of research you could probably find out.
I am sure it works very well for very long periods, but its a real pain to remove during any repair work.

A simpler do it yourself substitute might be to fill your plastic tube right up with fairly thick oil.
That would definitely keep the water out, and it should be pretty easy (if not messy) to completely drain the oil for any frequent maintenance or repairs. That might be better for short term use than the jelly. Some military equipment is completely oil filled.
The water simply cannot displace the oil, and that has proved to be quite effective.

Underground cables are sometimes pressurized with an inert gas, to keep out ground water, but I think that would be less than ideal for your purpose for several reasons.
Those are the only three methods that I am aware of that are actually used professionally, for keeping water under pressure out of electrical and electronic equipment.

 

[SolarForU2]

You could pot it in bees wax, melting temp 65c, any repairs needed warm it up and tip wax out.
But adds to the weight.

 

[Cronix]

They use some kind of clear jelly for potting. I don't know what that stuff is, its kind of soft rubbery in texture, but with a bit of research you could probably find out.
I am sure it works very well for very long periods, but its a real pain to remove during any repair work.

my guess is silicone glue/sealant, often the same stuff you use to seal around bath tubs/sinks/windows. That's what we used in our electronic enclosures to withstand sea mist/storms. It can get very hot/cold and remain pliable. Yes, it's also a PITA to remove for repairs, but that's pretty rare compared to how cheap/effective it is. We used just a commercial grade clear GE Silicone Sealant in huge drums.

 

[Warpspeed]

The stuff I have seen is more like a soft jelly and easier to remove.
The usual silicone roof and bathroom type sealers are more rubber like and harder.

But you are likely quite right, its probably some kind of specially formulated goo in the very broad silicone family.

 

[Hedges]

Better visit the threads on compression of LiFePO4 cells! Oh, "Li Ion", you may be referring to an "explody" chemistry.
Might be more amusing than hazardous at depth, but do carry a backup flashlight.

Liquid isn't compressible, but there may be headspace which could crush (reference Ocean's Gate), and you wouldn't want the cells to suffer Nitrogen Narcosis, either.

You might do some above-ground tests with a pressurized container to see if you can get away with just potting.
Otherwise, consider a container which is sealed and can withstand the depth.

 

[Warpspeed]

It might be interesting to know at what pressure the bursting discs on LiFePO4 cells "let go".
Its probably quite high.

Whatever pressure the cell cases can stand from within, its likely they may withstand similar pressures from without ?
But I agree with Hedges.

Some actual hydrostatic pressure testing may be in order.
All it might take is a brake master cylinder, a bolt with a long thread, and a suitable pressure gauge.

Three hundred feet works out to about 145 psi. Its high, but not a crazy high pressure.
https://www.omnicalculator.com/physics/hydrostatic-pressure

 

[DPC]

I have a preference for Urethane over Silicon.
Its probably not critical here, but in some electrical equipment Silicon causes problems, the fumes are corrosive and any contamination on brushes or slip rings will cause major problems

 

[v_green57]

Maybe this?

Etelec MPgel Plus [1 Liter/1 qt] Electrical Reenterable Encapsulating Potting Gel Seal Compound (Two Part) | Specto Technology

1 Liter (1 qt) - Re-enterable two-component silicone gel for insulated filling and sealing of casings and junction boxes housing electrical components and connections of up to 1 kV or electronic components.

www.spectotechnology.com

 

[DPC]

Reenterable is like Jello

 

[Hedges]

Three hundred feet works out to about 145 psi. Its high, but not a crazy high pressure.
https://www.omnicalculator.com/physics/hydrostatic-pressure

True, but cylindrical cells under external pressure not internal, concern is their getting crushed.
Spacers may apply force unevenly.

Stand on an empty Coke can. Then flick the side with your finger (but get it out of the way fast!)

 

[Warpspeed]

I assumed he was after big amp hours at low voltage for a lamp, and rectangular LiFePO4 cells.
But I may be dead wrong there.

 

[Q-Dog]

Does anyone make a hot melt sealant? Like hot glue?

 

[Solarstefan]

i have seen marine stuff filled with grease to keep the water out

 

[Hedges]

I figure rectangular cells have air filled headspace, won't press contents evenly in all directions (as a pouch might), and pressure bombing them might squeeze the jelly roll filling out. Reference compression recommendations.

 

[Warpspeed]

Ordinary "engine grease" breaks down in salt water over time.
But you can buy special marine wheel bearing grease for boat trailers which routinely back into salt water.
It probably does not cost any more than general purpose grease.

 

[Warpspeed]

i have seen marine stuff filled with grease to keep the water out

Ordinary "engine grease" breaks down in salt water over time.
But you can buy special marine wheel bearing grease for boat trailers which routinely back into salt water.
It probably does not cost any more than general purpose grease.

 

[Skypower]

Do not use common grade silicone around your electronic! Just the confined fumes will eat your bms alive. If it has a strong pungent vinegar smell, it’s the wrong stuff. I don’t personally know of a good electrical grade silicone brand that I could recommend so if someone knows, chime in.
I did pot some cells in for vibration and to secure them into a battery box. I used pourable urethane. I used aluminum duct tape to form a funnel around the perimeter (protect the terminals!) and three strips to form a dam (sides and bottom) to leave an end space for the bms. Pull the funnel off before it sets. The stuff I used has an extremely short pot life of about 5 minutes which doesn’t give you much time to mix thoroughly (sides and corners of container!) Use a submerged mixer to reduce bubbles.

 

[Warpspeed]

Some of this quick setting (add part A to part B stuff) often releases a lot of exothermic heat while curing.
That might be fine for your fiberglass whatever..... But it can cause problems for potting electrical components that contain thermoplastics.

Its very likely not an issue for polyurethane "rubbery type stuff" I really do not know.
But corrosive fumes are not the only potential issue to investigate when choosing a potting material.