Why not seal super capacitors?

[adjustttt]

This is probably an extraordinarily stupid question but if the longevity of super capactiors is limited by the electrolyte evaporating why can't they just be sealed off completely without evaporation they would last piratically forever right?
I am probably missing something very big here 

[glarsson]

It is difficult to make a perfect seal and keep the cost low.

[adjustttt]

So that means I could seal them completely myself?

[wraper]

So that means I could seal them completely myself?

Why do you think that with your added seal it will perform significantly better? If you think that covering it with silicone will completely prevent very slow electrolyte escape, you are wrong.

[adjustttt]

Why is that wrong?

[wraper]

Why is that wrong?

Why do you think that silicone cannot pass it through? If you use hard compound, it could even make things worse by adding mechanical stress to original seal.

[Brumby]

Why is that wrong?

What would you use as a sealant?  Silicone?  Glazed ceramic?

Silicone is good at keeping water out - but what about other substances?  It needs to cure to set - which happens when exposed to air.  If it was a perfect sealant, then only the surface would cure and the internal mass would remain uncured.

A solid glazed ceramic would have a better chance - but the capacitor isn't going to survive the process too well.  Even if it did, were there any expansion issues with the capacitor, you would get anything from a hairline crack (say goodbye to being sealed) to an explosion (say goodbye to the cap).

Maybe someone can tell us what caps NASA used on Voyager I and II.  They seem to be doing ok.

[wraper]

Silicone is good at keeping water out - but what about other substances?  It needs to cure to set - which happens when exposed to air.

Actually it isn't good at keeping water out. Curing process is based on water contained in the air, not oxygen or nitrogen. Of course there also are two part silicones but I don't think they are that good in this regard.

[JoeN]

Cerakote or Duracoat?  Is there some spray ceramic that might make a good seal that only needs to be lightly baked or not baked at all?  And you can choose really cool colors...

[David Hess]

If they were hermetically sealed, then they would be called bombs instead of capacitors.

[Paul Moir]

People wrongly think water doesn't migrate through plastics or rubber.  The only real options are metal, which doesn't work around the leads, and glass, which is tricky.
Hermetically sealed electrolytic caps do exist though.  Check this out:  https://www.digikey.com/product-detail/en/cornell-dubilier-electronics-cde/MLSH322M030JK0C/338-4248-ND/6096195

[Gyro]

If they were hermetically sealed, then they would be called bombs instead of capacitors.

No, you could still have the engineered 'weak spot' in the top, just like normal electrolytics, for safety venting. Still hermetic.

[bd139]

The vents barely work on half of the electrolytics made, even from named brands like Rubycon. I've blown up hundreds of the things over the years. No thanks:

[T3sl4co1l]

The vents barely work on half of the electrolytics made, even from named brands like Rubycon. I've blown up hundreds of the things over the years. No thanks:

On a related note, I have some film-in-oil can capacitors (not actually motor run caps, but that's the basic idea) which have a patented safety mechanism: the top is double-crimped with an extra bead, so that it's kind of scrunched down in length.  If gas generation occurs, the top poofs out, disconnecting the internal leads (which are only just long enough to connect).

Hopefully the top doesn't poof out too far, too rapidly, and explode and catch fire anyway.

Tim

[wraper]

The vents barely work on half of the electrolytics made, even from named brands like Rubycon. I've blown up hundreds of the things over the years. No thanks:

In the video only K50-18 cap has safety vent and it did not explode. Capacitor types which exploded violently, don't have any safety vent and you show this video as example of safety vent not working  .

[T3sl4co1l]

This is probably an extraordinarily stupid question but if the longevity of super capactiors is limited by the electrolyte evaporating why can't they just be sealed off completely without evaporation they would last piratically forever right?
I am probably missing something very big here

But they are!

Some, anyway.  I find a few results for military supercapacitors, although I can't seem to find a MIL STD for them (if it exists).  One picture shows a glossy backfill on the component, presumably an epoxy encapsulant.

Mil grade electrolytics do the same thing: either an aluminum can backfilled with epoxy (that always seems to be pink or red, on the ones I've seen?), or a metal can with a glass frit seal on one end and a soldered or swaged seal on the other (hermetically sealed, also dry and wet tantalums made the same way).

No need for high temperatures, solder does a fine job as hermetic seal.

Tim

[Gyro]

The vents barely work on half of the electrolytics made, even from named brands like Rubycon. I've blown up hundreds of the things over the years. No thanks:

They probably barely work half the time because of poor, non hermetic rubber seals at the bottom, allowing them to slowly spill their guts all over the PCB as the pressure builds instead. It ought to be possible to volume manufacture them to blow at a very consistent pressure.

[wraper]

But they are!

Some, anyway.  I find a few results for military supercapacitors, although I can't seem to find a MIL STD for them (if it exists).  One picture shows a glossy backfill on the component, presumably an epoxy encapsulant.

There were Soviet made K50-6 capacitors filled with epoxy. They had way lower reliability that Soviet capacitors which had rubber seal.

[David Hess]

Another reason they may not bother with better seals for supercapacitors is that they degrade at a rate dependent on the voltage.  It works out to about 1/10th the operating life for every 0.2 volt increase.  Most applications operate them at about the 10 year point where loss of electrolyte through the non-hermetic seal will not be the cause of failure.

[stj]

there are, or atleast was sealed ones from NEC,
i see them in stuff i repair backing up sram.

STEEL cylinders with a crimped base over a plastic bung with i think more behind it.

[Assafl]

Are the red Roederstein and Siemens "Bakelite" capacitors considered sealed?

Audiophiles seem to like them. They claim the sound is as if a veil is lifted and the transistor sound undergoes vastation.

[voltsandjolts]

But they are!

Some, anyway.  I find a few results for military supercapacitors, although I can't seem to find a MIL STD for them (if it exists).  One picture shows a glossy backfill on the component, presumably an epoxy encapsulant.

https://www.tecategroup.com/products/data_sheet.php?i=PC&t=SERIES

[Cliff Matthews]

I have some Litton 1980's el-caps sealed with glass-like resin below the rubber, then crimped, and then poly-wrapped. Marked 180uF/25v 105C but now they measure 30% high with 0.14 ESL. As can be seen, they're still full of electrolyte. Made in USA - can they still be trusted?

[T3sl4co1l]

Sure -- they may need reforming though.

I wonder if reforming is ill-advised for that type...

Tim

[wraper]

I have some Litton 1980's el-caps sealed with glass-like resin below the rubber, then crimped, and then poly-wrapped. Marked 180uF/25v 105C but now they measure 30% high with 0.14 ESL. As can be seen, they're still full of electrolyte. Made in USA - can they still be trusted?

No. Well, you can reform them but why to use such old stuff which already degraded. Aluminium oxide layer which is insulator partially dissolved over time thus becoming thinner. The result of this is increased capacitance and lower withstanding voltage.