An electrolytic capacitor keeps charging by itself

[Philip Koeck]

This is related to an earlier post (https://www.physicsforums.com/threads/an-electrolytic-capacitor-charges-by-itself.1008156/), but there's a new angle to it, therefore the new thread.

I have two 100 microF electrolyte capacitors in my office, one is connected to a volt meter, the other is not. I short circuit both of them every 1 to 2 weeks and I've been doing this for about half a year.
Within about a week they build up a potential of about 130 mV for the one connected to the volt meter and 150 mV for the disconnected one.
I just short circuit them, check that the voltage is zero, observe that the voltage is slowly building up and then wait a week before measuring the voltage again and it's always roughly the same value.
I've also put one capacitor into a metal container (thermos flask) to shield off EM-waves, but it still charges to roughly the same voltage.

Can this be explained by dielectric absorption?
Shouldn't the effect wear off after some time?

 

[Baluncore]

Can this be explained by dielectric absorption?
Shouldn't the effect wear off after some time?

During manufacture, an electrolytic capacitor is electrically conditioned to develop the polarity of the capacitor. The anode and cathode develop different surface chemistry, that is maintained by the operating voltage.

The capacitor can also be analysed as an electrochemical cell, with a very high internal resistance. The cell could be charged by thermal effects. That would explain why an electrolytic capacitor might repeatedly recover the same small cell voltage.

 

[tech99]

I believe capacitors generally exhibit a relaxation effect, whereby they acquire a small charge some time after a discharge.

 

[Philip Koeck]

The cell could be charged by thermal effects. That would explain why an electrolytic capacitor might repeatedly recover the same small cell voltage.

Are you saying that heat is converted into potential energy?

 

[DaveE]

That is weird. Do you see the series trending towards zero?
It's not enough to be a thermocouple like effect, that would be 30mV or less.
I'll vote for @Baluncore's idea, it's a capacitor AND a battery. How long do you discharge it? What if you left it shorted for longer, then did the test?

 

[Baluncore]

Are you saying that heat is converted into potential energy?

I don't know, we are all guessing.

The thermocouple Seebeck effect is too small, and appears between metallic connections where there is no series resistance, such as the electrolyte.

I am thinking here that this is more like the Peltier effect. Two chemically different electrodes have been electroformed, P and N, which is similar to an electrolytic rectifier.

My thermal conversion suspicion, comes from the band-gap junction voltage developed, 140 mV is long infra-red ≈ 9000 nm. It smells like a PiN diode.

Metal oxide rectifiers, that also need to be electroformed, had junction characteristics dependent on impurities. It may not be the aluminium that is used in the electrolytic capacitor, but the terminal material and bonding that is forming the critical junction with the electrolyte. The capacitor may just be efficiently storing the charge.

 

[Philip Koeck]

That is weird. Do you see the series trending towards zero?
It's not enough to be a thermocouple like effect, that would be 30mV or less.
I'll vote for @Baluncore's idea, it's a capacitor AND a battery. How long do you discharge it? What if you left it shorted for longer, then did the test?

I would also expect the voltage to decrease with time if it comes from some sort of chemical effect, but so far it doesn't seem to do that. It does fluctuate a bit, though.

 

[Philip Koeck]

I don't know, we are all guessing.

The thermocouple Seebeck effect is too small, and appears between metallic connections where there is no series resistance, such as the electrolyte.

I am thinking here that this is more like the Peltier effect. Two chemically different electrodes have been electroformed, P and N, which is similar to an electrolytic rectifier.

My thermal conversion suspicion, comes from the band-gap junction voltage developed, 140 mV is long infra-red ≈ 9000 nm. It smells like a PiN diode.

Metal oxide rectifiers, that also need to be electroformed, had junction characteristics dependent on impurities. It may not be the aluminium that is used in the electrolytic capacitor, but the terminal material and bonding that is forming the critical junction with the electrolyte. The capacitor may just be efficiently storing the charge.

The temperature in my office is far from constant. It gets warm during the day and I assume it cools down quite a bit during night.
Maybe the radial temperature gradient in the capacitor is the reason for the voltage.
This is definitely a working hypothesis one could test.

 

[Philip Koeck]

The temperature in my office is far from constant. It gets warm during the day and I assume it cools down quite a bit during night.
Maybe the radial temperature gradient in the capacitor is the reason for the voltage.
This is definitely a working hypothesis one could test.

I'll retract that idea. I can watch the voltage build up in the middle of the day when there can't really be any temperature gradient in the capacitor.

 

[Philip Koeck]

That is weird. Do you see the series trending towards zero?
It's not enough to be a thermocouple like effect, that would be 30mV or less.
I'll vote for @Baluncore's idea, it's a capacitor AND a battery. How long do you discharge it? What if you left it shorted for longer, then did the test?

I discharge them for a few seconds each time, but the voltage does go to zero and then it starts rising again immediately. To go above 100 mV takes 4 - 6 days, I would estimate.

I'll try shorting them for a few hours now and see what happens by the end of next week.

 

[DaveE]

I was interested enough to try this myself with 5 very old 100uF, 63V Al caps; in storage for at least 10 years. They were tested OK for capacitance and df and shorted for a day. Here's what I saw:

 

[Philip Koeck]

Here are my new results:
I shorted the two capacitors for about 2 hours on the 14th of March.
The disconnected one has reached about 145 mV today (after 12 days) but the one connected to a voltmeter is still below 110 mV.
It looks a bit like the process is slowing down, but very gradually.

It would be interesting to test this at very low temperature, such as in a -80 freezer, and also at high temperature, maybe +80 C.
I would think there's some sort of order in the electrolyte gel that is converted to a macroscopic charge separation due to thermal motion. Something like that has been hinted at in several posts in this thread.

 

[Philip Koeck]

I was interested enough to try this myself with 5 very old 100uF, 63V Al caps; in storage for at least 10 years. They were tested OK for capacitance and df and shorted for a day. Here's what I saw:

View attachment 342357

All 5 level off after about 200 hours. Number 2 is from a different production batch, maybe?

Are you going to repeat the measurements to see whether they reach the same voltages again?

 

[Philip Koeck]

Here are my new results:
I shorted the two capacitors for about 2 hours on the 14th of March.
The disconnected one has reached about 145 mV today (after 12 days) but the one connected to a voltmeter is still below 110 mV.
It looks a bit like the process is slowing down, but very gradually.

It would be interesting to test this at very low temperature, such as in a -80 freezer, and also at high temperature, maybe +80 C.
I would think there's some sort of order in the electrolyte gel that is converted to a macroscopic charge separation due to thermal motion. Something like that has been hinted at in several posts in this thread.

Now the one connected to the volt meter is finally giving up, it seems.
Yesterday it was above 100 mV but today it has been dropping steadily and was below 50 mV sometime in the afternoon.

 

[Swamp Thing]

They laughed when I said I could tap into the vacuum energy...

 

[ADesilets]

Maybe a differential work function/electron affinity effect. You might be seeing the potential difference between the EDLs of capacitor materials. Thermal ballistic motion of ions at the interface can exchange electrons differentially with each electrode even if no chemical reaction is happening. This forms a double layer of ions at the interface. Different material form different equilibrium potentials in their double layers.

Basically an extremely slow, stochastic accumulation of a very small static electric charge from thermal motion.

Could be something similar to the effect seen here: https://arxiv.org/abs/1203.0161

 

[bob012345]

I saw something like that awhile back. I would see small voltages generated across thin film capacitors plates. Ultimately I noticed that when all the lab equipment was turned off the effect went away. I think things were charging from 60Hz noise from lab equipment. My dad used to light small bulbs from his ham radio antenna.

 

[Philip Koeck]

I saw something like that awhile back. I would see small voltages generated across thin film capacitors plates. Ultimately I noticed that when all the lab equipment was turned off the effect went away. I think things were charging from 60Hz noise from lab equipment. My dad used to light small bulbs from his ham radio antenna.

I put a capacitor inside a metal thermos flask with a metal lid and it still charged.
Wouldn't that exclude 60Hz noise as a possible cause?

 

[Baluncore]

I put a capacitor inside a metal thermos flask with a metal lid and it still charged.
Wouldn't that exclude 60Hz noise as a possible cause?

The cause is not 60Hz noise, nor EMI of any sort. It only occurs in electrolytic capacitors, because they contain an electrolyte as the dielectric. If it was not due to the electrolyte chemistry, it would be seen in many other types of capacitor.

The electrolytic capacitor has a fast response from the free electrons on the conductive surfaces, but there is a very small amount of charge that is stored in the surface chemistry of the oil dielectric. After the capacitor has been rapidly discharged, it takes time for that deeper charge to return to the surface, where it becomes attached to the plates.

The phenomenon is called dielectric absorption, it is also known as dielectric relaxation, memory, soakage, or battery action.

 

[sophiecentaur]

Wouldn't that exclude 60Hz noise as a possible cause?

A very thin metallic coating would possibly screen an Electric Field but would have little effect on any Magnetic Field so that some Power can sneak in and induce a current into the circuit.

I'd be inclined to go for some electrochemical effect, rather than EM induction.

 

[DaveE]

I think things were charging from 60Hz noise from lab equipment.

This is a DC effect. 60Hz energy harvesting would require a rectifier of some sort (like dissimilar metals?). Yet dielectric absorption is evident in essentially all capacitors with a dielectric. This includes simple ceramic, paper/oil, or film capacitors which can have virtually identical metal plates. So, where's the rectifier? was this part of a more complex circuit. So far this discussion has only covered the open and short circuit configurations.

 

[DaveE]

It only occurs in electrolytic capacitors, because they contain an electrolyte as the dielectric.

Not precisely true. While larger in electrolytics, this is a well known effect in HV film and oil caps, which is why they are always stored with the terminals shorted.
https://en.wikipedia.org/wiki/Dielectric_absorption#Measurement

 

[sophiecentaur]

This is a DC effect. 60Hz energy harvesting would require a rectifier of some sort,

I always twitch when I read or hear the word "harvesting". Imo, the word should only be applied when an appreciable quantity of Energy or food is involved. i.e. Weeding the garden is not 'Harvesting'.

I can't find any numbers in this thread which could imply that there's some Energy coming from somewhere - a Q times V enough to be significant. There are a lot of processes that take place which involve energy changes. An electrolytic capacitor is formed by electrolysis; that involves slow energy transfer as the capacity builds up. Not relevant to the purpose that the component was made and unlikely to be of interest to the manufacturer.

 

[Baluncore]

This is a DC effect. 60Hz energy harvesting would require a rectifier of some sort (like dissimilar metals?).

Fifty years ago I remember reading about a device from fifty years before that. There can be rectifier action in every asymmetrically conditioned liquid electrolyte capacitor. The effect was exploited in the electrolytic rectifier, also called a chemical rectifier, or Nodon valve.
https://en.wikipedia.org/wiki/Rectifier#Electrolytic
These days, the effect is deliberately minimised by the manufacturers of electrolytic capacitors, by using pure metals, and avoiding internal soldered connections.

There are rectifiers everywhere that dissimilar metals contact in a chemically reactive situation. The best and worst example I know, is ZnO between the sheets of galvanised iron on a roof, that can generate the harmonics of local RF transmitters.

Not precisely true. While larger in electrolytics, this is a well known effect in HV film and oil caps, which is why they are always stored with the terminals shorted.

I agree. The effect is most significant in the largest capacitors, those that employ a liquid such as oil for the dielectric or the insulation. In that case any imperfect or damaged oil molecules can move to align with the impressed field, before returning the favour slowly, later.

 

[DaveE]

I always twitch when I read or hear the word "harvesting". Imo, the word should only be applied when an appreciable quantity of Energy or food is involved. i.e. Weeding the garden is not 'Harvesting'.

Wait. Are you disrespecting the 100pW of charging power my previous data clearly demonstrated (at least once, anyway)?

Yea, maybe a poor choice of words. I'd bet there are other, better, ways to collect picowatts.

 

[sophiecentaur]

I'd bet there are other, better, ways to collect picowatts.

You're right. Your ears probably do that sort of thing. I wonder what sort of rate of voltage drift it involves in that Capacitor. Could upset some measurements - along with all those other near-DC signals that people fight all the time.