What is the expected self discharge with time, for a disconnected 2200uF/16V? - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: February 23, 2025, 02:08:31 pm
content: The curious thing with this particular electrolytic is that it self-discharges very, very slow.I've unplugged it from a breadboard rail at 12V, put it aside, and some hours later thought of measuring its capacity. Usually one should discharge capacitors before measuring them, but I've said to myself that it must certainly be discharged after half a day, but I've measured its voltage anyway, out of curiosity. It was about 11.5V or so.So I've let it like that, curious for how long it will stay charged. Put it aside and forgot about it.A couple of days later, it was still charged at almost the same voltage. It's been about a week since I've unplugged that capacitor from 12V, and today found it again, it measures 11.233V.That would mean less than 5nA self discharge current for a 2200uF losing 1V in 5 days.I've searched a few datasheets at random, and application notes. They do not specify the self-discharge when disconnected, but there are other related charts showing plots with leakage currents under constant voltage applied. Yet, the values are about 1000 times bigger, in the range of uA, not nA. Random examples:Fig10 inhttps://www.cde.com/resources/technical-papers/AEappGuide.pdfFig16 inhttps://www.rubycon.co.jp/wp-content/uploads/products-aluminum/al-technical-note_en.pdfTable1 inhttps://www.vishay.com/docs/28313/013rlc.pdf- Is it common to self discharge only 1V after so many days?- Anybody encounter similar capacitors?Asking because the self-discharge rate seems way too low.I suspect this particular capacitor might probably rectify some radio wave captured from air, or maybe it acts as a battery, and consume its own internal structure in a chemical reaction, such that somehow it partially replenish its charge lost in leakage, leakage that is in fact much bigger (more like 5uA) than the calculated 5nA.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:45:29 pm
content: I have an old Wurlitzer electromechanical piano, a 200B, that used a 240 V photoflash battery to bias the electrostatic pickups. The battery was dead when I got it of course, and in order to test it I charged some salvaged 100 µF electrolytic to a few hundred volts and used that to power it. To my surprise, it worked for close to a year on the initial charge. The piano itself doesn't draw any current from this rail, aside from the charge transferred across the electrostatic pickup, but I would have expected self discharge of the cap to drain it in days to weeks. Since then I lost that capacitor, and the replacements I've tried only last for a month or two on a charge. I've been meaning to design a small converter to power it, but recharging the capacitor periodically is just not inconvenient enough for me to bother.

author: Kasper
author_rank: Frequent Contributor
timestamp: February 23, 2025, 03:37:07 pm
content: Maybe something to do with dielectric absorbtion or recovery voltage phenomenon? I dont know. Could compare 2:1) charge, discharge, measure.2) charge, disconnect for a while, discharge, measure.Does 1 recover more than 2?Maybe agitating it while measuring does something? Years ago I was testing drain of lithium thionyl chloride batteries. Was getting funny results, sometimes the voltage would increase. Eventually I realized moving the battery a bit while clipping on test leads caused the voltage to increase.

author: bostonman
author_rank: Super Contributor
timestamp: February 23, 2025, 05:04:22 pm
content: QuoteCapacitors can hold a charge for that long? Wow!

author: CosteC
author_rank: Regular Contributor
timestamp: February 23, 2025, 05:20:27 pm
content: Self discharge can take days easily for well formed capacitor. New or discharged for a long time will show more leakage. Worst case leakage is often stated in datasheet, usually applies to old capacitors in high temperature. Leakage gets worse with temperature, hence you got good results in room temp.Your result is normal for new, good quality capacitor.In general, 10 M input of multimeter shall cause far more discharge than capacitor has itself.Large, high voltage (e.g. 400 V) electrolytic capacitors are often sold with shortened leads to prove that are not charged/are safe.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2025, 06:18:05 pm
content: Quote from: RoGeorge on February 23, 2025, 02:08:31 pmThe leakage specifications are maximum allowable, usually at maximum voltage and perhaps maximum temperature and after only a short period. Leakage current will decline exponentially with temperature and voltage and probably asymptotically with time. And, of course, YMMV so one speciment could be be a lot better than spec and yours seems unusually good. There is no minimum and you'll find varying amounts of charge on capacitors that have been sitting for years. I suspect that a capacitor that lives at near room temperature and little to no ripple current and a voltage signficantly under its spec may last longer than one which is stored on a shelf.

author: Someone
author_rank: Super Contributor
timestamp: February 24, 2025, 12:54:11 am
content: Quote from: bostonman on February 23, 2025, 05:04:22 pmSMT Caps off the tape can still hold enough charge to light an LED for some time, years after manufacture.

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2025, 01:03:35 am
content: It's rare I work on anything high voltage anymore.Prior most stuff I worked on had discharge resistors, but I always thought leaving something unplugged a day or two was enough for a capacitor to self discharge. So normally I'd leave something unplugged for a few days before diving inside.Silly question, but can self charge time be calculated (ignoring whether a cap is new or old)?

author: Simmed
author_rank: Frequent Contributor
timestamp: February 24, 2025, 01:07:28 am
content: see this threadhttps://www.eevblog.com/forum/metrology/film-and-ceramic-capacitor-leakage-current/msg4023037/#msg4023037Quote from: MiDi on February 21, 2022, 05:26:05 pm

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2025, 03:18:08 am
content: The leakage current is not a constant. But I would second the dielectric absorption phenomenon here, which can be pretty high in electrolytic capacitors. You could further test this by charging the capacitor to a given voltage (say 12V here), then disconnect it, discharge it down to a given voltage (say 1V here) with a resistor, disconnect the resistor, leave the cap unconnected for a few hours, and measure the final voltage.

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2025, 03:30:44 am
content: Can the discharge be measured by connecting the capacitor to an op-amp?

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2025, 03:53:32 am
content: Quote from: bostonman on February 24, 2025, 03:30:44 amI suppose by that you mean measuring the "open cell" voltage of the capacitor? Using a very low input bias current opamp should work here, to limit any significant leakage to the opamp itself. In that case, limit obviously the charging voltage of the cap to below max output voltage of the opamp and mount it as a unity gain buffer. Be aware that most of these low input bias current opamps are low-voltage devices so that'll limit the max voltage at which you want to test the cap.The OPA396 with 20 fA would be such an opamp (5.5V max).Though if you're expecting leakage currents in the order of a few nA, you could go for a higher input bias current opamp such as the OPA196 (20 pA) which is 36V max. 20 pA of leakage through the opamp itself should be fine for measuring electrolytic capacitors.That would allow recording he voltage across the cap over an extended period of time.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2025, 04:25:54 am
content: Quote from: bostonman on February 24, 2025, 03:30:44 amSure, or you could just check it occasionally with a high-impedance voltmeter.

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2025, 04:38:11 am
content: QuoteTrueThis has somewhat given me concern for working on high voltage electronics. I know the rule is: even if it's unplugged, you can still get a shock.Prior, I never realized how long a capacitor can hold a lethal charge.

author: Smokey
author_rank: Super Contributor
timestamp: February 24, 2025, 04:53:31 am
content: Quote from: bostonman on February 24, 2025, 04:38:11 amOften big caps are across a power supply rail or something like that which suck the energy out once the supply is off. Good designers, if possible, add a discharge path if one does not exist. It would take a sort of special circuit (like the above mentioned electrostatic pickup) to have absolutely minimal leakage paths and big caps.(To be clear, I'm talking in-circuit stuff here. Not a random lone cap in a bin.)

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 24, 2025, 05:03:26 am
content: Sometimes they can hold the charge for very long times.Once I picked a random cap from my box of parts, instant bang! It had been lying there for months!

author: vk6zgo
author_rank: Super Contributor
timestamp: February 24, 2025, 08:16:12 am
content: Electrolytic capacitors may well show not much voltage change when tested with a 10Minput impedance DMM, but will drop precipitously with any real load.Very large old style oil filled paper capacitors are another thing altogether!

author: RoGeorge
author_rank: Super Contributor
timestamp: February 24, 2025, 08:45:38 am
content: Charged yesterday a few more 2200uF/16V for 5 minute at 12V, left them disconnected over night, and this morning they were between 2-10V. They are scrapped from various PC power supplies, including the very slow discharging one. Still good but they are all used, from at least a few years ago.I suspect the main difference between these and the low discharging one is that the former was kept at 12V for at least a few days, when I was testing a DIY Vref. Maybe I should let them all at 12V for a few days, then disconnect and measure the discharging rate again, out of curiosity.Most of already available measurements or specs are for leakage while the capacitor is connected, but I suspect this will not be exactly the same as the self-discharge current when disconnected. Found some discharging measurements of V in the other topic linked above, thank you.Found these measurements here:https://www.eevblog.com/forum/metrology/film-and-ceramic-capacitor-leakage-current/msg3899087/#msg3899087The OST capacitor that surprised me is still 10-100 better apparently.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2025, 09:02:14 am
content: One key factor may be the type of electrolyte used. If I'm not mistaken, non-solid electrolyte capacitors have higher dielectric absorption and that may account for the main difference. Possibly that's something you can look for in respective datasheets, although the type of electrolyte is not always mentioned.

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2025, 01:04:54 pm
content: QuoteDoes it matter how long they've been charging; assuming they reach the input voltage before removing the input voltage?Old computer monitors that I worked on sometimes got cracks in the power supply section causing the bleeder resistor(s) to open. That resulted in us having to use a screwdriver to check for any charged caps. Another time I worked on a flat screen TV and remember by coincidence finding a few caps had a charge on them. The flat screen was only unplugged for a day or two, but I thought, wow, guess caps can hold a charge for a few days.I read through the message thread, but it seems most discussions revolve around measuring versus being able to calculate how long a cap can hold a charge.I have some very high voltage caps in my collection, but placed resistors across them (screw terminals) as I believe they can pick up stray electrons and charge. I also have some (I believe) 450v caps from that flat screen TV I tried repairing, however, they don't have resistors across them (solder terminals).

author: RoGeorge
author_rank: Super Contributor
timestamp: February 24, 2025, 06:58:19 pm
content: Quote from: bostonman on February 24, 2025, 01:04:54 pmElectrolytic capacitor areformedat the factory for a given voltage and capacity. Theformingmeans that a layer of oxide is grown on one of the plates. That layer is formed by simply applying voltage on the unformed capacitors.When left unused for many months (at 0 volts), that oxide layer that was formed at the factory will (partially) dissolve itself back into electrolyte. Thinner oxide layer also means more leakage current, a decreased maxim voltage supported, and also an increase in capacity.For example, if you measure capacitors left unused for years, you may find some of them to have significantly bigger capacity than the nominal value written on them. Sometimes it can be a few times more than the nominal capacity (if the capacitor is not yet dry after all these years, of course).The oxide layer can be grown back by simply applying voltage on the capacitor, but it is a slow chemical process, so it may take hours or days to grow the oxide at the same thickness as it was from the factory. If you search for "forming old capacitors" you may read the details of how to do that.Now these that I've charged yesterday were from defective power supply scrapped years ago and left unused. Probably the oxide layer got thinner, the leakage increased, and the capacitance increased, too. To grow the oxide back, I should keep them for a few hours at their nominal voltage (or a little more than nominal V), to form the capacitor again.That is why keeping them for hours under voltage might make a difference. Either formed or unformed they will charge to 12V very fast, but while keeping them at constant 12V, the oxide layer grows thicker, and the leakage current becomes smaller.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 28, 2025, 08:38:21 pm
content: Well, I guess I was wrong estimating that that electrolytic should self discharge in only a couple of days.Measured it again today, more than a months later, 8.44V, which is more than 2/3 of the initial voltage.The other 6 electrolytic capacitors I've charged at 12V on 24 Feb (to compare with the first one) were not that low self-discharge, but still, they kept their charge much better than I would have expected from them. They measure now various voltages between 2...8V.

author: watchmaker
author_rank: Unknown
timestamp: March 29, 2025, 11:21:08 am
content: Thanks for bringing this up. I will include it as a demo in the STEM course we are preparing.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 12, 2025, 08:29:35 am
content: Two weeks later... 7.9472V (7 weeks in total, since last charged at 12V).

author: RoGeorge
author_rank: Super Contributor
timestamp: May 16, 2025, 11:19:40 am
content: 2025-05-16, measured 7.1595V (12 weeks in total since last charged).Hard for me to believe such a low leakage current is possible. I suspect this capacitor is somehow acting as an RF detector, and partially replenishes its leaked charge from the surrounding radio fields.Maybe I should keep it in a shielded metal box until the next measurement.

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2025, 11:35:16 am
content: I think what you are seeing is actually dielectric absorption. It isn't unknown for high voltage capacitors (including electrolytics), that have been used for a while at high voltage and then discharged resistively or shorted) and put in a drawer to give someone a nasty surprise months later.The same effect occurs at lower voltage too, that's why it's important to short capacitors immediately before testing on an LCR tester, particularly those little AVR based component testers. There can be enough recovered charge to kill them.I'm afraid the RF detector theory doesn't hold water. There is no rectifying element - once the capacitor has some DC charge on it, any rectifying effect from having higher reverse leakage current disappears because it can't become reverse biased.Edit: Dielectric absorption on it's own wouldn't be sufficient on its own to explain the magnitude if voltage that you are seeing, but because you didn't discharge the capacitor, I think it is causing you to see an artificially low leakage current.

author: shapirus
author_rank: Super Contributor
timestamp: May 16, 2025, 11:36:49 am
content: You got me interested and curious, too. I have a 35 V 10000 µF electrolytic cap here, which has never been used (for a prolonged time in a circuit, anyway). I'll have to find it, though, but when I do, I'll charge it and measure the voltage from time to time.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 04, 2025, 10:40:41 pm
content: Quote from: RoGeorge on May 16, 2025, 11:19:40 amSince 2025.05-16, kept it in a metal pan, with a metal lid, away from the lab. Doesn't look like it was charging from the surrounding RF fields. Today (after 3 weeks sited in the shielded metal pot) checked its voltage again:2025-06-05, measured 6.7930V