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I pulled a handful of electrolytic capacitors from a PCB and checked their capacitance on a multimeter. The number was not stable, but it also didn't average about some central value but rather steadily decreased. I didn't measure it for too long since the monotonic decrease was only a percent or so of the original value. Is this normal? Is this indicative of a bad capacitor? I included a bunch of relevant data below, the time table, the corresponding plot, and the first and last image I shot on my phone.

4700 uF nominal value

Time    Time[s] Capacitance[uF] %Decrease
8:31:21  00.00   4496             0.000 %
8:31:47  26.00   4443            -1.179 %
8:31:59  38.00   4440            -1.246 %
8:32:09  48.00   4438            -1.290 %
8:32:15  54.00   4436            -1.335 %
8:32:20  59.00   4435            -1.357 %

Decreasing capacitance measures for 4700uF electrolytic capacitor Before and after capacitance measures against multimeter

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    \$\begingroup\$ I won't write my answer bellow since I'm not 100% certain. If I had to take a guess: the capacitor is being charged by the multimeter through a rather big time constant (reinforced by the fact that this capacitor is quite big by itself). The voltage evolution is then measured over time and knowing what R was used, it is possible to mathematically deduce the C. The more you wait the more accurate the answer might become (maybe?). One thing for sure, the variance isn't extremely significant. It is rated as 4700 uF and you get a measured value that is within 10% of the rated value. \$\endgroup\$ – Simon Marcoux Apr 19 '18 at 3:49
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    \$\begingroup\$ The two readings are both within the capacitor tolerance, so the result doesn't have to be wrong: it really might drift just as the measurement shows. \$\endgroup\$ – Whit3rd Apr 19 '18 at 9:52
  • \$\begingroup\$ Check the multimeter against a new capacitor with the same value. I noticed strange behavoiur on my multimeter to with electrolityc capacitors. \$\endgroup\$ – Dorian Apr 19 '18 at 20:29
  • \$\begingroup\$ What is the tolerance of the capacitor and what is the precision of the multi-meter? \$\endgroup\$ – MathieuL Apr 19 '18 at 20:45
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    \$\begingroup\$ @Mathieul The value of a capacitor is given together with it's tolerance. A 100uF 20% capacitor can have a factory value between 80 and 120uF. Also a 100uF value measured with a 10% precision multimeter can in reality be anywhere between 90 and 110uF. Read your multimeter manual to see the precision for capacitance. I doubt that is 1% as you expect. More than 20% tolerance for electrolityc capacitors is common \$\endgroup\$ – Dorian Apr 21 '18 at 14:23
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Electrolytic capacitors have an electrolyte similar to a re-chargeable battery, but designed to instantly charge and discharge. In testing large values with a DVM it is the DVM supplying the current, thus the reading changes by a small amount as it charges up.

Like batteries capacitors do age in the sense the electrolyte breaks down at a microscopic scale, which expands with time. To maintain the shelf life of high-value capacitors it is good to charge them up every ten years or so. For a short time the leakage current is a bit high, but a good capacitor will recover with time and have much lower leakage.

You may find very old capacitors that will not hold a charge for anywhere near the expected time. The electrolyte has gone bad over large areas, basically turning into metallic salts, thus shorting out any charge.

You may find very old capacitors with a fraction of their original value. This is due to oxidation of the conductor (usually film) and is made worse if the capacitor has high AC or DC voltage applied for months or years non-stop.

This is a sign of a capacitor that did not have an air-tight seal from the factory. A conductor exposed to air and high-voltage disintegrates fast, less than a year. I will not name any names but there are those who make 'cheap' quality capacitors, with latent failures common.

NOTE: Batteries and capacitors are NOT the same chemically or design wise. Capacitors do not have a chemical reaction to produce current, as batteries do. If a capacitor has 'electrolytes' it is to enhance its storage ability, combined with certain dielectrics. A battery can produce current from the moment it is made, but a capacitor must be charged up. The total long term energy storage of a capacitor is a fraction of a battery of the same size.

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  • \$\begingroup\$ Sparky, the chemistry of batteries has nothing to do with the chemistry of the electrolytic capacitors because they actually don't have one. The electrolyte is simply used as a conductor, there isn't any chemical reaction inside a capacitor like in rechargeable batteries. \$\endgroup\$ – Dorian Apr 19 '18 at 9:47
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    \$\begingroup\$ I have added a 'NOTE' section to clarify that issue. \$\endgroup\$ – Sparky256 Apr 19 '18 at 20:08

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