Slightly Bulging Capacitors

[CapLeaker]

What capacitors physically look like on the outside doesn’t mean much. They can look perfectly fine but yet are garbage. I get it, the slight bulging or pregnant look is almost a dead giveaway, but doesn’t have to be the case 100% either. Take it as you want it, it is my personal observation repairing electronics.

[Hitto]

Is Nichicon CS an inherently unstable series? Or were the caps somehow overheated??

No, Nichicon CS is a very reliable serie with long lifetime and high ripple capability. It is the equivalent of Panasonic EE/ED.

[momaka]

Very strange if they are new. I wonder what made them bulge just from sitting in storage. According to the date codes, they are from 2017, which is relatively recent. A few years in storage shouldn't be a problem with just about any electrolytic cap (even some less-than-stellar cap brands.) FWIW, I have caps from 2009/2010 that still read OK, reform OK, pass leakage current tests, and have been fine when used for recapping equipment. So I wonder what gives here. Is Nichicon CS an inherently unstable series? Or were the caps somehow overheated??

I also have had caps bulge and still retain their specs... and done so for quite many years (actually, a few of these I still hold on to, just to see what will happen to them... but so far so good.) In my case through, a few were pulls from Xbox 360 motherboards, where someone overheated the caps while doing a GPU reflow. So the bulge on these was due to temporary expansion of contents inside the can. Once the caps cooled and I pressed on their bulged top, there was no more pressure and the caps had gone back to normal. Stj mentioned a good point, that this can sometimes cause the caps to leak through the bottom. But in the case of my overheated caps, this didn't happen.
Now I do also have a few very slightly bulged caps (ironically Nichicon again, but also a Panasonic) from very very old gear (1980's amplifier), and these too, are showing near perfect specs and low leakage current. However, when I pressed on their tops to check for pressure, they were still fully pressurized, so I can't make the bulge go away. At the same time, these caps have been like this for over 10 years now and still not lost their specs... and there's a few more bulging remaining in that amp, which up until recently has been seeing regular use.

While I find this occurrence interesting, I agree that generally it's probably not a good idea to use caps like that... or at least not in anything mission-critical.

As Per mentioned, you can try reforming them (thought a resistor) and also monitor their current as they reform. After 10-15 minutes of reforming, the current should settle to a value below the leakage current given in the datasheet. If it does, then the caps still might be OK to use (but again, advisable to do so only in less-than-critical applications, just in case.) What applications would that be? - Ones where you have clearly failed caps (that measure bad) and you just want to see/test if recapping the device can bring it back to life (i.e. a test recap, if you will )

[Mattz_GT]

Slightly bulging is like "slightly pregnant"

[stj]

they are done - throw them away.
the bulge is caused by gas pressure - that will eventually push the liquid out around the pins if the top doesnt open first.

[Per Hansson]

You could reform them with a high value resistor and diode directly from mains if you don't have a high voltage power supply.
If they take a charge of 325VDC though say a couple 100k resistors I would say they are fine: important thing is that current drops to zero and that they then retain that charge.
Usual caveats apply: mains is quite deadly and unforgiving about making mistakes

[Hitto]

I have seen capacitors bulging while being in storage but we are talking about one or two out of ten. This is the first time that I have seen a whole lot of identical capacitors failing en masse but still measuring perfectly (E.S.R. is around 1 Ohm which is quite an achievement for a 400V/22uF capacitor and loss of voltage is also perfectly in line compared to other capacitors of same class such as Panasonic EB).

I doubt that the vent is deformed/bulged per design so should I ignore the fact that they all measure perfectly and just assume that they are defective and going to gas out? More feedback from direct experience is welcomed.

[petehall347]

send them to me then i can dispose of them properly

[Hitto]

Would you consider a slightly bulging capacitor defective if it measures perfectly and it is brand-new? To be honest, they are all equally like this: vent is not totally flat but they have very low E.S.R. and VLOSS is negligible.

So, everything seems perfect apart from the vent looking a bit "strange". They are genuine capacitors, BTW.

The bad thing is that they are 400V capacitors so testing them just to check how much they last would mean using them on the primary of a power supply, which means the risk of damaging the whole circuit.

What do you think?