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I have a high voltage (750V) high value electrolytic cap about as big as my forearm. It has been in my storage box for about 20 years. In general, do electrolytics degrade under normal storage conditions ie room temperature over such a time? Not quite sure what type it is, but probably a standard Aluminium foil, not solid state. If they do degrade, what characteristics degrade?

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    \$\begingroup\$ I don't know enough to answer properly but it's a common problem on bringing old valve radio or test equipment back to life after years on the shelf. The normal practice is to use a variac (and, for you, a rectifier too) and slowly bring the voltage up - maybe with an inline current limiting resistor and an ammeter. Time to full voltage might be in the region of 20 minutes or so with "soakage" time at various points along the way. Elektor magazine had some articles on this in their Retronics section. There may be some freely available on line. \$\endgroup\$ – Transistor Mar 20 at 11:09
  • \$\begingroup\$ @Transistor Thanks. However, the energy storage is in the hundreds of joules and I don't want it popping! \$\endgroup\$ – Dirk Bruere Mar 20 at 11:27
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    \$\begingroup\$ While the experts on this site are busy answering the main question, I will answer the easy bit: if it gurgles or clucks then there is a risk it may produce other sound effects (such as bang or phizz) when you wire it up to the mains. I suggest you test it while wearing safety goggles and standing at least 100m away and you should be OK. Under these test conditions there is no need for the other precautions that would limit the rate of charge,as suggested by Transistor. \$\endgroup\$ – David Robinson Mar 21 at 13:13
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    \$\begingroup\$ When I was in high school electric shop around 1955 we played catch with those. But they were only charged to 150 Volts.😝 \$\endgroup\$ – richard1941 Mar 23 at 14:53
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    \$\begingroup\$ Some relevant products for this application bonitron.com/maintenance.html \$\endgroup\$ – Stephen Collings Mar 27 at 15:41
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The main thing that degrades is the aluminum oxide dielectric layer on the surface of the aluminum foil. It's also possible that electrolyte (or at least its more volatile components) has leaked through the seals.

You can try to re-form the oxide using the procedure that @Transistor has outlined: Use a variable DC voltage source to bring up the voltage on the capacitor gradually over the course of several hours. Monitor the leakage current carefully, and if it starts to rise out of proportion to the voltage, hold the voltage there and see if it goes back down.

The only thing that will make it "pop" is excessive power dissipation, so keep track of the power, and if it rises above a watt or two, you're probably out of luck.

If you get to full rated voltage successfully, you still need to measure the actual capacitance to verify that you haven't lost electrolyte. Be sure to discharge the high voltage first!

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