I have 6 x 500 farad super caps in series. I want to use them to smooth voltage dips in a 12v system that is powered by a DC-DC converter. Currently my application is a water pump (desalination) that builds pressure and releases it through a check valve, cycling like this every 3 or 4 seconds. As the load increases the voltage drops slightly, which slows the pump slightly, and over many hours would decrease the total output of the pump.

The DC-DC converter says it is "Suitable to buffer-charge a battery" Which makes me think it has some current limiting built in. But I have killed one before when I shorted the output, and i'm not keen to kill another one.

My intention is to put the cap bank in parallel close to the motor, with a 20 amp fuse and a large 0.5 ohm resistor inline with the positive cable coming from the Cap bank.

I will pre charge the caps, so as to limit the inrush anyway, but in a situation when they self discharge for some reason and they are cut off from the DC-DC converter I don't want them to turn the DC-DC converter inside out when I flip the switch.

As I say I REALLY don't want to kill my DC-DC converter, so I want to see what those in the know have to say about my plan.

  • \$\begingroup\$ That capacitor bank will hold the equivalent of almost 52 grams of TNT! Just keep that in mind! I'm not sufficiently capable of telling you how to safely manage this process. You probably know more than I do. So I'll leave this to others. It's dangerous stuff and I can easily imagine far, far too much wasted energy as well without using inductors in similarly seriously dangerous ways. So that puts me into watch and learn mode, I think. \$\endgroup\$
    – jonk
    May 1, 2021 at 3:44
  • \$\begingroup\$ Can we see the graphs of voltage dips? what does the oscilloscope show? \$\endgroup\$ May 1, 2021 at 9:18
  • 2
    \$\begingroup\$ Using a small lead acid battery in place of the supercaps would probably be a lot simpler... \$\endgroup\$
    – bobflux
    May 1, 2021 at 10:13

1 Answer 1


If the dips are rare enough, you can charge the capacitor through a resistor and place a big diode across the resistor so that when the voltage tries to dip, the capacitor can hold the voltage up once it falls by a diode drop.


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