Capacitor UPS

Hello,

I would like to wire several supercaps together to create a UPS of sorts. I found an Illinois 400F capacitor but the voltage rating is 2.7v. Is it possible to wire some of these in parallel to increase the voltage capability, say 10 or so to reach 27v?

TIA

You can wire capacitors in series to increase overall voltage ratings. Two 400F 2.7V capacitors in series will be good for 5.4V, but will only have 200F of equivalent capacitance (you don't get something for nothing!)

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What do you mean "UPS of sorts"? Are you going to build control hardware to handle the proper charging and discharging of the "super cap"? Or are you going to try and make a drop-in replacement for a battery?

In order to handle higher voltages, you will need to put the capacitors in series. However remember that capacitors in series reduces the overall capacitance. C = 1 / ((1/C1)+(1/Cn)). So if you put 4 of these in series they will handle 10.8V but your total capacitance drops to 100F.

Also, take note of what the ESR of each of the capacitors are. Since they are in series, these will add up. Calculate how much power they will be dissipating during charging and discharging by using the total ESR and the amount of current you plan to draw. Make sure if you cause their temperature to rise that their capacitance (or voltage rating) doesn't de-rate.

Lastly, pay special attention to the datasheet. Some capacitor technologies require de-rating, especially if the ambient + core temperature goes much above room temperature.

What do you mean "UPS of sorts"?

With a AC to DC power supply, power continues to flow for a few seconds (depending on load) after the AC is disconnected. What I wanted to do was to add a capacitor to the DC side to increase that effect to be able to provide 10 seconds of power for a 200 watt power draw. (at least 500F at 24v)

I see now that it would take a grid of capacitors linked in series for voltage rating, and then each series linked in parallel for capacitance; the end result would fill a cabinet and cost thousands of dollars! :astonished:

I guess I will just use a traditional UPS until capacitors are available in 1000F, 24v sizes. :(

I guess I will just use a traditional UPS until capacitors are available in 1000F, 24v sizes.

This is why super-capacitors are so interesting. The traditional capacitor’s voltage rated is determined by its dielectric thickness. This thickness also determines the capacitance of the device. A thicker dielectric gives you a higher voltage rating, but it means less capacitance.

In your application, you are asking for both of those to be very high! Which, is difficult to find in traditional materials.

Today the only (cost effective) component that can provide that kind of Power requirements you are asking for, is a battery.

And keep in mind that a capacitors output voltage when suppling a load will continously decreace with time rather then hold a nominal output voltage like a battery does until the battery reaches end of charge. This means you wil have analog input pin calibration problems from the start and eventually reach the brown out fuse setting which will then hold the arduino in reset.

Bottom line a super cap is a very poor choice to try and power an arduino board, just not designed for that kind of service.

Lefty