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# How much and how efficiently can a capacitor store charge? Answered

I recently found out all the items required to build this project: https://www.instructables.com/id/Electricity-Generating-Footwear/ .  I don't want to buy the power bank, so I though about using rechargeable batteries. But I had none, but I had capacitors and I thought of using them.  These capacitors 1000 uf . I am thinking about soldering 5 of them in paralell so that I have 5000 uf storage capacity, But I am concerned how well it would work, if at all.   Can this store upto the amount of charge that a powerbank can? Can it actually charge up at a constant voltage if I use it?  Thanks for helping!

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## 5 Replies

-max- (author)2014-07-06

A capacitor with low leakage can hold a charge for a very long time. Theoretically, they can hold a charge indefinitely. However, at the moment, not even supercapacitors ( Often around 0.5F-10F) are even close to the energy density of chemical-based batteries. However, they generally have much higher power densities, that is, to charge and discharge very fast.

Your best bet for charging a device is probably supercapacitors. They offer OK energy density, have decent power density, and are generally much more efficient than batteries.

Looking at the 'able with the shoes, the piezo elements produce small high voltage spikes of voltage at low currents. Since the output is high frequency AC, it needs to be rectified and have a small filtering capacitor. The voltage will probably be about 50V or so, so that needs to be stepped down with a constant-current switch mode converter and connected to a battery and/or supercapacitor. I am afraid there is still not enough effective power for charging a phone after all the conversions. It may charge an old phone or MP3 player but not a modern smartphone.

Calculating 5000uF of capacitance, at 5V, yields only 0.000069 watt-hours. To put this in perspective, a smartphone phone battery stores close to 5.5-10 watt hours.

seandogue (author)2014-06-30

No, Capacitors are transient devices, meant to hold charge only for a short period of time. Secondly, in terms of capacity, and aside from exotic "super caps (which are usually very very expensive and aren't salient to your post at all) batteries out-weigh them hands down. The basic purpose of a cap is to filter or act as an "instantaneous" reserve (the electronic analog to hydraulic's accumulator). The battery's purpose is bulk storage of electrical charge.

mpilchfamily (author)2014-06-30

Capacitors don't hold a charge vary well and they tend to release what charge they have all at once. You'll need to use batteries or buy supper caps.

steveastrouk (author)2014-06-30

Capacitors CAN hold a charge VERY well - you pays your money.... And they release their charge as fast as their loads permit.

steveastrouk (author)2014-06-30

The relevant formula is energy stored = 1/2 x C x V^2/10^6, if C is in uF, and V is in volts..

Plug the numbers in