# Discharging a capacitor at a battery like rate

## Main Question or Discussion Point

Discharging a capacitor at a "battery like" rate

We all know that capacitors discharge at a rate determine by the time constant of the circuit they are hooked up to (tau = RC). This discharge takes the form of a "dieing exponential" with respect to stored energy and likewise voltage. Capacitors would be much more useful for energy storage applications if they could also discharge at non-exponential rates.

It would seem to me that one does not have very much if any control on the shape of the "discharge curve" aside from using a larger or smaller resistance (which doesn't really change the shape of the curve only its dimensions). It would seem that one could use a high power transistor in series with the load to dynamically change the time constant of the circuit (applying a dynamic gate-source or base-emitter voltage to dynamically control the effective resistance of the transistor). Could one then achieve a linear discharge of energy/voltage? Are there significant drawbacks to this method of discharge?

If that method is impractical or useless, are there other methods for achieving "battery like" discharge or even simply non-exponential discharge? To my knowledge there are no such methods; it seems to me that the laws of physics would not permit it. If it sounds too good to be true it probably is, right?

The only other thought that immediately came to mind was using perhaps a capacitor and flywheel combination. One could discharge energy from the capacitor when needed, into the flywheel (spin it up). Perhaps one could then discharge the energy from the flywheel in a "battery like" fashion. However I also immediately doubted this thought because kinetic energy = .5*m*v^2 (quite like the capacitor equation E = .5*C*v^2) . This would mean if an object were to tap into the flywheels energy by physical contact the flywheel would loose its energy in a "dieing exponential" fashion just like the capacitor. Am I mistaken here?

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Just guessing, but perhaps this is an issue that capacitors "store" electricity whereas batteries "generate" electricity.

If that method is impractical or useless, are there other methods for achieving "battery like" discharge or even simply non-exponential discharge? To my knowledge there are no such methods; it seems to me that the laws of physics would not permit it. If it sounds too good to be true it probably is, right?
There are efficient dc-to-dc converters called SEPIC (buck-boost) converters that will efficiently convert the capacitor output power to a constant voltage. See

http://en.wikipedia.org/wiki/SEPIC_converter

Bob S

There are just a bunch of obstacles for capacitor energy storage to match that of a battery....lots of them economics based....a few insights from Wikipedia...would be a good idea to look up the energy density storage of some typical batteries....

Energy storage
A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.)

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technologies can provide more than 2.52 kilojoules per kilogram[20].

In car audio systems, large capacitors store energy for the amplifier to use on demand. Also for a flash tube a capacitor is used to hold the high voltage. In ceiling fans, capacitors play the important role of storing electrical energy to give the fan enough torque to start spinning.
http://en.wikipedia.org/wiki/Capacitor#Energy_storage

There are just a bunch of obstacles for capacitor energy storage to match that of a battery....lots of them economics based....a few insights from Wikipedia...would be a good idea to look up the energy density storage of some typical batteries....

http://en.wikipedia.org/wiki/Capacitor#Energy_storage
I understand that capacitors are still well behind batteries in terms of energy density. My question was not whether or not a capacitor could compete with batteries; it was how to discharge a capacitor in a similar way to a battery.

There are efficient dc-to-dc converters called SEPIC (buck-boost) converters that will efficiently convert the capacitor output power to a constant voltage. See

http://en.wikipedia.org/wiki/SEPIC_converter

Bob S
Thanks Bob. Any idea where I could find some examples of people using a SEPIC converter with an ultracapacitor or supercap as the source? A quick google search doesnt yield much. I can find plenty about the SEPIC converter, but little about its use with a capacitor as the source.

I couldn't find a specific application using ultra caps, but here is a typical SEPIC application using a Linear Technology chip:

http://pdg.lbl.gov/2009/reviews/rpp2009-rev-passage-particles-matter.pdf

That converts up to 40 volts into 24 volts (or whatever).

Here is a search page for finding a SEPIC converter for your app.

http://www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1003,C1042,C1036

Bob S
There has to be some serious limitation to this solution right? I mean if both you and I cant find anything online demonstrating that other people have tried this (ultracapacitor SEPIC setup), then something is up. Can these SEPIC converters be made to be user customizable (I.E provide variable output voltages)? Or is the output voltage of the SEPIC fixed? It would seem one would be able to make a very versatile DC source using one of these (if indeed their output is customizable within some range).

The SEPIC integrated circuit converters are dc voltage regulators, meaning that the output voltage can be adjusted, and will put out a stable flat preset fixed voltage, like for example 13.91 volts, independent of the input voltage.

Bob S

Here's a switching power supply that uses ultracapacitors for temporary storage.

http://www.gammaresearch.net/hps-1a.html

And here's a review with pictures.