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**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?