Got tired of replacing the lead-acid battery in a UPS (Uninterruptible Power Supply), so put together a super-capacitor array to go in its place.
Such units are now commercially available
Step 1: Super-Capacitor array
Started with six used 2600F 2.5V Maxwell super-capacitors that I found for $70 with postage and cross-bars included (from www.goldmine-elec.com, but they no longer stock them). The six are wired in series to allow for the voltages seen by a 12V battery; fortunately the array was found to be so well balanced by itself, that no extra circuitry was needed to ensure that no capacitor experienced an over-voltage. A UPS that trickle charges its battery will overcharge such an array, so a protective Zener diode would need to be added; the UPS I used it with did not have this problem, so there was no need for any extra components.