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.
Step 2: Carrier Design
Designed a wood carrier that would take the place of the lead-acid battery. Tried to keep the design simple, and got it down to two dowels and two end-plates that needed holes drilled. The carrier is glued together permanently for strength, but still allows the capacitors to be removed and replaced. The two central dowels hold things together and stop the capacitors moving around; added a final cross-bar of wood across the output for physical strength.
Step 3: Possible Alternative Configuration
Considered the possibility of reconfiguring the array if the space available was narrower but higher; in my case the original design fitted well and was preferred for the support the inner dowels gave. In both designs the bolt heads reside part way into the end-plates, ensuring that the array cannot unfold.
Step 4: Final Product
Made the carrier of oak; a tough wood to work with, and not cheap (the ends were made from a plank sold for stair steps). The array is equivalent to a 1Ah 12V battery, and can hold up an average computer for a couple of minutes, more than enough to carry it through short glitches, and should endure a lot longer than the lead-acid battery it replaced. One can see here how the capacitor bolts are accessed through holes in the carrier end-plates (for assembly and dis-assembly). The rear plate is taller than the front to match the original battery mounts.
Super-capacitors can hold a lot of charge, so a disclaimer is necessary, and one must proceed at ones own risk.
Step 5: In Situ
Fortunately there was enough room inside my UPS to house the capacitor array.