Here's a very effective earth battery that's easy (and cheap) to build. This won't run your AC in a Zombie Apocalypse but it can be used to run small electronics direct, or charge the (rechargeable) batteries in you tactical flashlight, tactical night-vision, or tactical time machine. Or even an automobile battery, if the installation was appropriately scaled up. It can also light up LEDs without a lot of fuss or fancy circuitry. The ice-tray version is super easy to build and use, but the larger the electrode area the more amps you get, and the more cells, the higher the voltage. And the larger the cells, the more energy contained in them. You can use flowerpots full of dirt just as easily as ice trays. Or mason jars. Or dead Tupperware containers. Rather than using zinc-plated (galvanized) bolts I'd recommend using scraps of drywall corner strip. It's galvanized, and has a lot of surface area for the buck. It's also punched along the edges, so you can tie the copper wire right on. In the 1800's, half the telegraph system in the US was powered entirely by earth batteries, although the more advanced telluric version was used.