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.
I cannot believe it is this easy. I have all the stuff. Really cool. So you would just hold the wire to the battery area? Seems too simple.
It's easy if you have one of those cheap voltmeters from Harbor Freight ($6.00) so you can be sure you have enough dirt cells tototal up to the needed voltage. An AA runs at 1.5 volts, so charge it at 1.75. If it takes all night, that just means your ice-tray is good for two fully charged batteries per day. Add water as necessary to keep the dirt damp, replace the dirt when it's spirit flies up the the Great Sandpile in the Sky. A 12-V car battery normally charges at 13.5 volts--but you'll need 2 amps for a decent trickle charge. That's either going to take some some fairly large batteries (think 5-gal buckets) or a longer time. If you have 0.2 amps, it'll take ten times longer to get a full charge. But even a slow charge is better than no charge. If you google multi-meters and basic electricity, you'll find you can either measure the amps directly, or calculate them very easily. Few people initially stop to consider that a working time machine (aka clock) is a real necessity in Interesting Times. You need one or several to coordinate sentry shifts, meals, and a thousand other things.
It is simple, and hard to go wrong with. With the ice-tray battery, you could just tape a rechargeable AA battery to the two output wires. Just like any charger: the wires go pos to pos and neg to neg. On your ice-tray battery the ( -) wire is the one from the first zinc and the (+) wire is the one from the last copper. Think of it as putting two batteries together and letting the larger amount of electricity in one even out in both. After a while the battery you are charging will get as full as it can. Then it's good to go. (In the video, the long screw should have a long copper wire added to make taping it to a battery easy, & ditto for the copper arch.)
the dirt must be acid base , so lots of ceder tenner red soil is good , in PH neutral it's poor. Remember to charge a 5V item you need 7V with amps to maintain a higher voltage , If you read 5v open circuit & then add a charging device & see zip , just add seeds & grow something..