So I have an old cellphone and charger that I'ld like to charge off of earth batteries. Not practical but an amusing project. Advice on how to proceed?
As you say, not practical. You'd need a DC/DC boost converter to get to 5V which the cellphone could use, but the current available would probably be small, especially after electrolytic action and soil contaminants attacked the earth battery electrodes.
Two different metal rods (I use scrap copper wire and aluminum cans with the plastic seal sanded off) in wet dirt. Roughly a volt per battery, amperage depends on the size of the rods, closer together without touching the better. I can get volts in series no problem. Amps are my big hurdle at 200μA
Simple 5 groups of 5000 of these cells (Battery is a number of single cells connected in series.) connected in parallel connected in series. this will give 5 volts at 1 amp which is about what you want.
When I was a kid a lemon made a better battery than a potato. But the metal rods disappeared quicker.
Why not charge the cellphone normally using mains electricity like everybody else? Oh, you live in a cave with no electricity? How about using solar power or a windmill generator?