i made a potato battery for pack meeting (for cub scouts). anyway, here's the best way i found on the internet on how to light up an LED.
i ended up using eight potatoes in series and parallel. and what i heard from some of the kids was that they could barely see it. but the LED was very tiny.
we also tested apples, pears, bananas, grapefruit, and lemons and compared them. the potato in parallel had the most potential, and i think the potato, lemon, and grapefruit were slightly more powerful than the apple, pear and banana, but not enough to make a significant difference.