So, this week we did a student designed experiment in my Physics 145 class. We decided that we were going to find out the internal resistance of a potatoe battery. So we brought a bundle (26) potatoes and one lemon, brought out the pennies and zink-screws and built a potatoe battery! When we connected them all together we got a 20.7 volt battery! Sadly, the internal resistance is huge (we aren't quite sure yet what it is; we are going to finalize it this week) that there is practically no current. We measured the current in micro-Amps.
This is the full battery, minus the lemon, that we built. We tried to light a diode with it, but there wasn't enough current. So, we charged a capacitor for about 12 seconds, and discharged it through the diode, and it turned on for a moment. It was lots of fun.