I thought the lemon battery would last much longer than the potato battery. my potato battery ran my clock for 16 hours; I was lucky to get 5 hours out of the lemon. based on what i've read its the acid or the liquid in the fruit/veggie. did I just do something wrong?

You are, I think, confusing the fact that the thing works at all (a function of the total electrolyte content, which is similar for a potato or a lemon) and the length of time that it works, which is a function of how long your electrodes last. How long an electrode -- more specifically, the anode -- lasts depends partly on what sort of electrolyte it is immersed in.

A lemon, where the electrolyte has a lot of acid, is much more corrosive than a potato, which has more neutral pH. Your anode, which is made of a reactive metal, will corrode in either the potato or the lemon, whether or not you are drawing power from it -- but it will corrode much faster in the lemon. This corrosion is in addition to the corrosion caused by drawing electricity from the electrodes.

See the food batteries FAQ.

Dan Berger