Can I eat the lemons after I have used them as a battery?
In a lemon battery, the zinc from the galvanized nail is giving up electrons and transitioning into an aqueous state:

$$\ce{Zn → Zn^2+ + 2e−}$$ 

While zinc is entering the electrolyte, two positively charged hydrogen ions ($\ce{H+}$) from the citric acid with two electrons at the copper electrode's surface and form an uncharged hydrogen molecule $\ce{H2}$:

$$\ce{2H+ + 2e− → H2}$$

While zinc is a mineral our bodies need, I wouldn't recommend using this as a source of zinc. Also, zinc toxicity is a thing, so without knowing the dose you're taking, you risk poisoning yourself. 



Lemon Battery Wikipedia