Use for overly ripe limes?
We used lemons in high school.
Whichever is more acidic, you can test them with litmus paper (probably you don't have any of that though). Lemon is more likely to be the better choice and copper works well.
While a lime is more acidic, a lemon is a better choice because it strips copper ions from the one electrode generating energy I would test them both and use a voltmeter to find out for yourself ^_~
