Lime Battery

We wanted to see how a battery worked. we put a zinc nail and a piece of copper into a lime and connected it to a volt meter. 

1 lime = 0.95 volts 

2 limes = 1.96 volts 

3 limes = 2.9 volts and it lit up a LED.  

Does it depend on the size of the lime?
What if I put more nails and copper in?
Can I use different nails? 

Can you put more nails in?

A: Yes, you can as long as you have a balanced amount of nails. 

The link to the youtube video: https://www.youtube.com/watch?v=GhbuhT1GDpI