lime Battery

We wanted to see how a battery worked. We put zinc nail and a piece of copper into a lime and connected to a volt meter.
1 Banana=1 volt
1 Lime=0.95 volts 
2 Limes=1.96 volts
3 Limes= 2.9 volts and it lit up a LED

So does it depend on the size of the lime?
What if I put more nails and copper in?
Can I use different metals? 
Can you put more wires in? Yes

Electronics

Website  tells us we need to have a torch and spare batteries.