Batteries are electrochemical cells. Electrochemical cells are two electrochemical half-reactions coupled to each other. A zinc-hydrogen battery, which is what you make if you make a citrus battery, has:
a hydrogen-forming half-reaction that happens at 0 V
a zinc-dissolving half-reaction that happens at -0.76 V
In reality, half-reactions cannot exist independently. They must always be coupled to make a cell. This is because otherwise you have electrons without a home. 

Batteries are electrochemical cells. Electrochemical cells are two electrochemical half-reactions coupled to each other. A zinc-hydrogen battery, which is what you make if you make a citrus battery, has:

  1. a hydrogen-forming half-reaction that happens at 0 V
  2. a zinc-dissolving half-reaction that happens at -0.76 V

In reality, half-reactions cannot exist independently. They must always be coupled to make a cell. This is because otherwise you have electrons without a home.