Running the world on tomatoes

We do not get all the energy out of a battery we would expect from the chemistry. All cells (the correct term of a battery; more than one cell makes a battery) have an internal resistance that stops the emf (electromotive force the potential difference produced by the cell) from getting out into the real world.
The is another potential divider problem. Some energy is taken up by the internal resistance so the voltage we measure outside the cell is less than the emf. The lower the current the closer V out comes to the emf.
Stick some metal into fruit or veg and you have a cell which you can measure a potential difference from. They have a large internal resistance so very little current flows meaning very little power output make them useless as a power source for most things.

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