Hello
No matter the size I always get the same voltage (approx 0.5 V)
with copper and aluminium in salty water solution.
I can only power 1 LED with a joule thief.
Why do I get always the same voltage ?
What do you think about it ?
How to increase efficiency ?

 

Using the same electrode materials and electrolyte will always give the same potential. Increasing the area and reducing the spacing will reduce the internal resistance of the cell which will allow you to draw more current from the cell. Technically what you have is a single cell. A battery is a number of cells in series although many people use the term battery when they mean a single cell.

Les.

 

You will get higher voltages the further apart your electrode materials are on the galvanic series, see link below.
Thus AA cells used to use zinc and carbon to get 1.5V which are about as far apart as you can get. Copper and aluminium are pretty close in this table.
https://en.wikipedia.org/wiki/Galvanic_series