6 volt air battery! Very easy to make.
This is an air battery test experiment I have had running for the last few weeks. The outer masking tape is just to hold the magnesium ribbon in place. The battery works fine without the outer masking tape layer. If you do not have cotton you can also use masking tape for the layer between the carbon rod and the magnesium ribbon . Masking tape seems to work almost as good as the cotton with only a slight voltage drop.