I'm thinking the aluminum plates of the same metal water battery are like magnets. where a magnet would have a north pole and a south pole I find that the aluminum plates also have something like a north pole and a south pole. I've made a video showing that when I put one end of the plate into the water I get a negative voltage (south pole) and when I switch to the other end of the plate I get a positive voltage (north pole). Also You may notice that I get a higher voltage when I'm in the positive range and thats due to the other plate being attracted to the plate, Like a north pole of magnet being attracted to a south pole. So You could say that these cells are magnetic.

here's the video. http://www.youtube.com/watch?v=n-To_TadRGI