I've been playing with the magnetic idea of the plates on the same metal water battery. I been swiping a magnet on a plate as to make it magnetic, but of course aluminum is not ferrous metal.
It does seem that when done right I can make the plate loose power by swiping a magnet on it. I had the cell starting out at 450mV but swiping the plate brought it to 350mV. I also the voltage increase too when swiping in a different way but more testing needs to be done to confirm all of this.