Subject: Why does my potato battery increase voltage with time?

Thank you for your assistance with building a potato battery.  I used a 
nickel and a penny in the end of my potato.  I recorded the mV.  Later I 
went back to confirm my results.  The voltage had increased not decreased 
as I anticipated it would.  It continued to increase over the next several 
days.  Why?  I suspect it is because the different types (-,+) ions in the 
potato have aligned or grouped themselves together.  Am I correct or have 
I totally missed the mark?

Re: Why does my potato battery increase voltage with time?