These batteries are simply voltaic piles made by stacking alternating types of coins with filter paper soaked in saturated salt water solution. The metal compositions of pennies are sufficiently different from nickels so that these coins may be used to make weak batteries. More coins can produce more voltage, but there is a tremendous variability in the actual voltage measured. This is likely due to effects such as internal resistance and degree of corrosion on the coins.
See: Scharlin, P.; Battino, R.; Boschman, E. J. Chem. Educ.1991,68, 665.
ABOVE: (FIRST) A simple coin battery - a nickel on salt-water soaked filter paper on a penny. (SECOND) Measuring the potential of a single pair of coins (in millivolts). (THIRD) Measuring the potential of multiple pairs of coins (in millivolts).
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