Electromotive Force Batteries, by their very nature, introduce resistance as well as a potential difference.
This script "E" is called the "electromotive force" or "emf"; it is the voltage of the battery when there is no load, so the current is zero.
When there is a load on the battery -- that is, when there is a current -- the voltage across the terminals of the battery is reduced by an amount I Rint -- the " I R - drop " due to the internal resistance of the battery.
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(c) Doug Davis, 2002; all rights reserved