An electric battery is an electrochemical device that converts stored chemical energy into usable electrical energy through spontaneous redox reactions. Functionally, it consists of two dissimilar electrodes—an anode and a cathode—separated by a conductive material called an electrolyte. When a voltage difference exists between the electrodes, chemical reactions occur at the anode, causing it to oxidize and releasing electrons. These electrons flow through an external circuit to the cathode, where they facilitate a reduction reaction. Batteries are categorized generally as primary cells, which are designed for single use, or secondary cells, which are designed to be recharged by reversing the chemical reaction. The efficiency and capacity of a battery are determined by the inherent chemical potentials of the materials used and the electrochemical processes that occur within the cell structure.