Publisher Summary This chapter deals with the basic theory behind the operation of batteries. A galvanic or voltaic cell consists of two dissimilar electrodes immersed in a conducting material such as a liquid electrolyte or a fused salt; when the two electrodes are connected by a wire a current flows. Each electrode, in general, involves an electronic (metallic) and an ionic conductor in contact. At the surface of separation between the metal and the solution there exists a difference in electrical potential, called the electrode potential. The electromotive force (e.m.f.) of the cell is then equal to the algebraic sum of the two electrode potentials, appropriate allowance being made for the sign of each potential difference as follows. The electromotive force of a cell is defined as the potential difference between the poles when no current is flowing through the cell. The electromotive force of a cell is usually measured by the compensation method. Further, the chapter discusses the effects of couple materials, electrolyte composition, concentration, and temperature on battery performance, followed by the factors that affect the discharge rate on battery capacity.