The voltage rating is based on the number of cells connected in series and the nominal voltage of each cell( 2.0V for lead acid cells, and 1.2 V. for Ni-cad ) The most common voltage rating for aircraft batteries is 24 V. A 24-V lead-acid battery contains 12 cells, while a 24-V nickel-cadmium battery contains either 19 or 20 cells.