This recommended practice describes methods for electrical sizing of nickel-cadmium batteries for application on rail passenger vehicles used for battery back-up of low voltage dc auxiliary power systems. It encompasses various factors that govern the electrical size of the battery including but not limited to number of cells, rated capacity, maximum system voltage, minimum system voltage, correction factors, and margins. It does not describe methods for defining the dc load profile nor address the physical sizing, installation or maintenance of the batteries. Nor does it apply to batteries used for motive power or starting internal combustion engines. Design of the low voltage dc system and sizing of the low voltage power supply and battery charger(s), and consideration of battery types other than nickel-cadmium batteries are also beyond the scope of this recommended practice.