This recommended practice prescribes a method for electrical sizing of nickel-cadmium batteries for use on passenger rail cars used for battery back up of low voltage dc auxiliary power systems. It encompasses all factors that influence the electrical battery capacity requirements including loads@ temperature@ cycling@ charging and discharging profiles. It does not address physical sizing@ qualification or testing of the batteries@ and it does not apply to batteries used for motive power or engine starting. Purpose This recommended practice provides a standardized method for electrical sizing of nickel-cadmium batteries for application and use aboard rail passenger vehicles.