SAE J 539:1993-11-23

This SAE Recommended Practice is intended to apply to lamps, batteries, heaters, radios, and similar equipment for operation with mobile or automotive diesel engines. Twenty-four V systems have long been used for heavy duty services because 24 V permit operating 12 V systems in series-parallel. Thirty-two V systems have been used for marine, railroad-car lighting, and other uses. Generators, storage batteries, starting motors, lighting, and auxiliary electrical equipment shall be for nominal system ratings of 12, 24, or 32 V as determined by the power requirements of the application. It is recommended that no intermediate voltages be considered. The combination of a 24 V starting motor and two 12 V batteries connected in series for cranking is considered practice where it can be adapted to the installation. The batteries are reconnected in parallel for charging from a 12 V generator/ alternator and for operating lights and other auxiliary equipment, or charged separately and used individually for lights and other electrical equipment.