Method and systems for an auxiliary power unit

A method for a vehicle, comprising:

• operating an engine of an auxiliary power unit (APU) of the vehicle to supply electrical power to a battery of the vehicle in response to shutting down a main engine of the vehicle and:
• a state of charge (SOC) of the battery being below a determined SOC threshold level, or
• a drain load being applied to the battery that will deplete the battery to a SOC level that is less than the determined SOC threshold level in less time than a determined period;
• and
• operating the engine of the APU to supply compressed air to an air reservoir of the vehicle in response to shutting down the main engine and an air pressure level of the air reservoir of the vehicle being below a determined air pressure threshold level.

A system for a vehicle, comprising:

• a main engine;
• a compressed air system including an air reservoir coupled to air brakes of the vehicle;
• a battery adapted to power one or more electrical loads of the vehicle;
• an auxiliary power unit (APU) including an APU engine rotationally connected to a compressor and an alternator of the APU, the alternator adapted to supply power to the battery and the compressor adapted to supply compressed air to the air reservoir;
• and
• a controller with computer readable instructions stored on non-transitory memory that when executed during operation of the vehicle cause the controller to:
• enable operation of the APU engine in response to shutting down the main engine;
• provide air to the air reservoir from the compressor and provide electrical power to the battery via the alternator of the APU;
• and
• automatically adjust an operating speed of the APU engine, the alternator, and the compressor based on a charge level of the battery and an air level of the air reservoir.