Abstract: An electric vehicle power supply system and cargo box includes an electric vehicle assembly, for location on the vehicle. The assembly includes a drivetrain, vehicle electric arrangements, including a low voltage arrangement having a vehicle system battery, for powering vehicle electric systems, and a vehicle first converter, converting power from traction battery pack to vehicle system battery. The assembly includes traction battery pack, for powering the electric arrangements and drivetrain. A cargo box assembly includes cargo box electric arrangements, including a low voltage arrangement with cargo box system battery, for powering box electric systems, and a cargo box first converter, converting power from box battery pack to cargo box system battery. The box assembly includes cargo box battery pack, for powering the box electric arrangements and traction battery pack, and a cargo box second converter for converting power from a charging system to the cargo box battery pack.