An electrical power supply system for powering an electric load of a vehicle includes a main battery, a support battery and a battery control module. The battery control module includes a first terminal for connection to the main battery, a second terminal for connection to the support battery and a third terminal for connecting to the electric load. A switch is arranged between said second terminal and said third terminal. The battery control module is arranged to connect the support battery to the load by turning the switch conducting in case the voltage at the third terminal drops below a predetermined voltage. The system provides the automatic activation of a support energy source when the output voltage falls below a specified voltage threshold. The support battery supports the main battery when needed or acts as a redundant power source.