Power supply for powering an electric load of a vehicle

The present invention relates to an electrical power supply system (1) for powering an electric load (2,3) of a vehicle (60) comprises a main battery (6), a support battery (7) and a battery control module (8). The battery control module (8) comprises a first terminal (11) for connection to said main battery (6), a second terminal (12) for connection to said support battery (7) and a third terminal (13) for connecting to said electric load (2,3). Furthermore the module (8) comprises a voltage comparator(16) arranged to determine a voltage at the third terminal (13), and output a high output level when said determined voltage is less than a predetermined critical lowest voltage level. A first switch (21) is arranged between said first terminal (11) and said third terminal (13), said first switch (21) being controllable by a first control signal (S1). A second switch (22',22") is arranged between said second terminal (12) and said third terminal (13). The battery control module (8) is arranged to connect the support battery (7) to the load (2,3) by turning the second switch (22',22") conducting in case the voltage at the third terminal drops below a predetermined critical voltage. The invention provides the automatic activation of a support energy source when the output voltage falls below a specified voltage threshold. The support battery (7) supports the main battery (6) when needed or acts as a redundant power source. It is purely needed as a backup battery and not used in normal situations, so charging of this battery is less frequently needed, increasing its lifetime.