Dual voltage electrical installation of a heavy vehicle comprising: - high voltage energy storage means (BTT), - at least one supercapacitor (SC) or an electrical storage device with similar features, defining low voltage energy storage means (12V or 24V), - a first high voltage line (48V dir) permanently connected with said high voltage energy storage means (BTT), - a first low voltage line (12V dir) always connected with said at least one supercapacitor (SC), - a second high voltage line (48V K) connected with said first high voltage line, subordinate to a state of activation of a vehicle ignition key (KSW), - a second low voltage line (12V K) connected with said first low voltage line, subordinate to a state of activation of said vehicle ignition key (KSW), - charging means (C) comprising a DC-DC voltage converter (C) connected to said second high voltage line (48V K) and adapted to charge said at least one supercapacitor (SC) by means of the energy stored in said high voltage energy storage means (BTT), - comparator means (R2, K2), configured to ▪ measure a state of charge of the supercapacitor (SC) and, based on said state of charge, control said charging means (C) of the supercapacitor (SC) regardless of a state of activation of a vehicle key (KSW), ▪ completely disconnect said charging means (C) from the power supply when it is not necessary to charge said supercapacitor (SC); wherein said charging means (C) consist of adjustable voltage limiting means and consist of said DC-DC voltage converter (C) connected between said first high voltage line (48V dir) and said first low voltage line (12V dir) by means of said comparator means (R2, K2, R3, K3) and characterized in that said comparator means (R2, K2, R3, K3) comprise a first relay (R2) connected between said first low voltage line (12V dir) and a vehicle mass (ground) that controls a first coil (K2) arranged in series with a second relay (R3) between said first high voltage line (48V dir) and said vehicle mass (ground), the second relay (R3) controlling a second coil (K3) adapted to switch a power supply port of said DC-DC voltage converter (C) between said first high voltage line (48V dir) and said second high voltage line (48V k). - high voltage energy storage means (BTT), - at least one supercapacitor (SC) or an electrical storage device with similar features, defining low voltage energy storage means (12V or 24V), - a first high voltage line (48V dir) permanently connected with said high voltage energy storage means (BTT), - a first low voltage line (12V dir) always connected with said at least one supercapacitor (SC), - a second high voltage line (48V K) connected with said first high voltage line, subordinate to a state of activation of a vehicle ignition key (KSW), - a second low voltage line (12V K) connected with said first low voltage line, subordinate to a state of activation of said vehicle ignition key (KSW), - charging means (C) comprising a DC-DC voltage converter (C) connected to said second high voltage line (48V K) and adapted to charge said at least one supercapacitor (SC) by means of the energy stored in said high voltage energy storage means (BTT), - comparator means (R2, K2), configured to ▪ measure a state of charge of the supercapacitor (SC) and, based on said state of charge, control said charging means (C) of the supercapacitor (SC) regardless of a state of activation of a vehicle key (KSW), ▪ completely disconnect said charging means (C) from the power supply when it is not necessary to charge said supercapacitor (SC); ▪ measure a state of charge of the supercapacitor (SC) and, based on said state of charge, control said charging means (C) of the supercapacitor (SC) regardless of a state of activation of a vehicle key (KSW), ▪ completely disconnect said charging means (C) from the power supply when it is not necessary to charge said supercapacitor (SC); <EOS>