Starting and electrical generation system for cooperating with the bipolar onboard network of a motor vehicle with heat engine comprising: - an electromechanical device allowing for the combined functions of electricity production - operation in electrical generator mode - and of mechanical power production - operation in electric motor mode - notably for starting the heat engine, comprising two terminals of different polarity; - a first and a second distinct electrochemical conversion units, such as electrochemical batteries, comprising two terminals of different polarity; - a switching unit (K1, K2, K3) intended to connect one or other of the batteries to the onboard network, - said switching unit comprising: - a switch intended to be connected between a first polarity of the onboard network and a first terminal of the second electrochemical unit; - a changeover switch intended to be connected to a second terminal of the first electrochemical conversion unit brought to a second polarity and on the onboard network and a first position of which is linked both to a second terminal of the electromechanical device and to a second terminal of the second electrochemical unit and a second position of which is linked to the common point between a second terminal of the switch and the first terminal of the second electrochemical conversion unit; characterized in that the onboard network is connected to the terminals of the first electrochemical conversion unit and in that, to allow the two electrochemical units (10, 11) to operate at different operating temperatures, notably during the recharging of one of them, said system comprises an additional switch inserted between the second terminal at a second polarity of the first electrochemical unit and the common point between the changeover switch and the onboard network. - an electromechanical device allowing for the combined functions of electricity production - operation in electrical generator mode - and of mechanical power production - operation in electric motor mode - notably for starting the heat engine, comprising two terminals of different polarity; - a first and a second distinct electrochemical conversion units, such as electrochemical batteries, comprising two terminals of different polarity; - a switching unit (K1, K2, K3) intended to connect one or other of the batteries to the onboard network, - said switching unit comprising: - a switch intended to be connected between a first polarity of the onboard network and a first terminal of the second electrochemical unit; - a changeover switch intended to be connected to a second terminal of the first electrochemical conversion unit brought to a second polarity and on the onboard network and a first position of which is linked both to a second terminal of the electromechanical device and to a second terminal of the second electrochemical unit and a second position of which is linked to the common point between a second terminal of the switch and the first terminal of the second electrochemical conversion unit; - a switch intended to be connected between a first polarity of the onboard network and a first terminal of the second electrochemical unit; - a changeover switch intended to be connected to a second terminal of the first electrochemical conversion unit brought to a second polarity and on the onboard network and a first position of which is linked both to a second terminal of the electromechanical device and to a second terminal of the second electrochemical unit and a second position of which is linked to the common point between a second terminal of the switch and the first terminal of the second electrochemical conversion unit;